This is a TeXinfo version of Berkeley Logo User Manual

Original written by: Brian Harvey

Introduction
************

Overview
========

Copyright © 1993 by the Regents of the University of California

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

This is a program that is still being written.  Many things are missing, including adequate documentation.  This manual assumes that you already know how to program in Logo, and merely presents the details of this new implementation.

Read Computer_Science_Logo_Style, Volume_1:_ _Symbolic_Computing_ by Brian Harvey (MIT Press, 1997) for a tutorial on Logo programming with emphasis on symbolic computation.

Here are the special features of this dialect of Logo:

     Source file compatible among Unix, DOS, Windows, and Mac platforms.

     Random-access arrays.

     Variable number of inputs to user-defined procedures.

     Mutators for list structure (dangerous).

     Pause on error, and other improvements to error handling.

     Comments and continuation lines; formatting is preserved when
     procedure definitions are saved or edited.

     Terrapin-style tokenization (e.g., [2+3] is a list with one member)
     but LCSI-style syntax (no special forms except TO).  The best of
     both worlds.

     First-class instruction and expression templates (see APPLY).

     Macros.

Features not found in Berkeley Logo include robotics, music, GUIs, animation, parallelism, and multimedia.  For those, buy a commercial version.

Getter/Setter Variable Syntax
=============================

Logo distinguishes PROCEDURES from VARIABLES.  A procedure is a set of instructions to carry out some computation; a variable is a named container that holds a data value such as a number, word, list, or array.

In traditional Logo syntax, a non-numeric word typed without punctuation represents a request to invoke the procedure named by that word.  A word typed with a preceding quotation mark represents the word itself.  For example, in the instruction

     	PRINT FIRST "WORD

the procedures named FIRST and PRINT are invoked, but the procedure named WORD is not invoked; the word W-O-R-D is the input to FIRST.

What about variables?  There are two things one can do with a variable: give it a value, and find out its value.  To give a variable a value, Logo provides the primitive procedure MAKE, which requires two inputs: the name of the variable and the new value to be assigned.  The first input, the name of the variable, is just a word, and if (as is almost always the case) the programmer wants to assign a value to a specific variable whose name is known in advance, that input is quoted, just as any known specific word would be:

     	MAKE "MY.VAR FIRST "WORD

gives the variable named MY.VAR the value W (the first letter of WORD).

To find the value of a variable, Logo provides the primitive procedure THING, which takes a variable name as its input, and outputs the value of the accessible variable with that name.  Thus

     	PRINT THING "MY.VAR

will print W (supposing the MAKE above has been done).  Since finding the value of a specific, known variable name is such a common operation, Logo also provides an abbreviated notation that combines THING with quote:

     	PRINT :MY.VAR

The colon (which Logo old-timers pronounce "dots") replaces THING and " in the earlier version of the instruction.

Newcomers to Logo often complain about the need for all this punctuation. In particular, Logo programmers who learned about dots and quotes without also learning about THING wonder why an instruction such as

     	MAKE "NEW.VAR :OLD.VAR

uses two different punctuation marks to identify the two variables. (Having read the paragraphs above, you will understand that actually both variable names are quoted, but the procedure THING is invoked to find the value of OLD.VAR, since it's that value, not OLD.VAR's name, that MAKE needs to know.  It wouldn't make sense to ask for THING of NEW.VAR, since we haven't given NEW.VAR a value yet.)

Although Logo's punctuation rules make sense once understood, they do form a barrier to entry for the Logo beginner.  Why, then, couldn't Logo be designed so that an unpunctuated word would represent a procedure if there is a procedure by that name, or a variable if there is a variable by that name?  Then we could say

     	PRINT MY.VAR

and Logo would realize that MY.VAR is the name of a variable, not of a procedure.  The traditional reason not to use this convention is that Logo allows the same word to name a procedure and a variable at the same time.  This is most often important for words that name data types, as in the following procedure:

     	TO PLURAL :WORD
     	OUTPUT WORD :WORD "S
     	END

Here the name WORD is a natural choice for the input to PLURAL, since it describes the kind of input that PLURAL expects.  Within the procedure, we use WORD to represent Logo's primitive procedure that combines two input words to form a new, longer word; we use :WORD to represent the variable containing the input, whatever actual word is given when PLURAL is invoked.

     	? PRINT PLURAL "COMPUTER
     	COMPUTERS

However, if a Logo instruction includes an unquoted word that is not the name of a procedure, Logo could look for a variable of that name instead.  This would allow a "punctuationless" Logo, * PROVIDED THAT USERS WHO WANT TO WORK WITHOUT COLONS FOR VARIABLES CHOOSE VARIABLE NAMES THAT ARE NOT ALSO PROCEDURE NAMES. *

What about assigning a value to a variable?  Could we do without the quotation mark on MAKE's first input?  Alas, no.  Although the first input to MAKE is usually a constant, known variable name, sometimes it isn't, as in this example:

     	TO INCREMENT :VAR
     	MAKE :VAR (THING :VAR)+1	; Note: it's not "VAR here!
     	END

     	? MAKE "X 5
     	? INCREMENT "X
     	? PRINT :X
     	6

The procedure INCREMENT takes a variable name as its input and changes the value of that variable.  In this example there are two variables; the variable whose name is VAR, and whose value is the word X; and the variable whose name is X and whose value changes from 5 to 6.  Suppose we changed the behavior of MAKE so that it took the word after MAKE as the name of the variable to change; we would be unable to write INCREMENT:

     	TO INCREMENT :VAR		; nonworking!
     	MAKE VAR (THING VAR)+1
     	END

This would assign a new value to VAR, not to X.

What we can do is to allow an alternative to MAKE, a "setter" procedure for a particular variable.  The notation will be

     	? SETFOO 7
     	? PRINT FOO
     	7

SETFOO is a "setter procedure" that takes one input (in this case the input 7) and assigns its value to the variable named FOO.

Berkeley Logo allows users to choose either the traditional notation, in which case the same name can be used both for a procedure and for a variable, or the getter/setter notation, in which variable FOO is set with SETFOO and examined with FOO, but the same name can't be used for procedure and variable.

Here is how this choice is allowed: Berkeley Logo uses traditional notation, with procedures distinct from variables.  However, if there is a variable named ALLOWGETSET whose value is TRUE (which there is, by default, when Logo starts up), then if a Logo instruction refers to a nonexistent procedure (so that the error message "I don't know how to ..." would result), Logo tries the following two steps:

     	1.  If the name is at least four characters long, and the first three
     	characters are the letters SET (upper or lower case), and if the name
     	is followed in the instruction by another value, and if the name
     	without the SET is the name of a variable that already exists, then
     	Logo will invoke MAKE with its first input being the name without the
     	SET, and its second input being the following value.

     	2.  If step 1's conditions are not met, but the name is
     	the name of an accessible variable, then Logo will invoke
     	THING with that name as input, to find the variable's value.

Step 1 requires that the variable already exist so that misspellings of names of SETxxx primitives (e.g., SETHEADING) will still be caught, instead of silently creating a new variable.  The command GLOBAL can be used to create a variable without giving it a value.

One final point:  The TO command in Logo has always been a special case; the rest of the line starting with TO is not evaluated as ordinary Logo expressions are.  In particular, the colons used to mark the names of inputs to the procedure do not cause THING to be invoked.  They are merely mnemonic aids, reminding the Logo user that these words are names of variables.  (Arguably, this nonstantard behavior of TO adds to Logo beginners' confusion about colons.) To a programmer using colonless variable references, the colons in the TO line are unnecessary and meaningless.  Berkeley Logo therefore makes the colons optional:

     	TO FOO :IN1 :IN2

and

     	TO FOO IN1 IN2

are both allowed.

Entering and Leaving Logo
=========================

The process to start Logo depends on your operating system:

Unix:
     Type the word logo to the shell.  (The directory in which you've installed Logo must be in your path.)

DOS:
     Change directories to the one containing Logo (probably C:\UCBLOGO).  Then type UCBLOGO for the large memory version, or BL for the 640K version.

Mac:
     Double-click on the LOGO icon within the "UCB Logo" folder.

Windows:
     Double-click on the UCBWLOGO icon in the UCBLOGO folder.

To leave Logo, enter the command bye.

After initialization, Logo looks for a file in the current working directory named startup.lg and, if one is found, executes the Logo instructions in it. Then, under Unix, DOS, or Windows, if you include one or more filenames on the command line when starting Logo, those files will be loaded before the interpreter starts reading commands from your terminal.  If you load a file that executes some program that includes a BYE command, Logo will run that program and exit.  You can therefore write standalone programs in Logo and run them with shell/batch scripts.  To support this technique, Logo does not print its usual welcoming and parting messages if you give file arguments to the logo command.

If you type your interrupt character (see table below) Logo will stop what it's doing and return to top-level, as if you did THROW "TOPLEVEL.  If you type your quit character Logo will pause as if you did PAUSE.

                     Unix           DOS/Windows          Mac

     toplevel    usually ctrl-C       ctrl-Q      command-. (period)

     pause       usually ctrl-\       ctrl-W      command-, (comma)

If you have an environment variable called LOGOLIB whose value is the name of a directory, then Logo will use that directory instead of the default library.  If you invoke a procedure that has not been defined, Logo first looks for a file in the current directory named proc.lg where proc is the procedure name in lower case letters.  If such a file exists, Logo loads that file.  If the missing procedure is still undefined, or if there is no such file, Logo then looks in the library directory for a file named proc (no .lg) and, if it exists, loads it.  If neither file contains a definition for the procedure, then Logo signals an error.  Several procedures that are primitive in most versions of Logo are included in the default library, so if you use a different library you may want to include some or all of the default library in it.

Tokenization
============

Names of procedures, variables, and property lists are case-insensitive.  So are the special words END, TRUE, and FALSE.  Case of letters is preserved in everything you type, however.

Within square brackets, words are delimited only by spaces and square brackets.  [2+3] is a list containing one word.  Note, however, that the Logo primitives that interpret such a list as a Logo instruction or expression (RUN, IF, etc.) reparse the list as if it had not been typed inside brackets.

After a quotation mark outside square brackets, a word is delimited by a space, a square bracket, or a parenthesis.

A word not after a quotation mark or inside square brackets is delimited by a space, a bracket, a parenthesis, or an infix operator +-*/=<>.  Note that words following colons are in this category. Note that quote and colon are not delimiters. Each infix operator character is a word in itself, except that the two-character sequences <= >= and <> (the latter meaning not-equal) with no intervening space are recognized as a single word.

A word consisting of a question mark followed by a number (e.g., ?37), when runparsed (i.e., where a procedure name is expected), is treated as if it were the sequence

     ( ? 37 )

making the number an input to the ? procedure.  (See the discussion of templates, below.)  This special treatment does not apply to words read as data, to words with a non-number following the question mark, or if the question mark is backslashed.

A line (an instruction line or one read by READLIST or READWORD) can be continued onto the following line if its last character is a tilde (~). READWORD preserves the tilde and the newline; READLIST does not.

Lines read with READRAWLINE are never continued.

An instruction line or a line read by READLIST (but not by READWORD) is automatically continued to the next line, as if ended with a tilde, if there are unmatched brackets, parentheses, braces, or vertical bars pending.  However, it's an error if the continuation line contains only the word END; this is to prevent runaway procedure definitions. Lines explicitly continued with a tilde avoid this restriction.

If a line being typed interactively on the keyboard is continued, either with a tilde or automatically, Logo will display a tilde as a prompt character for the continuation line.

A semicolon begins a comment in an instruction line.  Logo ignores characters from the semicolon to the end of the line.  A tilde as the last character still indicates a continuation line, but not a continuation of the comment.  For example, typing the instruction

     print "abc;comment ~
     def

will print the word abcdef.  Semicolon has no special meaning in data lines read by READWORD or READLIST, but such a line can later be reparsed using RUNPARSE and then comments will be recognized.

The two-character sequence #! at the beginning of a line also starts a comment.  Unix users can therefore write a file containing Logo commands, starting with the line

     #! /usr/local/bin/logo

(or wherever your Logo executable lives) and the file will be executable directly from the shell.

To include an otherwise delimiting character (including semicolon or tilde) in a word, precede it with backslash (\).  If the last character of a line is a backslash, then the newline character following the backslash will be part of the last word on the line, and the line continues onto the following line.  To include a backslash in a word, use \\.  If the combination backslash-newline is entered at the terminal, Logo will issue a backslash as a prompt character for the continuation line.  All of this applies to data lines read with READWORD or READLIST as well as to instruction lines.  A character entered with backslash is EQUALP to the same character without the backslash, but can be distinguished by the BACKSLASHEDP predicate. (However, BACKSLASHEDP recognizes backslashedness only on characters for which it is necessary: whitespace, parentheses, brackets, infix operators, backslash, vertical bar, tilde, quote, question mark, colon, and semicolon.)

A line read with READRAWLINE has no special quoting mechanism; both backslash and vertical bar (described below) are just ordinary characters.

An alternative notation to include otherwise delimiting characters in words is to enclose a group of characters in vertical bars.  All characters between vertical bars are treated as if they were letters.  In data read with READWORD the vertical bars are preserved in the resulting word.  In data read with READLIST (or resulting from a PARSE or RUNPARSE of a word) the vertical bars do not appear explicitly; all potentially delimiting characters (including spaces, brackets, parentheses, and infix operators) appear as though entered with a backslash.  Within vertical bars, backslash may still be used; the only characters that must be backslashed in this context are backslash and vertical bar themselves.

Characters entered between vertical bars are forever special, even if the word or list containing them is later reparsed with PARSE or RUNPARSE. Characters typed after a backslash are treated somewhat differently:  When a quoted word containing a backslashed character is runparsed, the backslashed character loses its special quality and acts thereafter as if typed normally. This distinction is important only if you are building a Logo expression out of parts, to be RUN later, and want to use parentheses.  For example,

     PRINT RUN (SE "\( 2 "+ 3 "\))

will print 5, but

     RUN (SE "MAKE ""|(| 2)

will create a variable whose name is open-parenthesis.  (Each example would fail if vertical bars and backslashes were interchanged.)

Data Structure Primitives
*************************

Constructors
============

word
----

     WORD word1 word2
     (WORD word1 word2 word3 ...)

outputs a word formed by concatenating its inputs.

list
----

     LIST thing1 thing2
     (LIST thing1 thing2 thing3 ...)

outputs a list whose members are its inputs, which can be any Logo datum (word, list, or array).

sentence
--------

     SENTENCE thing1 thing2
     SE thing1 thing2
     (SENTENCE thing1 thing2 thing3 ...)
     (SE thing1 thing2 thing3 ...)

outputs a list whose members are its inputs, if those inputs are not lists, or the members of its inputs, if those inputs are lists.

fput
----

     FPUT thing list

outputs a list equal to its second input with one extra member, the first input, at the beginning.  If the second input is a word, then the first input must be a one-letter word, and FPUT is equivalent to WORD.

lput
----

     LPUT thing list

outputs a list equal to its second input with one extra member, the first input, at the end.  If the second input is a word, then the first input must be a one-letter word, and LPUT is equivalent to WORD with its inputs in the other order.

array
-----

     ARRAY size
     (ARRAY size origin)

outputs an array of size members (must be a positive integer), each of which initially is an empty list.  Array members can be selected with ITEM and changed with SETITEM.  The first member of the array is member number 1 unless an origin input (must be an integer) is given, in which case the first member of the array has that number as its index. (Typically 0 is used as the origin if anything.)  Arrays are printed by PRINT and friends, and can be typed in, inside curly braces; indicate an origin with {a b c}@0.

See ITEM , SETITEM , PRINT .

mdarray
-------

     MDARRAY sizelist				(library procedure)
     (MDARRAY sizelist origin)

outputs a multi-dimensional array.  The first input must be a list of one or more positive integers.  The second input, if present, must be a single integer that applies to every dimension of the array.

Ex: (MDARRAY [3 5] 0) outputs a two-dimensional array whose members range from [0 0] to [2 4].

listtoarray
-----------

     LISTTOARRAY list
     (LISTTOARRAY list origin)

outputs an array of the same size as the input list, whose members are the members of the input list.

arraytolist
-----------

     ARRAYTOLIST array

outputs a list whose members are the members of the input array.  The first member of the output is the first member of the array, regardless of the array's origin.

combine
-------

     COMBINE thing1 thing2				(library procedure)

if thing2 is a word, outputs WORD thing1 thing2.  If thing2 is a list, outputs FPUT thing1 thing2.

See WORD , FPUT

reverse
-------

     REVERSE list					(library procedure)

outputs a list whose members are the members of the input list, in reverse order.

gensym
------

     GENSYM						(library procedure)

outputs a unique word each time it's invoked.  The words are of the form G1, G2, etc.

Data Selectors
==============

first
-----

     FIRST thing

if the input is a word, outputs the first character of the word.  If the input is a list, outputs the first member of the list.  If the input is an array, outputs the origin of the array (that is, the INDEX OF the first member of the array).

firsts
------

     FIRSTS list

outputs a list containing the FIRST of each member of the input list. It is an error if any member of the input list is empty.  (The input itself may be empty, in which case the output is also empty.)  This could be written as

     to firsts :list
     output map "first :list
     end

but is provided as a primitive in order to speed up the iteration tools MAP, MAP.SE, and FOREACH.

     to transpose :matrix
     if emptyp first :matrix [op []]
     op fput firsts :matrix transpose bfs :matrix
     end

See MAP , MAPdSE , FOREACH

last
----

     LAST wordorlist

if the input is a word, outputs the last character of the word.  If the input is a list, outputs the last member of the list.

butfirst
--------

     BUTFIRST wordorlist
     BF wordorlist

if the input is a word, outputs a word containing all but the first character of the input.  If the input is a list, outputs a list containing all but the first member of the input.

butfirsts
---------

     BUTFIRSTS list
     BFS list

outputs a list containing the BUTFIRST of each member of the input list. It is an error if any member of the input list is empty or an array. (The input itself may be empty, in which case the output is also empty.) This could be written as

     to butfirsts :list
     output map "butfirst :list
     end

but is provided as a primitive in order to speed up the iteration tools MAP, MAP.SE, and FOREACH.

See MAP , MAPdSE , FOREACH

butlast
-------

     BUTLAST wordorlist
     BL wordorlist

if the input is a word, outputs a word containing all but the last character of the input.  If the input is a list, outputs a list containing all but the last member of the input.

item
----

     ITEM index thing

if the thing is a word, outputs the indexth character of the word. If the thing is a list, outputs the indexth member of the list.  If the thing is an array, outputs the indexth member of the array. Index starts at 1 for words and lists; the starting index of an array is specified when the array is created.

mditem
------

     MDITEM indexlist array				(library procedure)

outputs the member of the multidimensional array selected by the list of numbers indexlist.

pick
----

     PICK list					(library procedure)

outputs a randomly chosen member of the input list.

remove
------

     REMOVE thing list				(library procedure)

outputs a copy of list with every member equal to thing removed.

remdup
------

     REMDUP list					(library procedure)

outputs a copy of list with duplicate members removed.  If two or more members of the input are equal, the rightmost of those members is the one that remains in the output.

quoted
------

     QUOTED thing					(library procedure)

outputs its input, if a list; outputs its input with a quotation mark prepended, if a word.

Data Mutators
=============

setitem
-------

     SETITEM index array value

command.  Replaces the indexth member of array with the new value. Ensures that the resulting array is not circular, i.e., value may not be a list or array that contains array.

mdsetitem
---------

     MDSETITEM indexlist array value			(library procedure)

command.  Replaces the member of array chosen by indexlist with the new value.

.setfirst
---------

     .SETFIRST list value

command.  Changes the first member of list to be value.

WARNING: Primitives whose names start with a period are DANGEROUS. Their use by non-experts is not recommended.  The use of .SETFIRST can lead to circular list structures, which will get some Logo primitives into infinite loops; unexpected changes to other data structures that share storage with the list being modified; and the loss of memory if a circular structure is released.

.setbf
------

     .SETBF list value

command.  Changes the butfirst of list to be value.

WARNING: Primitives whose names start with a period are DANGEROUS. Their use by non-experts is not recommended.  The use of .SETBF can lead to circular list structures, which will get some Logo primitives into infinite loops; unexpected changes to other data structures that share storage with the list being modified; Logo crashes and coredumps if the butfirst of a list is not itself a list; and the loss of memory if a circular structure is released.

.setitem
--------

     .SETITEM index array value

command.  Changes the indexth member of array to be value, like SETITEM, but without checking for circularity.

WARNING: Primitives whose names start with a period are DANGEROUS. Their use by non-experts is not recommended.  The use of .SETITEM can lead to circular arrays, which will get some Logo primitives into infinite loops; and the loss of memory if a circular structure is released.

See SETITEM.

push
----

     PUSH stackname thing				(library procedure)

command.  Adds the thing to the stack that is the value of the variable whose name is stackname.  This variable must have a list as its value; the initial value should be the empty list.  New members are added at the front of the list.

pop
---

     POP stackname					(library procedure)

outputs the most recently PUSHed member of the stack that is the value of the variable whose name is stackname and removes that member from the stack.

queue
-----

     QUEUE queuename thing				(library procedure)

command.  Adds the thing to the queue that is the value of the variable whose name is queuename.  This variable must have a list as its value; the initial value should be the empty list.  New members are added at the back of the list.

dequeue
-------

     DEQUEUE queuename				(library procedure)

outputs the least recently QUEUEd member of the queue that is the value of the variable whose name is queuename and removes that member from the queue.

Predicates
==========

wordp
-----

     WORDP thing
     WORD? thing

outputs TRUE if the input is a word, FALSE otherwise.

listp
-----

     LISTP thing
     LIST? thing

outputs TRUE if the input is a list, FALSE otherwise.

arrayp
------

     ARRAYP thing
     ARRAY? thing

outputs TRUE if the input is an array, FALSE otherwise.

emptyp
------

     EMPTYP thing
     EMPTY? thing

outputs TRUE if the input is the empty word or the empty list, FALSE otherwise.

equalp
------

     EQUALP thing1 thing2
     EQUAL? thing1 thing2
     thing1 = thing2

outputs TRUE if the inputs are equal, FALSE otherwise.  Two numbers are equal if they have the same numeric value.  Two non-numeric words are equal if they contain the same characters in the same order.  If there is a variable named CASEIGNOREDP whose value is TRUE, then an upper case letter is considered the same as the corresponding lower case letter. (This is the case by default.)  Two lists are equal if their members are equal.  An array is only equal to itself; two separately created arrays are never equal even if their members are equal.  (It is important to be able to know if two expressions have the same array as their value because arrays are mutable; if, for example, two variables have the same array as their values then performing SETITEM on one of them will also change the other.)

See CASEIGNOREDP , SETITEM

notequalp
---------

     NOTEQUALP thing1 thing2
     NOTEQUAL? thing1 thing2
     thing1 <> thing2

outputs FALSE if the inputs are equal, TRUE otherwise.  See EQUALP for the meaning of equality for different data types.

beforep
-------

     BEFOREP word1 word2
     BEFORE? word1 word2

outputs TRUE if word1 comes before word2 in ASCII collating sequence (for words of letters, in alphabetical order).  Case-sensitivity is determined by the value of CASEIGNOREDP.  Note that if the inputs are numbers, the result may not be the same as with LESSP; for example, BEFOREP 3 12 is false because 3 collates after 1.

See CASEIGNOREDP , LESSP

.eq
---

     .EQ thing1 thing2

outputs TRUE if its two inputs are the same datum, so that applying a mutator to one will change the other as well.  Outputs FALSE otherwise, even if the inputs are equal in value.

WARNING: Primitives whose names start with a period are DANGEROUS. Their use by non-experts is not recommended.  The use of mutators can lead to circular data structures, infinite loops, or Logo crashes.

memberp
-------

     MEMBERP thing1 thing2
     MEMBER? thing1 thing2

if thing2 is a list or an array, outputs TRUE if thing1 is EQUALP to a member of thing2, FALSE otherwise.  If thing2 is a word, outputs TRUE if thing1 is a one-character word EQUALP to a character of thing2, FALSE otherwise.

See EQUALP .

substringp
----------

     SUBSTRINGP thing1 thing2
     SUBSTRING? thing1 thing2

if thing1 or thing2 is a list or an array, outputs FALSE.  If thing2 is a word, outputs TRUE if thing1 is EQUALP to a substring of thing2, FALSE otherwise.

See EQUALP .

numberp
-------

     NUMBERP thing
     NUMBER? thing

outputs TRUE if the input is a number, FALSE otherwise.

backslashedp
------------

     BACKSLASHEDP char
     BACKSLASHED? char

outputs TRUE if the input character was originally entered into Logo with a backslash (\) before it or within vertical bars (|) to prevent its usual special syntactic meaning, FALSE otherwise.  (Outputs TRUE only if the character is a backslashed space, tab, newline, or one of ()[]+-*/=<>":;\~?| )

Queries
=======

count
-----

     COUNT thing

outputs the number of characters in the input, if the input is a word; outputs the number of members in the input, if it is a list or an array. (For an array, this may or may not be the index of the last member, depending on the array's origin.)

ascii
-----

     ASCII char

outputs the integer (between 0 and 255) that represents the input character in the ASCII code.  Interprets control characters as representing backslashed punctuation, and returns the character code for the corresponding punctuation character without backslash.  (Compare RAWASCII.)

rawascii
--------

     RAWASCII char

outputs the integer (between 0 and 255) that represents the input character in the ASCII code.  Interprets control characters as representing themselves.  To find out the ASCII code of an arbitrary keystroke, use RAWASCII RC.

char
----

     CHAR int

outputs the character represented in the ASCII code by the input, which must be an integer between 0 and 255.

See ASCII .

member
------

     MEMBER thing1 thing2

if thing2 is a word or list and if MEMBERP with these inputs would output TRUE, outputs the portion of thing2 from the first instance of thing1 to the end.  If MEMBERP would output FALSE, outputs the empty word or list according to the type of thing2.  It is an error for thing2 to be an array.

See MEMBERP .

lowercase
---------

     LOWERCASE word

outputs a copy of the input word, but with all uppercase letters changed to the corresponding lowercase letter.

uppercase
---------

     UPPERCASE word

outputs a copy of the input word, but with all lowercase letters changed to the corresponding uppercase letter.

standout
--------

     STANDOUT thing

outputs a word that, when printed, will appear like the input but displayed in standout mode (boldface, reverse video, or whatever your terminal does for standout).  The word contains terminal-specific magic characters at the beginning and end; in between is the printed form (as if displayed using TYPE) of the input.  The output is always a word, even if the input is of some other type, but it may include spaces and other formatting characters.  Note: a word output by STANDOUT while Logo is running on one terminal will probably not have the desired effect if printed on another type of terminal.

On the Macintosh, the way that standout works is incompatible with the use of characters whose ASCII code is greater than 127.  Therefore, you have a choice to make:  The instruction

     CANINVERSE 0

disables standout, but enables the display of ASCII codes above 127, and the instruction

     CANINVERSE 1

restores the default situation in which standout is enabled and the extra graphic characters cannot be printed.

parse
-----

     PARSE word

outputs the list that would result if the input word were entered in response to a READLIST operation.  That is, PARSE READWORD has the same value as READLIST for the same characters read.

See READLIST , READWORD

runparse
--------

     RUNPARSE wordorlist

outputs the list that would result if the input word or list were entered as an instruction line; characters such as infix operators and parentheses are separate members of the output.  Note that sublists of a runparsed list are not themselves runparsed.

Communication
*************

Transmitters
============

Note:  If there is a variable named PRINTDEPTHLIMIT with a nonnegative integer value, then complex list and array structures will be printed only to the allowed depth.  That is, members of members of... of members will be allowed only so far.  The members omitted because they are just past the depth limit are indicated by an ellipsis for each one, so a too-deep list of two members will print as [... ...].

If there is a variable named PRINTWIDTHLIMIT with a nonnegative integer value, then only the first so many members of any array or list will be printed.  A single ellipsis replaces all missing data within the structure.  The width limit also applies to the number of characters printed in a word, except that a PRINTWIDTHLIMIT between 0 and 9 will be treated as if it were 10 when applied to words.  This limit applies not only to the top-level printed datum but to any substructures within it.

See PRINTDEPTHLIMIT , PRINTWIDTHLIMIT

If there is a variable named FULLPRINTP whose value is TRUE, then words that were created using backslash or vertical bar (to include characters that would otherwise not be treated as part of a word) are printed with the backslashes or vertical bars shown, so that the printed result could be re-read by Logo to produce the same value.  If FULLPRINTP is TRUE then the empty word (however it was created) prints as ||.  (Otherwise it prints as nothing at all.)

See FULLPRINTP .

print
-----

     PRINT thing
     PR thing
     (PRINT thing1 thing2 ...)
     (PR thing1 thing2 ...)

command.  Prints the input or inputs to the current write stream (initially the terminal).  All the inputs are printed on a single line, separated by spaces, ending with a newline.  If an input is a list, square brackets are not printed around it, but brackets are printed around sublists.  Braces are always printed around arrays.

type
----

     TYPE thing
     (TYPE thing1 thing2 ...)

command.  Prints the input or inputs like PRINT, except that no newline character is printed at the end and multiple inputs are not separated by spaces.  Note: printing to the terminal is ordinarily "line buffered"; that is, the characters you print using TYPE will not actually appear on the screen until either a newline character is printed (for example, by PRINT or SHOW) or Logo tries to read from the keyboard (either at the request of your program or after an instruction prompt).  This buffering makes the program much faster than it would be if each character appeared immediately, and in most cases the effect is not disconcerting. To accommodate programs that do a lot of positioned text display using TYPE, Logo will force printing whenever SETCURSOR is invoked.  This solves most buffering problems.  Still, on occasion you may find it necessary to force the buffered characters to be printed explicitly; this can be done using the WAIT command.  WAIT 0 will force printing without actually waiting.

See SETCURSOR , WAIT

show
----

     SHOW thing
     (SHOW thing1 thing2 ...)

command.  Prints the input or inputs like PRINT, except that if an input is a list it is printed inside square brackets.

See PRINT .

Receivers
=========

readlist
--------

     READLIST
     RL

reads a line from the read stream (initially the terminal) and outputs that line as a list.  The line is separated into members as though it were typed in square brackets in an instruction.  If the read stream is a file, and the end of file is reached, READLIST outputs the empty word (not the empty list).  READLIST processes backslash, vertical bar, and tilde characters in the read stream; the output list will not contain these characters but they will have had their usual effect.  READLIST does not, however, treat semicolon as a comment character.

readword
--------

     READWORD
     RW

reads a line from the read stream and outputs that line as a word.  The output is a single word even if the line contains spaces, brackets, etc. If the read stream is a file, and the end of file is reached, READWORD outputs the empty list (not the empty word).  READWORD processes backslash, vertical bar, and tilde characters in the read stream.  In the case of a tilde used for line continuation, the output word DOES include the tilde and the newline characters, so that the user program can tell exactly what the user entered.  Vertical bars in the line are also preserved in the output.  Backslash characters are not preserved in the output, but the character following the backslash has 128 added to its representation.  Programs can use BACKSLASHEDP to check for this code.  (Backslashedness is preserved only for certain characters.)

See BACKSLASHEDP .

readrawline
-----------

     READRAWLINE

reads a line from the read stream and outputs that line as a word.  The output is a single word even if the line contains spaces, brackets, etc. If the read stream is a file, and the end of file is reached, READRAWLINE outputs the empty list (not the empty word).  READRAWLINE outputs the exact string of characters as they appear in the line, with no special meaning for backslash, vertical bar, tilde, or any other formatting characters.

See READWORD .

readchar
--------

     READCHAR
     RC

reads a single character from the read stream and outputs that character as a word.  If the read stream is a file, and the end of file is reached, READCHAR outputs the empty list (not the empty word).  If the read stream is a terminal, echoing is turned off when READCHAR is invoked, and remains off until READLIST or READWORD is invoked or a Logo prompt is printed.  Backslash, vertical bar, and tilde characters have no special meaning in this context.

See READLIST .

readchars
---------

     READCHARS num
     RCS num

reads num characters from the read stream and outputs those characters as a word.  If the read stream is a file, and the end of file is reached, READCHARS outputs the empty list (not the empty word).  If the read stream is a terminal, echoing is turned off when READCHARS is invoked, and remains off until READLIST or READWORD is invoked or a Logo prompt is printed.  Backslash, vertical bar, and tilde characters have no special meaning in this context.

See READLIST , READWORD

shell
-----

     SHELL command
     (SHELL command wordflag)

Under Unix, outputs the result of running command as a shell command. (The command is sent to /bin/sh, not csh or other alternatives.)  If the command is a literal list in the instruction line, and if you want a backslash character sent to the shell, you must use \\ to get the backslash through Logo's reader intact.  The output is a list containing one member for each line generated by the shell command.  Ordinarily each such line is represented by a list in the output, as though the line were read using READLIST.  If a second input is given, regardless of the value of the input, each line is represented by a word in the output as though it were read with READWORD.  Example:

     to dayofweek
     output first first shell [date]
     end

This is "first first" to extract the first word of the first (and only) line of the shell output.

Under DOS, SHELL is a command, not an operation; it sends its input to a DOS command processor but does not collect the result of the command.

The Macintosh, of course, is not programmable (unless you are running the Unix version of UCBLogo under OS X).

File Access
===========

setprefix
---------

     SETPREFIX string

command.  Sets a prefix that will be used as the implicit beginning of filenames in OPENREAD, OPENWRITE, OPENAPPEND, OPENUPDATE, LOAD, and SAVE commands.  Logo will put the appropriate separator character (slash for Unix, backslash for DOS/Windows, colon for MacOS) between the prefix and the filename entered by the user.  The input to SETPREFIX must be a word, unless it is the empty list, to indicate that there should be no prefix.

See OPENREAD , see OPENWRITE , see OPENAPPEND , see OPENUPDATE , see LOAD , see SAVE .

prefix
------

     PREFIX

outputs the current file prefix, or [] if there is no prefix.

See SETPREFIX .

openread
--------

     OPENREAD filename

command.  Opens the named file for reading.  The read position is initially at the beginning of the file.

openwrite
---------

     OPENWRITE filename

command.  Opens the named file for writing.  If the file already existed, the old version is deleted and a new, empty file created.

OPENWRITE, but not the other OPEN variants, will accept as input a two-element list, in which the first element must be a variable name, and the second must be a positive integer.  A character buffer of the specified size will be created.  When a SETWRITE is done with this same list (in the sense of .EQ, not a copy, so you must do something like

     ? make "buf [foo 100]
     ? openwrite :buf
     ? setwrite :buf
         [...]
     ? close :buf

and not just

     ? openwrite [foo 100]
     ? setwrite [foo 100]

and so on), the printed characters are stored in the buffer; when a CLOSE is done with the same list as input, the characters from the buffer (treated as one long word, even if spaces and newlines are included) become the value of the specified variable.

openappend
----------

     OPENAPPEND filename

command.  Opens the named file for writing.  If the file already exists, the write position is initially set to the end of the old file, so that newly written data will be appended to it.

openupdate
----------

     OPENUPDATE filename

command.  Opens the named file for reading and writing.  The read and write position is initially set to the end of the old file, if any. Note: each open file has only one position, for both reading and writing.  If a file opened for update is both READER and WRITER at the same time, then SETREADPOS will also affect WRITEPOS and vice versa. Also, if you alternate reading and writing the same file, you must SETREADPOS between a write and a read, and SETWRITEPOS between a read and a write.

See READER , WRITER , SETREADPOS , SETWRITEPOS

close
-----

     CLOSE filename

command.  Closes the named file.  If the file was currently the reader or writer, then the reader or writer is changed to the keyboard or screen, as if SETREAD [] or SETWRITE [] had been done.

allopen
-------

     ALLOPEN

outputs a list whose members are the names of all files currently open. This list does not include the dribble file, if any.

closeall
--------

     CLOSEALL					(library procedure)

command.  Closes all open files.  Abbreviates FOREACH ALLOPEN [CLOSE ?]

See FOREACH , CLOSE

erasefile
---------

     ERASEFILE filename
     ERF filename

command.  Erases (deletes, removes) the named file, which should not currently be open.

dribble
-------

     DRIBBLE filename

command.  Creates a new file whose name is the input, like OPENWRITE, and begins recording in that file everything that is read from the keyboard or written to the terminal.  That is, this writing is in addition to the writing to WRITER.  The intent is to create a transcript of a Logo session, including things like prompt characters and interactions.

See OPENWRITE , WRITER

nodribble
---------

     NODRIBBLE

command.  Stops copying information into the dribble file, and closes the file.

setread
-------

     SETREAD filename

command.  Makes the named file the read stream, used for READLIST, etc. The file must already be open with OPENREAD or OPENUPDATE.  If the input is the empty list, then the read stream becomes the terminal, as usual. Changing the read stream does not close the file that was previously the read stream, so it is possible to alternate between files.

See READLIST , OPENREAD , OPENUPDATE

setwrite
--------

     SETWRITE filename

command.  Makes the named file the write stream, used for PRINT, etc.  The file must already be open with OPENWRITE, OPENAPPEND, or OPENUPDATE.  If the input is the empty list, then the write stream becomes the terminal, as usual.  Changing the write stream does not close the file that was previously the write stream, so it is possible to alternate between files.

If the input is a list, then its first element must be a variable name, and its second and last element must be a positive integer; a buffer of that many characters will be allocated, and will become the writestream.  If the same list (same in the .EQ sense, not a copy) has been used as input to OPENWRITE, then the already-allocated buffer will be used, and the writer can be changed to and from this buffer, with all the characters accumulated as in a file.  When the same list is used as input to CLOSE, the contents of the buffer (as an unparsed word, which may contain newline characters) will become the value of the named variable.  For compatibility with earlier versions, if the list has not been opened when the SETWRITE is done, it will be opened implicitly, but the first SETWRITE after this one will implicitly close it, setting the variable and freeing the allocated buffer.

See PRINT , OPENWRITE ; OPENAPPEND ; OPENUPDATE

reader
------

     READER

outputs the name of the current read stream file, or the empty list if the read stream is the terminal.

writer
------

     WRITER

outputs the name of the current write stream file, or the empty list if the write stream is the terminal.

setreadpos
----------

     SETREADPOS charpos

command.  Sets the file pointer of the read stream file so that the next READLIST, etc., will begin reading at the charposth character in the file, counting from 0.  (That is, SETREADPOS 0 will start reading from the beginning of the file.)  Meaningless if the read stream is the terminal.

See READLIST .

setwritepos
-----------

     SETWRITEPOS charpos

command.  Sets the file pointer of the write stream file so that the next PRINT, etc., will begin writing at the charposth character in the file, counting from 0.  (That is, SETWRITEPOS 0 will start writing from the beginning of the file.)  Meaningless if the write stream is the terminal.

See PRINT .

readpos
-------

     READPOS

outputs the file position of the current read stream file.

writepos
--------

     WRITEPOS

outputs the file position of the current write stream file.

eofp
----

     EOFP
     EOF?

predicate, outputs TRUE if there are no more characters to be read in the read stream file, FALSE otherwise.

filep
-----

     FILEP filename
     FILE? filename					(library procedure)

predicate, outputs TRUE if a file of the specified name exists and can be read, FALSE otherwise.

Terminal Access
===============

keyp
----

     KEYP
     KEY?

predicate, outputs TRUE if there are characters waiting to be read from the read stream.  If the read stream is a file, this is equivalent to NOT EOFP.  If the read stream is the terminal, then echoing is turned off and the terminal is set to CBREAK (character at a time instead of line at a time) mode.  It remains in this mode until some line-mode reading is requested (e.g., READLIST).  The Unix operating system forgets about any pending characters when it switches modes, so the first KEYP invocation will always output FALSE.

See EOFP , READLIST

cleartext
---------

     CLEARTEXT
     CT

command.  Clears the text screen of the terminal.

setcursor
---------

     SETCURSOR vector

command.  The input is a list of two numbers, the x and y coordinates of a screen position (origin in the upper left corner, positive direction is southeast).  The screen cursor is moved to the requested position. This command also forces the immediate printing of any buffered characters.

cursor
------

     CURSOR

outputs a list containing the current x and y coordinates of the screen cursor.  Logo may get confused about the current cursor position if, e.g., you type in a long line that wraps around or your program prints escape codes that affect the terminal strangely.

setmargins
----------

     SETMARGINS vector

command.  The input must be a list of two numbers, as for SETCURSOR. The effect is to clear the screen and then arrange for all further printing to be shifted down and to the right according to the indicated margins.  Specifically, every time a newline character is printed (explicitly or implicitly) Logo will type x_margin spaces, and on every invocation of SETCURSOR the margins will be added to the input x and y coordinates.  (CURSOR will report the cursor position relative to the margins, so that this shift will be invisible to Logo programs.)  The purpose of this command is to accommodate the display of terminal screens in lecture halls with inadequate TV monitors that miss the top and left edges of the screen.

See SETCURSOR .

settextcolor
------------

     SETTEXTCOLOR foreground background
     SETTC foreground background

Command (Windows and DOS extended only).  The inputs are color numbers, as for turtle graphics.  Future printing to the text window will use the specified colors for foreground (the characters printed) and background (the space under those characters).  Using STANDOUT will revert to the default text window colors.  In the DOS extended (ucblogo.exe) version, colors in textscreen mode are limited to numbers 0-7, and the coloring applies only to text printed by the program, not to the echoing of text typed by the user.  Neither limitation applies to the text portion of splitscreen mode, which is actually drawn as graphics internally.

See STANDOUT .

Arithmetic
**********

Numeric Operations
==================

sum
---

     SUM num1 num2
     (SUM num1 num2 num3 ...)
     num1 + num2

outputs the sum of its inputs.

difference
----------

     DIFFERENCE num1 num2
     num1 - num2

outputs the difference of its inputs.  Minus sign means infix difference in ambiguous contexts (when preceded by a complete expression), unless it is preceded by a space and followed by a nonspace.  (See also MINUS.)

minus
-----

     MINUS num
     - num

outputs the negative of its input.  Minus sign means unary minus if the previous token is an infix operator or open parenthesis, or it is preceded by a space and followed by a nonspace.  There is a difference in binding strength between the two forms:

     MINUS 3 + 4     means   -(3+4)
     - 3 + 4         means   (-3)+4

product
-------

     PRODUCT num1 num2
     (PRODUCT num1 num2 num3 ...)
     num1 * num2

outputs the product of its inputs.

quotient
--------

     QUOTIENT num1 num2
     (QUOTIENT num)
     num1 / num2

outputs the quotient of its inputs.  The quotient of two integers is an integer if and only if the dividend is a multiple of the divisor.  (In other words, QUOTIENT 5 2 is 2.5, not 2, but QUOTIENT 4 2 is 2, not 2.0 --- it does the right thing.)  With a single input, QUOTIENT outputs the reciprocal of the input.

remainder
---------

     REMAINDER num1 num2

outputs the remainder on dividing num1 by num2; both must be integers and the result is an integer with the same sign as num1.

modulo
------

     MODULO num1 num2

outputs the remainder on dividing num1 by num2; both must be integers and the result is an integer with the same sign as num2.

int
---

     INT num

outputs its input with fractional part removed, i.e., an integer with the same sign as the input, whose absolute value is the largest integer less than or equal to the absolute value of the input.

round
-----

     ROUND num

outputs the nearest integer to the input.

sqrt
----

     SQRT num

outputs the square root of the input, which must be nonnegative.

power
-----

     POWER num1 num2

outputs num1 to the num2 power.  If num1 is negative, then num2 must be an integer.

exp
---

     EXP num

outputs e (2.718281828+) to the input power.

log10
-----

     LOG10 num

outputs the common logarithm of the input.

ln
--

     LN num

outputs the natural logarithm of the input.

sin
---

     SIN degrees

outputs the sine of its input, which is taken in degrees.

radsin
------

     RADSIN radians

outputs the sine of its input, which is taken in radians.

cos
---

     COS degrees

outputs the cosine of its input, which is taken in degrees.

radcos
------

     RADCOS radians

outputs the cosine of its input, which is taken in radians.

arctan
------

     ARCTAN num
     (ARCTAN x y)

outputs the arctangent, in degrees, of its input.  With two inputs, outputs the arctangent of y/x, if x is nonzero, or 90 or --90 depending on the sign of y, if x is zero.

radarctan
---------

     RADARCTAN num
     (RADARCTAN x y)

outputs the arctangent, in radians, of its input.  With two inputs, outputs the arctangent of y/x, if x is nonzero, or pi/2 or --pi/2 depending on the sign of y, if x is zero.

The expression 2*(RADARCTAN 0 1) can be used to get the value of pi.

iseq
----

     ISEQ from to					(library procedure)

outputs a list of the integers from FROM to TO, inclusive.

     ? show iseq 3 7
     [3 4 5 6 7]
     ? show iseq 7 3
     [7 6 5 4 3]

rseq
----

     RSEQ from to count				(library procedure)

outputs a list of COUNT equally spaced rational numbers between FROM and TO, inclusive.

     ? show rseq 3 5 9
     [3 3.25 3.5 3.75 4 4.25 4.5 4.75 5]
     ? show rseq 3 5 5
     [3 3.5 4 4.5 5]

Numeric Predicates
==================

lessp
-----

     LESSP num1 num2
     LESS? num1 num2
     num1 < num2

outputs TRUE if its first input is strictly less than its second.

greaterp
--------

     GREATERP num1 num2
     GREATER? num1 num2
     num1 > num2

outputs TRUE if its first input is strictly greater than its second.

lessequalp
----------

     LESSEQUALP num1 num2
     LESSEQUAL? num1 num2
     num1 <= num2

outputs TRUE if its first input is less than or equal to its second.

greaterequalp
-------------

     GREATEREQUALP num1 num2
     GREATEREQUAL? num1 num2
     num1 >= num2

outputs TRUE if its first input is greater than or equal to its second.

Random Numbers
==============

random
------

     RANDOM num
     (RANDOM start end)

with one input, outputs a random nonnegative integer less than its input, which must be a positive integer.

With two inputs, RANDOM outputs a random integer greater than or equal to the first input, and less than or equal to the second input.  Both inputs must be integers, and the first must be less than the second.  (RANDOM 0 9) is equivalent to RANDOM 10; (RANDOM 3 8) is equivalent to (RANDOM 6)+3.

rerandom
--------

     RERANDOM
     (RERANDOM seed)

command.  Makes the results of RANDOM reproducible.  Ordinarily the sequence of random numbers is different each time Logo is used.  If you need the same sequence of pseudo-random numbers repeatedly, e.g. to debug a program, say RERANDOM before the first invocation of RANDOM.  If you need more than one repeatable sequence, you can give RERANDOM an integer input; each possible input selects a unique sequence of numbers.

Print Formatting
================

form
----

     FORM num width precision

outputs a word containing a printable representation of num, possibly preceded by spaces (and therefore not a number for purposes of performing arithmetic operations), with at least width characters, including exactly precision digits after the decimal point.  (If precision is 0 then there will be no decimal point in the output.)

As a debugging feature, (FORM num -1 format) will print the floating point num according to the C printf format, to allow

     to hex :num
     op form :num -1 "|%08X %08X|
     end

to allow finding out the exact result of floating point operations.  The precise format needed may be machine-dependent.

Bitwise Operations
==================

bitand
------

     BITAND num1 num2
     (BITAND num1 num2 num3 ...)

outputs the bitwise AND of its inputs, which must be integers.

See AND .

bitor
-----

     BITOR num1 num2
     (BITOR num1 num2 num3 ...)

outputs the bitwise OR of its inputs, which must be integers.

See OR .

bitxor
------

     BITXOR num1 num2
     (BITXOR num1 num2 num3 ...)

outputs the bitwise EXCLUSIVE OR of its inputs, which must be integers.

See OR .

bitnot
------

     BITNOT num

outputs the bitwise NOT of its input, which must be an integer.

See NOT .

ashift
------

     ASHIFT num1 num2

outputs num1 arithmetic-shifted to the left by num2 bits.  If num2 is negative, the shift is to the right with sign extension.  The inputs must be integers.

lshift
------

     LSHIFT num1 num2

outputs num1 logical-shifted to the left by num2 bits.  If num2 is negative, the shift is to the right with zero fill.  The inputs must be integers.

Logical Operations
******************

and
---

     AND tf1 tf2
     (AND tf1 tf2 tf3 ...)

outputs TRUE if all inputs are TRUE, otherwise FALSE.  All inputs must be TRUE or FALSE.  (Comparison is case-insensitive regardless of the value of CASEIGNOREDP.  That is, true or True or TRUE are all the same.)  An input can be a list, in which case it is taken as an expression to run; that expression must produce a TRUE or FALSE value.  List expressions are evaluated from left to right; as soon as a FALSE value is found, the remaining inputs are not examined.  Example:

     MAKE "RESULT AND [NOT (:X = 0)] [(1 / :X) > .5]

to avoid the division by zero if the first part is false.

See CASEIGNOREDP .

or
--

     OR tf1 tf2
     (OR tf1 tf2 tf3 ...)

outputs TRUE if any input is TRUE, otherwise FALSE.  All inputs must be TRUE or FALSE.  (Comparison is case-insensitive regardless of the value of CASEIGNOREDP.  That is, true or True or TRUE are all the same.)  An input can be a list, in which case it is taken as an expression to run; that expression must produce a TRUE or FALSE value.  List expressions are evaluated from left to right; as soon as a TRUE value is found, the remaining inputs are not examined.  Example:

     IF OR :X=0 [some.long.computation] [...]

to avoid the long computation if the first condition is met.

See CASEIGNOREDP .

not
---

     NOT tf

outputs TRUE if the input is FALSE, and vice versa.  The input can be a list, in which case it is taken as an expression to run; that expression must produce a TRUE or FALSE value.

Graphics
********

Berkeley Logo provides traditional Logo turtle graphics with one turtle. Multiple turtles, dynamic turtles, and collision detection are not supported. This is the most hardware-dependent part of Logo; some features may exist on some machines but not others.  Nevertheless, the goal has been to make Logo programs as portable as possible, rather than to take fullest advantage of the capabilities of each machine.  In particular, Logo attempts to scale the screen so that turtle coordinates [--100 --100] and [100 100] fit on the graphics window, and so that the aspect ratio is 1:1.

The center of the graphics window (which may or may not be the entire screen, depending on the machine used) is turtle location [0 0].  Positive X is to the right; positive Y is up.  Headings (angles) are measured in degrees clockwise from the positive Y axis.  (This differs from the common mathematical convention of measuring angles counterclockwise from the positive X axis.)  The turtle is represented as an isoceles triangle; the actual turtle position is at the midpoint of the base (the short side). However, the turtle is drawn one step behind its actual position, so that the display of the base of the turtle's triangle does not obscure a line drawn perpendicular to it (as would happen after drawing a square).

Colors are, of course, hardware-dependent.  However, Logo provides partial hardware independence by interpreting color numbers 0 through 7 uniformly on all computers:

     	0  black        1  blue         2  green        3  cyan
     	4  red          5  magenta      6  yellow       7 white

Where possible, Logo provides additional user-settable colors; how many are available depends on the hardware and operating system environment. If at least 16 colors are available, Logo tries to provide uniform initial settings for the colors 8-15:

     	 8  brown        9  tan         10  forest      11  aqua
     	12  salmon      13  purple      14  orange      15  grey

Logo begins with a black background and white pen.

Turtle Motion
=============

forward
-------

     FORWARD dist
     FD dist

moves the turtle forward, in the direction that it's facing, by the specified distance (measured in turtle steps).

back
----

     BACK dist
     BK dist

moves the turtle backward, i.e., exactly opposite to the direction that it's facing, by the specified distance.  (The heading of the turtle does not change.)

left
----

     LEFT degrees
     LT degrees

turns the turtle counterclockwise by the specified angle, measured in degrees (1/360 of a circle).

right
-----

     RIGHT degrees
     RT degrees

turns the turtle clockwise by the specified angle, measured in degrees (1/360 of a circle).

setpos
------

     SETPOS pos

moves the turtle to an absolute screen position.  The input is a list of two numbers, the X and Y coordinates.

setxy
-----

     SETXY xcor ycor

moves the turtle to an absolute screen position.  The two inputs are numbers, the X and Y coordinates.

setx
----

     SETX xcor

moves the turtle horizontally from its old position to a new absolute horizontal coordinate.  The input is the new X coordinate.

sety
----

     SETY ycor

moves the turtle vertically from its old position to a new absolute vertical coordinate.  The input is the new Y coordinate.

setheading
----------

     SETHEADING degrees
     SETH degrees

turns the turtle to a new absolute heading.  The input is a number, the heading in degrees clockwise from the positive Y axis.

home
----

     HOME

moves the turtle to the center of the screen.  Equivalent to SETPOS [0 0] SETHEADING 0.

See SETPOS , see SETHEADING .

arc
---

     ARC angle radius

draws an arc of a circle, with the turtle at the center, with the specified radius, starting at the turtle's heading and extending clockwise through the specified angle.  The turtle does not move.

Turtle Motion Queries
=====================

pos
---

     POS

outputs the turtle's current position, as a list of two numbers, the X and Y coordinates.

xcor
----

     XCOR						(library procedure)

outputs a number, the turtle's X coordinate.

ycor
----

     YCOR						(library procedure)

outputs a number, the turtle's Y coordinate.

heading
-------

     HEADING

outputs a number, the turtle's heading in degrees.

towards
-------

     TOWARDS pos

outputs a number, the heading at which the turtle should be facing so that it would point from its current position to the position given as the input.

scrunch
-------

     SCRUNCH

outputs a list containing two numbers, the X and Y scrunch factors, as used by SETSCRUNCH.  (But note that SETSCRUNCH takes two numbers as inputs, not one list of numbers.)

See SETSCRUNCH .

Turtle and Window Control
=========================

showturtle
----------

     SHOWTURTLE
     ST

makes the turtle visible.

hideturtle
----------

     HIDETURTLE
     HT

makes the turtle invisible.  It's a good idea to do this while you're in the middle of a complicated drawing, because hiding the turtle speeds up the drawing substantially.

clean
-----

     CLEAN

erases all lines that the turtle has drawn on the graphics window.  The turtle's state (position, heading, pen mode, etc.) is not changed.

clearscreen
-----------

     CLEARSCREEN
     CS

erases the graphics window and sends the turtle to its initial position and heading.  Like HOME and CLEAN together.

See HOME .

wrap
----

     WRAP

tells the turtle to enter wrap mode:  From now on, if the turtle is asked to move past the boundary of the graphics window, it will "wrap around" and reappear at the opposite edge of the window.  The top edge wraps to the bottom edge, while the left edge wraps to the right edge. (So the window is topologically equivalent to a torus.)  This is the turtle's initial mode.  Compare WINDOW and FENCE.

See FENCE .

window
------

     WINDOW

tells the turtle to enter window mode:  From now on, if the turtle is asked to move past the boundary of the graphics window, it will move offscreen.  The visible graphics window is considered as just part of an infinite graphics plane; the turtle can be anywhere on the plane.  (If you lose the turtle, HOME will bring it back to the center of the window.)  Compare WRAP and FENCE.

See HOME .

fence
-----

     FENCE

tells the turtle to enter fence mode:  From now on, if the turtle is asked to move past the boundary of the graphics window, it will move as far as it can and then stop at the edge with an "out of bounds" error message.  Compare WRAP and WINDOW.

See WRAP .

fill
----

     FILL

fills in a region of the graphics window containing the turtle and bounded by lines that have been drawn earlier.  This is not portable; it doesn't work for all machines, and may not work exactly the same way on different machines.

label
-----

     LABEL text

takes a word or list as input, and prints the input on the graphics window, starting at the turtle's position.

textscreen
----------

     TEXTSCREEN
     TS

rearranges the size and position of windows to maximize the space available in the text window (the window used for interaction with Logo).  The details differ among machines.  Compare SPLITSCREEN and FULLSCREEN.

See SPLITSCREEN .

fullscreen
----------

     FULLSCREEN
     FS

rearranges the size and position of windows to maximize the space available in the graphics window.  The details differ among machines. Compare SPLITSCREEN and TEXTSCREEN.

In the DOS version, switching from fullscreen to splitscreen loses the part of the picture that's hidden by the text window.  Also, since there must be a text window to allow printing (including the printing of the Logo prompt), Logo automatically switches from fullscreen to splitscreen whenever anything is printed.  [This design decision follows from the scarcity of memory, so that the extra memory to remember an invisible part of a drawing seems too expensive.]

splitscreen
-----------

     SPLITSCREEN
     SS

rearranges the size and position of windows to allow some room for text interaction while also keeping most of the graphics window visible.  The details differ among machines.  Compare TEXTSCREEN and FULLSCREEN.

See TEXTSCREEN .

setscrunch
----------

     SETSCRUNCH xscale yscale

adjusts the aspect ratio and scaling of the graphics display.  After this command is used, all further turtle motion will be adjusted by multiplying the horizontal and vertical extent of the motion by the two numbers given as inputs.  For example, after the instruction SETSCRUNCH 2 1 motion at a heading of 45 degrees will move twice as far horizontally as vertically.  If your squares don't come out square, try this.  (Alternatively, you can deliberately misadjust the aspect ratio to draw an ellipse.)

For Unix machines and Macintoshes, both scale factors are initially 1. For DOS machines, the scale factors are initially set according to what the hardware claims the aspect ratio is, but the hardware sometimes lies.  The values set by SETSCRUNCH are remembered in a file (called SCRUNCH.DAT) and are automatically put into effect when a Logo session begins.

refresh
-------

     REFRESH

tells Logo to remember the turtle's motions so that they can be reconstructed in case the graphics window is overlayed.  The effectiveness of this command may depend on the machine used.

norefresh
---------

     NOREFRESH

tells Logo not to remember the turtle's motions.  This will make drawing faster, but prevents recovery if the window is overlayed.

Turtle and Window Queries
=========================

shownp
------

     SHOWNP
     SHOWN?

outputs TRUE if the turtle is shown (visible), FALSE if the turtle is hidden.  See SHOWTURTLE and HIDETURTLE.

See SHOWTURTLE , HIDETURTLE .

screenmode
----------

     SCREENMODE

outputs the word TEXTSCREEN, SPLITSCREEN, or FULLSCREEN depending on the current screen mode.

turtlemode
----------

     TURTLEMODE

outputs the word WRAP, FENCE, or WINDOW depending on the current turtle mode.

Pen and Background Control
==========================

The turtle carries a pen that can draw pictures.  At any time the pen can be UP (in which case moving the turtle does not change what's on the graphics screen) or DOWN (in which case the turtle leaves a trace). If the pen is down, it can operate in one of three modes: PAINT (so that it draws lines when the turtle moves), ERASE (so that it erases any lines that might have been drawn on or through that path earlier), or REVERSE (so that it inverts the status of each point along the turtle's path).

pendown
-------

     PENDOWN
     PD

sets the pen's position to DOWN, without changing its mode.

penup
-----

     PENUP
     PU

sets the pen's position to UP, without changing its mode.

penpaint
--------

     PENPAINT
     PPT

sets the pen's position to DOWN and mode to PAINT.

penerase
--------

     PENERASE
     PE

sets the pen's position to DOWN and mode to ERASE.

See ERASE .

penreverse
----------

     PENREVERSE
     PX

sets the pen's position to DOWN and mode to REVERSE.  (This may interact in hardware-dependent ways with use of color.)

See REVERSE .

setpencolor
-----------

     SETPENCOLOR colornumber.or.rgblist
     SETPC colornumber.or.rgblist

sets the pen color to the given number, which must be a nonnegative integer.  There are initial assignments for the first 16 colors:

      0  black	 1  blue	 2  green	 3  cyan
      4  red		 5  magenta	 6  yellow	 7 white
      8  brown	 9  tan		10  forest	11  aqua
     12  salmon	13  purple	14  orange	15  grey

but other colors can be assigned to numbers by the PALETTE command. Alternatively, sets the pen color to the given RGB values (a list of three nonnegative integers less than 64K (65536) specifying the amount of red, green, and blue in the desired color).

setpalette
----------

     SETPALETTE colornumber rgblist

sets the actual color corresponding to a given number, if allowed by the hardware and operating system.  Colornumber must be an integer greater than or equal to 8.  (Logo tries to keep the first 8 colors constant.) The second input is a list of three nonnegative integers less than 64K (65536) specifying the amount of red, green, and blue in the desired color.  The actual color resolution on any screen is probably less than 64K, but Logo scales as needed.

setpensize
----------

     SETPENSIZE size

sets the thickness of the pen.  The input is either a single positive integer or a list of two positive integers (for horizontal and vertical thickness).  Some versions pay no attention to the second number, but always have a square pen.

     SETPENPATTERN pattern

sets hardware-dependent pen characteristics.  This command is not guaranteed compatible between implementations on different machines.

setpenpattern
-------------

     SETPENSIZE size
     SETPENPATTERN pattern

set hardware-dependent pen characteristics.  These commands are not guaranteed compatible between implementations on different machines.

setpen
------

     SETPEN list					(library procedure)

sets the pen's position, mode, thickness, and hardware-dependent characteristics according to the information in the input list, which should be taken from an earlier invocation of PEN.

See PEN .

setbackground
-------------

     SETBACKGROUND colornumber.or.rgblist
     SETBG colornumber.or.rgblist

set the screen background color by slot number or RGB values. See SETPENCOLOR for details.

See SETPENCOLOR .

Pen Queries
===========

pendownp
--------

     PENDOWNP
     PENDOWN?

outputs TRUE if the pen is down, FALSE if it's up.

penmode
-------

     PENMODE

outputs one of the words PAINT, ERASE, or REVERSE according to the current pen mode.

See ERASE , REVERSE .

pencolor
--------

     PENCOLOR
     PC

outputs a color number, a nonnegative integer that is associated with a particular color, or a list of RGB values if such a list was used as the most recent input to SETPENCOLOR.  There are initial assignments for the first 16 colors:

     	 0  black        1  blue         2  green        3  cyan
     	 4  red          5  magenta      6  yellow       7 white
     	 8  brown        9  tan         10  forest      11  aqua
     	12  salmon      13  purple      14  orange      15  grey

but other colors can be assigned to numbers by the PALETTE command.

palette
-------

     PALETTE colornumber

outputs a list of three integers, each in the range 0-65535, representing the amount of red, green, and blue in the color associated with the given number.

pensize
-------

     PENSIZE

outputs a list of two positive integers, specifying the horizontal and vertical thickness of the turtle pen.  (In some implementations the two numbers may always be equal.)

     PENPATTERN

outputs hardware-specific pen information.

pen
---

     PEN						(library procedure)

outputs a list containing the pen's position, mode, thickness, and hardware-specific characteristics, for use by SETPEN.

See SETPEN .

background
----------

     BACKGROUND
     BG

outputs the graphics background color, either as a slot number or as an RGB list, whichever way it was set.  (See PENCOLOR.)

saving and loading pictures
===========================

savepict
--------

     SAVEPICT filename

command.  Writes a file with the specified name containing the state of the graphics window, including any nonstandard color palette settings, in Logo's internal format.  This picture can be restored to the screen using LOADPICT.  The format is not portable between platforms, nor is it readable by other programs.  EPSPICT to export Logo graphics for other programs.

loadpict
--------

     LOADPICT filename

command.  Reads the specified file, which must have been written by a SAVEPICT command, and restores the graphics window and color palette settings to the values stored in the file.  Any drawing previously on the screen is cleared.

See SAVEPICT .

epspict
-------

     EPSPICT filename

command.  Writes a file with the specified name, containing an Encapsulated Postscript (EPS) representation of the state of the graphics window.  This file can be imported into other programs that understand EPS format.  Restrictions: the drawing cannot use ARC, FILL, PENERASE, or PENREVERSE; any such instructions will be ignored in the translation to Postscript form.

See ARC , see FILL , see PENERASE , see PENREVERSE .

Mouse Queries
=============

mousepos
--------

     MOUSEPOS

outputs the coordinates of the mouse, provided that it's within the graphics window, in turtle coordinates.  If the mouse is outside the graphics window, then the last position within the window is returned. Exception:  If a mouse button is pressed within the graphics window and held while the mouse is dragged outside the window, the mouse's position is returned as if the window were big enough to include it.

buttonp
-------

     BUTTONP
     BUTTON?

outputs TRUE if a mouse button is down and the mouse is over the graphics window.  Once the button is down, BUTTONP remains true until the button is released, even if the mouse is dragged out of the graphics window.

buttonp
-------

     BUTTON

outputs 0 if BUTTONP would output FALSE; otherwise, it outputs an integer between 1 and 3 indicating which button was pressed. Ordinarily 1 means left, 2 means right, and 3 means center, but operating systems may reconfigure these.

Workspace Management
********************

Procedure Definition
====================

to
--

     TO procname :input1 :input2 ...			(special form)

command.  Prepares Logo to accept a procedure definition.  The procedure will be named procname and there must not already be a procedure by that name.  The inputs will be called input1 etc.  Any number of inputs are allowed, including none.  Names of procedures and inputs are case-insensitive.

Unlike every other Logo procedure, TO takes as its inputs the actual words typed in the instruction line, as if they were all quoted, rather than the results of evaluating expressions to provide the inputs. (That's what "special form" means.)

This version of Logo allows variable numbers of inputs to a procedure. After the procedure name come four kinds of things, *in this order*:

     	    1.   0 or more REQUIRED inputs    :FOO :FROBOZZ
     	    2.   0 or more OPTIONAL inputs    [:BAZ 87] [:THINGO 5+9]
     	    3.   0 or 1 REST input            [:GARPLY]
     	    4.   0 or 1 DEFAULT number        5

Every procedure has a MINIMUM, DEFAULT, and MAXIMUM number of inputs. (The latter can be infinite.)

The MINIMUM number of inputs is the number of required inputs, which must come first.  A required input is indicated by the

     :inputname

notation.

After all the required inputs can be zero or more optional inputs, each of which is represented by the following notation:

     [:inputname default.value.expression]

When the procedure is invoked, if actual inputs are not supplied for these optional inputs, the default value expressions are evaluated to set values for the corresponding input names.  The inputs are processed from left to right, so a default value expression can be based on earlier inputs.  Example:

     to proc :inlist [:startvalue first :inlist]

If the procedure is invoked by saying

     proc [a b c]

then the variable inlist will have the value [A B C] and the variable startvalue will have the value A.  If the procedure is invoked by saying

     (proc [a b c] "x)

then inlist will have the value [A B C] and startvalue will have the value X.

After all the required and optional input can come a single rest input, represented by the following notation:

     [:inputname]

This is a rest input rather than an optional input because there is no default value expression.  There can be at most one rest input.  When the procedure is invoked, the value of this input will be a list containing all of the actual inputs provided that were not used for required or optional inputs.  Example:

     to proc :in1 [:in2 "foo] [:in3 "baz] [:in4]

If this procedure is invoked by saying

     proc "x

then in1 has the value X, in2 has the value FOO, in3 has the value BAZ,and in4 has the value [] (the empty list).  If it's invoked by saying

     (proc "a "b "c "d "e)

then in1 has the value A, in2 has the value B, in3 has the value C, and in4 has the value [D E].

The MAXIMUM number of inputs for a procedure is infinite if a rest input is given; otherwise, it is the number of required inputs plus the number of optional inputs.

The DEFAULT number of inputs for a procedure, which is the number of inputs that it will accept if its invocation is not enclosed in parentheses, is ordinarily equal to the minimum number.  If you want a different default number you can indicate that by putting the desired default number as the last thing on the TO line.  example:

     to proc :in1 [:in2 "foo] [:in3] 3

This procedure has a minimum of one input, a default of three inputs, and an infinite maximum.

Logo responds to the TO command by entering procedure definition mode. The prompt character changes from ? to > and whatever instructions you type become part of the definition until you type a line containing only the word END.

define
------

     DEFINE procname text

command.  Defines a procedure with name procname and text text.  If there is already a procedure with the same name, the new definition replaces the old one.  The text input must be a list whose members are lists.  The first member is a list of inputs; it looks like a TO line but without the word TO, without the procedure name, and without the colons before input names.  In other words, the members of this first sublist are words for the names of required inputs and lists for the names of optional or rest inputs.  The remaining sublists of the text input make up the body of the procedure, with one sublist for each instruction line of the body.  (There is no END line in the text input.) It is an error to redefine a primitive procedure unless the variable REDEFP has the value TRUE.

See REDEFP .

text
----

     TEXT procname

outputs the text of the procedure named procname in the form expected by DEFINE: a list of lists, the first of which describes the inputs to the procedure and the rest of which are the lines of its body.  The text does not reflect formatting information used when the procedure was defined, such as continuation lines and extra spaces.

fulltext
--------

     FULLTEXT procname

outputs a representation of the procedure procname in which formatting information is preserved.  If the procedure was defined with TO, EDIT, or LOAD, then the output is a list of words.  Each word represents one entire line of the definition in the form output by READWORD, including extra spaces and continuation lines.  The last member of the output represents the END line.  If the procedure was defined with DEFINE, then the output is a list of lists.  If these lists are printed, one per line, the result will look like a definition using TO.  Note: the output from FULLTEXT is not suitable for use as input to DEFINE!

See TO , EDIT , LOAD , DEFINE .

copydef
-------

     COPYDEF newname oldname

command.  Makes newname a procedure identical to oldname.  The latter may be a primitive.  If newname was already defined, its previous definition is lost.  If newname was already a primitive, the redefinition is not permitted unless the variable REDEFP has the value TRUE.

Note: dialects of Logo differ as to the order of inputs to COPYDEF. This dialect uses "MAKE order," not "NAME order."

See REDEFP , SAVE , PO , POT .

Variable Definition
===================

make
----

     MAKE varname value

command.  Assigns the value value to the variable named varname, which must be a word.  Variable names are case-insensitive.  If a variable with the same name already exists, the value of that variable is changed.  If not, a new global variable is created.

name
----

     NAME value varname				(library procedure)

command.  Same as MAKE but with the inputs in reverse order.

local
-----

     LOCAL varname
     LOCAL varnamelist
     (LOCAL varname1 varname2 ...)

command.  Accepts as inputs one or more words, or a list of words.  A variable is created for each of these words, with that word as its name. The variables are local to the currently running procedure.  Logo variables follow dynamic scope rules; a variable that is local to a procedure is available to any subprocedure invoked by that procedure. The variables created by LOCAL have no initial value; they must be assigned a value (e.g., with MAKE) before the procedure attempts to read their value.

See MAKE .

localmake
---------

     LOCALMAKE varname value				(library procedure)

command.  Makes the named variable local, like LOCAL, and assigns it the given value, like MAKE.

See LOCAL , see MAKE .

thing
-----

     THING varname
     :quoted.varname

outputs the value of the variable whose name is the input.  If there is more than one such variable, the innermost local variable of that name is chosen.  The colon notation is an abbreviation not for THING but for the combination

     thing "

so that :FOO means THING "FOO.

global
------

     GLOBAL varname
     GLOBAL varnamelist
     (GLOBAL varname1 varname2 ...)

command.  Accepts as inputs one or more words, or a list of words.  A global variable is created for each of these words, with that word as its name.  The only reason this is necessary is that you might want to use the "setter" notation SETXYZ for a variable XYZ that does not already have a value; GLOBAL "XYZ makes that legal. Note: If there is currently a local variable of the same name, this command does *not* make Logo use the global value instead of the local one.

Property Lists
==============

Note: Names of property lists are always case-insensitive.  Names of individual properties are case-sensitive or case-insensitive depending on the value of CASEIGNOREDP, which is TRUE by default.

See CASEIGNOREDP .

In principle, every possible name is the name of a property list, which is initially empty.  So Logo never gives a "no such property list" error, as it would for undefined procedure or variable names.  But the primitive procedures that deal with "all" property lists (CONTENTS, PLISTS, etc.) list only nonempty ones.  To "erase" a property list ERASE means to make it empty, removing all properties from it.

pprop
-----

     PPROP plistname propname value

command.  Adds a property to the plistname property list with name propname and value value.

gprop
-----

     GPROP plistname propname

outputs the value of the propname property in the plistname property list, or the empty list if there is no such property.

remprop
-------

     REMPROP plistname propname

command.  Removes the property named propname from the property list named plistname.

plist
-----

     PLIST plistname

outputs a list whose odd-numbered members are the names, and whose even-numbered members are the values, of the properties in the property list named plistname.  The output is a copy of the actual property list; changing properties later will not magically change a list output earlier by PLIST.

Workspace Predicates
====================

procedurep
----------

     PROCEDUREP name
     PROCEDURE? name

outputs TRUE if the input is the name of a procedure.

primitivep
----------

     PRIMITIVEP name
     PRIMITIVE? name

outputs TRUE if the input is the name of a primitive procedure (one built into Logo).  Note that some of the procedures described in this document are library procedures, not primitives.

definedp
--------

     DEFINEDP name
     DEFINED? name

outputs TRUE if the input is the name of a user-defined procedure, including a library procedure.  (However, Logo does not know about a library procedure until that procedure has been invoked.)

namep
-----

     NAMEP name
     NAME? name

outputs TRUE if the input is the name of a variable.

plistp
------

     PLISTP name
     PLIST? name

outputs TRUE if the input is the name of a *nonempty* property list. (In principle every word is the name of a property list; if you haven't put any properties in it, PLIST of that name outputs an empty list, rather than giving an error message.)

Workspace Queries
=================

contents
--------

     CONTENTS

outputs a "contents list," i.e., a list of three lists containing names of defined procedures, variables, and property lists respectively.  This list includes all unburied named items in the workspace.

buried
------

     BURIED

outputs a contents list including all buried named items in the workspace.

traced
------

     TRACED

outputs a contents list including all traced named items in the workspace.

stepped
-------

     STEPPED

outputs a contents list including all stepped named items in the workspace.

procedures
----------

     PROCEDURES

outputs a list of the names of all unburied user-defined procedures in the workspace.  Note that this is a list of names, not a contents list. (However, procedures that require a contents list as input will accept this list.)

primitives
----------

     PRIMITIVES

outputs a list of the names of all primitive procedures in the workspace.  Note that this is a list of names, not a contents list.  (However, procedures that require a contents list as input will accept this list.)

names
-----

     NAMES

outputs a contents list consisting of an empty list (indicating no procedure names) followed by a list of all unburied variable names in the workspace.

plists
------

     PLISTS

outputs a contents list consisting of two empty lists (indicating no procedures or variables) followed by a list of all unburied nonempty property lists in the workspace.

namelist
--------

     NAMELIST varname				(library procedure)
     NAMELIST varnamelist

outputs a contents list consisting of an empty list followed by a list of the name or names given as input.  This is useful in conjunction with workspace control procedures that require a contents list as input.

pllist
------

     PLLIST plname					(library procedure)
     PLLIST plnamelist

outputs a contents list consisting of two empty lists followed by a list of the name or names given as input.  This is useful in conjunction with workspace control procedures that require a contents list as input.

Note:  All procedures whose input is indicated as contentslist will accept a single word (taken as a procedure name), a list of words (taken as names of procedures), or a list of three lists as described under the CONTENTS command above.

See CONTENTS .

arity
-----

     ARITY procedurename

outputs a list of three numbers: the minimum, default, and maximum number of inputs for the procedure whose name is the input.  It is an error if there is no such procedure.  A maximum of -1 means that the number of inputs is unlimited.

nodes
-----

     NODES

outputs a list of two numbers.  The first represents the number of nodes of memory currently in use.  The second shows the maximum number of nodes that have been in use at any time since the last invocation of NODES.  (A node is a small block of computer memory as used by Logo. Each number uses one node.  Each non-numeric word uses one node, plus some non-node memory for the characters in the word.  Each array takes one node, plus some non-node memory, as well as the memory required by its elements.  Each list requires one node per element, as well as the memory within the elements.)  If you want to track the memory use of an algorithm, it is best if you invoke GC at the beginning of each iteration, since otherwise the maximum will include storage that is unused but not yet collected.

Workspace Inspection
====================

po
--

     PRINTOUT contentslist
     PO contentslist

command.  Prints to the write stream the definitions of all procedures, variables, and property lists named in the input contents list.

poall
-----

     POALL						(library procedure)

command.  Prints all unburied definitions in the workspace.  Abbreviates PO CONTENTS.

See CONTENTS .

pops
----

     POPS						(library procedure)

command.  Prints the definitions of all unburied procedures in the workspace.  Abbreviates PO PROCEDURES.

See PO , PROCEDURES .

pons
----

     PONS						(library procedure)

command.  Prints the definitions of all unburied variables in the workspace.  Abbreviates PO NAMES.

See PO , NAMES .

popls
-----

     POPLS						(library procedure)

command.  Prints the contents of all unburied nonempty property lists in the workspace.  Abbreviates PO PLISTS.

See PO , PLISTS .

pon
---

     PON varname					(library procedure)
     PON varnamelist

command.  Prints the definitions of the named variable(s). Abbreviates PO NAMELIST varname(list).

See PO , NAMELIST .

popl
----

     POPL plname					(library procedure)
     POPL plnamelist

command.  Prints the definitions of the named property list(s). Abbreviates PO PLLIST plname(list).

See PO , PLLIST .

pot
---

     POT contentslist

command.  Prints the title lines of the named procedures and the definitions of the named variables and property lists.  For property lists, the entire list is shown on one line instead of as a series of PPROP instructions as in PO.

See PPROP , PO .

pots
----

     POTS						(library procedure)

command.  Prints the title lines of all unburied procedures in the workspace.  Abbreviates POT PROCEDURES.

See PROCEDURES .

Workspace Control
=================

erase
-----

     ERASE contentslist
     ER contentslist

command.  Erases from the workspace the procedures, variables, and property lists named in the input.  Primitive procedures may not be erased unless the variable REDEFP has the value TRUE.

See REDEFP .

erall
-----

     ERALL

command.  Erases all unburied procedures, variables, and property lists from the workspace.  Abbreviates ERASE CONTENTS.

See CONTENTS .

erps
----

     ERPS

command.  Erases all unburied procedures from the workspace. Abbreviates ERASE PROCEDURES.

See ERASE , PROCEDURES .

erns
----

     ERNS

command.  Erases all unburied variables from the workspace.  Abbreviates ERASE NAMES.

See ERASE , NAMES .

erpls
-----

     ERPLS

command.  Erases all unburied property lists from the workspace. Abbreviates ERASE PLISTS.

See ERASE , PLISTS .

ern
---

     ERN varname					(library procedure)
     ERN varnamelist

command.  Erases from the workspace the variable(s) named in the input. Abbreviates ERASE NAMELIST varname(list).

See ERASE , NAMELIST .

erpl
----

     ERPL plname					(library procedure)
     ERPL plnamelist

command.  Erases from the workspace the property list(s) named in the input.  Abbreviates ERASE PLLIST plname(list).

See ERASE , PLLIST .

bury
----

     BURY contentslist

command.  Buries the procedures, variables, and property lists named in the input.  A buried item is not included in the lists output by CONTENTS, PROCEDURES, VARIABLES, and PLISTS, but is included in the list output by BURIED.  By implication, buried things are not printed by POALL or saved by SAVE.

See CONTENTS , PROCEDURES , PONS , PLISTS , POALL , SAVE .

buryall
-------

     BURYALL                                         (library procedure)

command.  Abbreviates BURY CONTENTS.

See CONTENTS .

buryname
--------

     BURYNAME varname				(library procedure)
     BURYNAME varnamelist

command.  Abbreviates BURY NAMELIST varname(list).

See BURY , NAMELIST .

unbury
------

     UNBURY contentslist

command.  Unburies the procedures, variables, and property lists named in the input.  That is, the named items will be returned to view in CONTENTS, etc.

See CONTENTS .

unburyall
---------

     UNBURYALL					(library procedure)

command.  Abbreviates UNBURY BURIED.

See BURIED .

unburyname
----------

     UNBURYNAME varname				(library procedure)
     UNBURYNAME varnamelist

command.  Abbreviates UNBURY NAMELIST varname(list).

See UNBURY , NAMELIST .

buriedp
-------

     BURIEDP contentslist
     BURIED? contentslist

outputs TRUE if the first procedure, variable, or property list named in the contents list is buried, FALSE if not.  Only the first thing in the list is tested; the most common use will be with a word as input, naming a procedure, but a contents list is allowed so that you can BURIEDP [[] [VARIABLE]] or BURIEDP [[] [] [PROPLIST]].

trace
-----

     TRACE contentslist

command.  Marks the named items for tracing.  A message is printed whenever a traced procedure is invoked, giving the actual input values, and whenever a traced procedure STOPs or OUTPUTs.  A message is printed whenever a new value is assigned to a traced variable using MAKE.  A message is printed whenever a new property is given to a traced property list using PPROP.

See STOP , OUTPUT , MAKE , PPROP .

untrace
-------

     UNTRACE contentslist

command.  Turns off tracing for the named items.

tracedp
-------

     TRACEDP contentslist
     TRACED? contentslist

outputs TRUE if the first procedure, variable, or property list named in the contents list is traced, FALSE if not.  Only the first thing in the list is tested; the most common use will be with a word as input, naming a procedure, but a contents list is allowed so that you can TRACEDP [[] [VARIABLE]] or TRACEDP [[] [] [PROPLIST]].

step
----

     STEP contentslist

command.  Marks the named items for stepping.  Whenever a stepped procedure is invoked, each instruction line in the procedure body is printed before being executed, and Logo waits for the user to type a newline at the terminal.  A message is printed whenever a stepped variable name is shadowed because a local variable of the same name is created either as a procedure input or by the LOCAL command.

See LOCAL .

unstep
------

     UNSTEP contentslist

command.  Turns off stepping for the named items.

steppedp
--------

     STEPPEDP contentslist
     STEPPED? contentslist

outputs TRUE if the first procedure, variable, or property list named in the contents list is stepped, FALSE if not.  Only the first thing in the list is tested; the most common use will be with a word as input, naming a procedure, but a contents list is allowed so that you can STEPPEDP [[] [VARIABLE]] or STEPPEDP [[] [] [PROPLIST]].

edit
----

     EDIT contentslist
     ED contentslist
     (EDIT)
     (ED)

command.  If invoked with an input, EDIT writes the definitions of the named items into a temporary file and edits that file, using your favorite editor as determined by the EDITOR environment variable. If you don't have an EDITOR variable, edits the definitions using jove.  If invoked without an input, EDIT edits the same file left over from a previous EDIT or EDITFILE instruction.  When you leave the editor, Logo reads the revised definitions and modifies the workspace accordingly.  It is not an error if the input includes names for which there is no previous definition.

If there is a variable LOADNOISILY whose value is TRUE, then, after leaving the editor, TO commands in the temporary file print "PROCNAME defined" (where PROCNAME is the name of the procedure being defined); if LOADNOISILY is FALSE or undefined, TO commands in the file are carried out silently.

If there is an environment variable called TEMP, then Logo uses its value as the directory in which to write the temporary file used for editing.

Exceptionally, the EDIT command can be used without its default input and without parentheses provided that nothing follows it on the instruction line.

See LOADNOISILY , see EDITFILE .

editfile
--------

     EDITFILE filename

command.  Starts the Logo editor, like EDIT, but instead of editing a temporary file it edits the file specified by the input.  When you leave the editor, Logo reads the revised file, as for EDIT.  EDITFILE also remembers the filename, so that a subsequent EDIT command with no input will re-edit the same file.

EDITFILE is intended as an alternative to LOAD and SAVE.  You can maintain a workspace file yourself, controlling the order in which definitions appear, maintaining comments in the file, and so on.

edall
-----

     EDALL						(library procedure)

command.  Abbreviates EDIT CONTENTS.

See CONTENTS .

edps
----

     EDPS						(library procedure)

command.  Abbreviates EDIT PROCEDURES.

See EDIT , PROCEDURES .

edns
----

     EDNS						(library procedure)

command.  Abbreviates EDIT NAMES.

See EDIT , NAMES .

edpls
-----

     EDPLS						(library procedure)

command.  Abbreviates EDIT PLISTS.

See EDIT , PLISTS .

edn
---

     EDN varname					(library procedure)
     EDN varnamelist

command.  Abbreviates EDIT NAMELIST varname(list).

See EDIT , NAMELIST .

edpl
----

     EDPL plname					(library procedure)
     EDPL plnamelist

command.  Abbreviates EDIT PLLIST plname(list).

See EDIT , PLLIST .

save
----

     SAVE filename

command.  Saves the definitions of all unburied procedures, variables, and nonempty property lists in the named file.  Equivalent to

     to save :filename
     local "oldwriter
     make "oldwriter writer
     openwrite :filename
     setwrite :filename
     poall
     setwrite :oldwriter
     close :filename
     end

savel
-----

     SAVEL contentslist filename			(library procedure)

command.  Saves the definitions of the procedures, variables, and property lists specified by contentslist to the file named filename.

load
----

     LOAD filename

command.  Reads instructions from the named file and executes them.  The file can include procedure definitions with TO, and these are accepted even if a procedure by the same name already exists.  If the file assigns a list value to a variable named STARTUP, then that list is run as an instructionlist after the file is loaded. If there is a variable LOADNOISILY whose value is TRUE, then TO commands in the file print "PROCNAME defined" (where PROCNAME is the name of the procedure being defined); if LOADNOISILY is FALSE or undefined, TO commands in the file are carried out silently.

See STARTUP , see LOADNOISILY .

cslsload
--------

     CSLSLOAD name

command.  Loads the named file, like LOAD, but from the directory containing the Computer Science Logo Style programs instead of the current user's directory.

See LOAD .

help
----

     HELP name
     (HELP)

command.  Prints information from the reference manual about the primitive procedure named by the input.  With no input, lists all the primitives about which help is available.  If there is an environment variable LOGOHELP, then its value is taken as the directory in which to look for help files, instead of the default help directory.

If HELP is called with the name of a defined procedure for which there is no help file, it will print the title line of the procedure followed by lines from the procedure body that start with semicolon, stopping when a non-semicolon line is seen.

Exceptionally, the HELP command can be used without its default input and without parentheses provided that nothing follows it on the instruction line.

seteditor
---------

     SETEDITOR path

command.  Tells Logo to use the specified program as its editor instead of the default editor.  The format of a path depends on your operating system.

setlibloc
---------

     SETLIBLOC path

command.  Tells Logo to use the specified directory as its library instead of the default.  (Note that many Logo "primitive" procedures are actually found in the library, so they may become unavailable if your new library does not include them!)  The format of a path depends on your operating system.

setcslsloc
----------

     SETCSLSLOC path

command.  Tells Logo to use the specified directory for the CSLSLOAD command, instead of the default directory.  The format of a path depends on your operating system.

See CSLSLOAD .

sethelploc
----------

     SETHELPLOC path

command.  Tells Logo to look in the specified directory for the information provided by the HELP command, instead of the default directory. The format of a path depends on your operating system.

settemploc
----------

     SETTEMPLOC path

command.  Tells Logo to write editor temporary files in the specified directory rather than in the default directory.  You must have write permission for this directory.  The format of a path depends on your operating system.

gc
--

     GC
     (GC anything)

command.  Runs the garbage collector, reclaiming unused nodes.  Logo does this when necessary anyway, but you may want to use this command to control exactly when Logo does it.  In particular, the numbers output by the NODES operation will not be very meaningful unless garbage has been collected.  Another reason to use GC is that a garbage collection takes a noticeable fraction of a second, and you may want to schedule collections for times before or after some time-critical animation.  If invoked with an argument (of any value), GC runs a full garbage collection, including GCTWA (Garbage Collect Truly Worthless Atoms, which means that it removes from Logo's memory words that used to be procedure or variable names but aren't any more); without an argument, GC does a generational garbage collection, which means that only recently created nodes are examined.  (The latter is usually good enough.)

.setsegmentsize
---------------

     .SETSEGMENTSIZE num

command.  Sets the number of nodes that Logo allocates from the operating system at once to num, which mush be a positive integer. The name is dotted because bad things will happen if you use a number that's too small or too large for your computer.  The initial value is 16,000 for most systems, but is smaller for 68000-based Macs.  Making it larger will speed up computations (by reducing the number of garbage collections) at the cost of allocating more memory than necessary.

Control Structures
******************

Control
=======

Note: in the following descriptions, an instructionlist can be a list or a word.  In the latter case, the word is parsed into list form before it is run.  Thus, RUN READWORD or RUN READLIST will work.  The former is slightly preferable because it allows for a continued line (with ~) that includes a comment (with ;) on the first line.

A tf input must be the word TRUE, the word FALSE, or a list.  If it's a list, then it must be a Logo expression, which will be evaluated to produce a value that must be TRUE or FALSE.  The comparisons with TRUE and FALSE are always case-insensitive.

run
---

     RUN instructionlist

command or operation.  Runs the Logo instructions in the input list; outputs if the list contains an expression that outputs.

See READWORD , READLIST .

runresult
---------

     RUNRESULT instructionlist

runs the instructions in the input; outputs an empty list if those instructions produce no output, or a list whose only member is the output from running the input instructionlist.  Useful for inventing command-or-operation control structures:

     local "result
     make "result runresult [something]
     if emptyp :result [stop]
     output first :result

repeat
------

     REPEAT num instructionlist

command.  Runs the instructionlist repeatedly, num times.

forever
-------

     FOREVER instructionlist

command.  Runs the "instructionlist" repeatedly, until something inside the instructionlist (such as STOP or THROW) makes it stop.

See STOP , see THROW .

repcount
--------

     REPCOUNT

outputs the repetition count of the innermost current REPEAT or FOREVER, starting from 1.  If no REPEAT or FOREVER is active, outputs --1.

if
--

     IF tf instructionlist
     (IF tf instructionlist1 instructionlist2)

command.  If the first input has the value TRUE, then IF runs the second input.  If the first input has the value FALSE, then IF does nothing. (If given a third input, IF acts like IFELSE, as described below.)  It is an error if the first input is not either TRUE or FALSE.

For compatibility with earlier versions of Logo, if an IF instruction is not enclosed in parentheses, but the first thing on the instruction line after the second input expression is a literal list (i.e., a list in square brackets), the IF is treated as if it were IFELSE, but a warning message is given.  If this aberrant IF appears in a procedure body, the warning is given only the first time the procedure is invoked in each Logo session.

ifelse
------

     IFELSE tf instructionlist1 instructionlist2

command or operation.  If the first input has the value TRUE, then IFELSE runs the second input.  If the first input has the value FALSE, then IFELSE runs the third input.  IFELSE outputs a value if the instructionlist contains an expression that outputs a value.

test
----

     TEST tf

command.  Remembers its input, which must be TRUE or FALSE, for use by later IFTRUE or IFFALSE instructions.  The effect of TEST is local to the procedure in which it is used; any corresponding IFTRUE or IFFALSE must be in the same procedure or a subprocedure.

See IFFALSE .

iftrue
------

     IFTRUE instructionlist
     IFT instructionlist

command.  Runs its input if the most recent TEST instruction had a TRUE input.  The TEST must have been in the same procedure or a superprocedure.

iffalse
-------

     IFFALSE instructionlist
     IFF instructionlist

command.  Runs its input if the most recent TEST instruction had a FALSE input.  The TEST must have been in the same procedure or a superprocedure.

See TEST .

stop
----

     STOP

command.  Ends the running of the procedure in which it appears. Control is returned to the context in which that procedure was invoked. The stopped procedure does not output a value.

output
------

     OUTPUT value
     OP value

command.  Ends the running of the procedure in which it appears.  That procedure outputs the value value to the context in which it was invoked.  Don't be confused: OUTPUT itself is a command, but the procedure that invokes OUTPUT is an operation.

catch
-----

     CATCH tag instructionlist

command or operation.  Runs its second input.  Outputs if that instructionlist outputs.  If, while running the instructionlist, a THROW instruction is executed with a tag equal to the first input (case-insensitive comparison), then the running of the instructionlist is terminated immediately.  In this case the CATCH outputs if a value input is given to THROW.  The tag must be a word.

If the tag is the word ERROR, then any error condition that arises during the running of the instructionlist has the effect of THROW "ERROR instead of printing an error message and returning to toplevel.  The CATCH does not output if an error is caught.  Also, during the running of the instructionlist, the variable ERRACT is temporarily unbound.  (If there is an error while ERRACT has a value, that value is taken as an instructionlist to be run after printing the error message.  Typically the value of ERRACT, if any, is the list [PAUSE].)

See ERROR , ERRACT , PAUSE .

throw
-----

     THROW tag
     (THROW tag value)

command.  Must be used within the scope of a CATCH with an equal tag. Ends the running of the instructionlist of the CATCH.  If THROW is used with only one input, the corresponding CATCH does not output a value. If THROW is used with two inputs, the second provides an output for the CATCH.

THROW "TOPLEVEL can be used to terminate all running procedures and interactive pauses, and return to the toplevel instruction prompt. Typing the system interrupt character (normally control-C for Unix, control-Q for DOS, or command-period for Mac) has the same effect.

THROW "ERROR can be used to generate an error condition.  If the error is not caught, it prints a message (THROW "ERROR) with the usual indication of where the error (in this case the THROW) occurred.  If a second input is used along with a tag of ERROR, that second input is used as the text of the error message instead of the standard message. Also, in this case, the location indicated for the error will be, not the location of the THROW, but the location where the procedure containing the THROW was invoked.  This allows user-defined procedures to generate error messages as if they were primitives.  Note: in this case the corresponding CATCH "ERROR, if any, does not output, since the second input to THROW is not considered a return value.

THROW "SYSTEM immediately leaves Logo, returning to the operating system, without printing the usual parting message and without deleting any editor temporary file written by EDIT.

See EDIT .

error
-----

     ERROR

outputs a list describing the error just caught, if any.  If there was not an error caught since the last use of ERROR, the empty list will be output.  The error list contains four members: an integer code corresponding to the type of error, the text of the error message, the name of the procedure in which the error occurred, and the instruction line on which the error occurred.

pause
-----

     PAUSE

command or operation.  Enters an interactive pause.  The user is prompted for instructions, as at toplevel, but with a prompt that includes the name of the procedure in which PAUSE was invoked.  Local variables of that procedure are available during the pause.  PAUSE outputs if the pause is ended by a CONTINUE with an input.

If the variable ERRACT exists, and an error condition occurs, the contents of that variable are run as an instructionlist.  Typically ERRACT is given the value [PAUSE] so that an interactive pause will be entered on the event of an error.  This allows the user to check values of local variables at the time of the error.

Typing the system quit character (normally control-\ for Unix, control-W for DOS, or command-comma for Mac) will also enter a pause.

See ERRACT .

continue
--------

     CONTINUE value
     CO value
     (CONTINUE)
     (CO)

command.  Ends the current interactive pause, returning to the context of the PAUSE invocation that began it.  If CONTINUE is given an input, that value is used as the output from the PAUSE.  If not, the PAUSE does not output.

Exceptionally, the CONTINUE command can be used without its default input and without parentheses provided that nothing follows it on the instruction line.

wait
----

     WAIT time

command.  Delays further execution for time 60ths of a second.  Also causes any buffered characters destined for the terminal to be printed immediately.  WAIT 0 can be used to achieve this buffer flushing without actually waiting.

bye
---

     BYE

command.  Exits from Logo; returns to the operating system.

.maybeoutput
------------

     .MAYBEOUTPUT value				(special form)

works like OUTPUT except that the expression that provides the input value might not, in fact, output a value, in which case the effect is like STOP.  This is intended for use in control structure definitions, for cases in which you don't know whether or not some expression produces a value.  Example:

     to invoke :function [:inputs] 2
     .maybeoutput apply :function :inputs
     end

     ? (invoke "print "a "b "c)
     a b c
     ? print (invoke "word "a "b "c)
     abc

This is an alternative to RUNRESULT.  It's fast and easy to use, at the cost of being an exception to Logo's evaluation rules.  (Ordinarily, it should be an error if the expression that's supposed to provide an input to something doesn't have a value.)

See OUTPUT , STOP , RUNRESULT .

goto
----

     GOTO word

command.  Looks for a TAG command with the same input in the same procedure, and continues running the procedure from the location of that TAG.  It is meaningless to use GOTO outside of a procedure.

tag
---

     TAG quoted.word

command.  Does nothing.  The input must be a literal word following a quotation mark ("), not the result of a computation.  Tags are used by the GOTO command.

ignore
------

     IGNORE value					(library procedure)

command.  Does nothing.  Used when an expression is evaluated for a side effect and its actual value is unimportant.

`
-

     ` list						(library procedure)

outputs a list equal to its input but with certain substitutions.  If a member of the input list is the word , (comma) then the following member should be an instructionlist that produces an output when run. That output value replaces the comma and the instructionlist.  If a member of the input list is the word ,@ (comma atsign) then the following member should be an instructionlist that outputs a list when run.  The members of that list replace the ,@ and the instructionlist. Example:

     show `[foo baz ,[bf [a b c]] garply ,@[bf [a b c]]]

will print

     [foo baz [b c] garply b c]

A word starting with , or ,@ is treated as if the rest of the word were a one-word list, e.g., ,:foo is equivalent to ,[:Foo].

A word starting with ", (quote comma) or :, (colon comma) becomes a word starting with " or : but with the result of running the substitution (or its first word, if the result is a list) replacing what comes after the comma.

Backquotes can be nested.  Substitution is done only for commas at the same depth as the backquote in which they are found:

     ? show `[a `[b ,[1+2] ,[foo ,[1+3] d] e] f]
     [a ` [b , [1+2] , [foo 4 d] e] f]

     ?make "name1 "x
     ?make "name2 "y
     ? show `[a `[b ,:,:name1 ,",:name2 d] e]
     [a ` [b , [:x] , ["y] d] e]

for
---

     FOR forcontrol instructionlist			(library procedure)

command.  The first input must be a list containing three or four members: (1) a word, which will be used as the name of a local variable; (2) a word or list that will be evaluated as by RUN to determine a number, the starting value of the variable; (3) a word or list that will be evaluated to determine a number, the limit value of the variable; (4) an optional word or list that will be evaluated to determine the step size.  If the fourth member is missing, the step size will be 1 or --1 depending on whether the limit value is greater than or less than the starting value, respectively.

The second input is an instructionlist.  The effect of FOR is to run that instructionlist repeatedly, assigning a new value to the control variable (the one named by the first member of the forcontrol list) each time.  First the starting value is assigned to the control variable. Then the value is compared to the limit value.  FOR is complete when the sign of (current-limit) is the same as the sign of the step size.  (If no explicit step size is provided, the instructionlist is always run at least once.  An explicit step size can lead to a zero-trip FOR, e.g., FOR [I 1 0 1] ...). Otherwise, the instructionlist is run, then the step is added to the current value of the control variable and FOR returns to the comparison step.

     ? for [i 2 7 1.5] [print :i]
     2
     3.5
     5
     6.5
     ?

See RUN .

do.while
--------

     DO.WHILE instructionlist tfexpression		(library procedure)

command.  Repeatedly evaluates the instructionlist as long as the evaluated tfexpression remains TRUE.  Evaluates the first input first, so the instructionlist is always run at least once.  The tfexpression must be an expressionlist whose value when evaluated is TRUE or FALSE.

while
-----

     WHILE tfexpression instructionlist		(library procedure)

command.  Repeatedly evaluates the instructionlist as long as the evaluated tfexpression remains TRUE.  Evaluates the first input first, so the instructionlist may never be run at all.  The tfexpression must be an expressionlist whose value when evaluated is TRUE or FALSE.

do.until
--------

     DO.UNTIL instructionlist tfexpression		(library procedure)

command.  Repeatedly evaluates the instructionlist as long as the evaluated tfexpression remains FALSE.  Evaluates the first input first, so the instructionlist is always run at least once.  The tfexpression must be an expressionlist whose value when evaluated is TRUE or FALSE.

until
-----

     UNTIL tfexpression instructionlist		(library procedure)

command.  Repeatedly evaluates the instructionlist as long as the evaluated tfexpression remains FALSE.  Evaluates the first input first, so the instructionlist may never be run at all.  The tfexpression must be an expressionlist whose value when evaluated is TRUE or FALSE.

case
----

     CASE value clauses					(library procedure)

command or operation.  The second input is a list of lists (clauses); each clause is a list whose first element is either a list of values or the word ELSE and whose butfirst is a Logo expression or instruction.  CASE examines the clauses in order.  If a clause begins with the word ELSE (upper or lower case), then the butfirst of that clause is evaluated and CASE outputs its value, if any.  If the first input to CASE is a member of the first element of a clause, then the butfirst of that clause is evaluated and CASE outputs its value, if any.  If neither of these conditions is met, then CASE goes on to the next clause.  If no clause is satisfied, CASE does nothing.  Example:

     to vowelp :letter
     output case :letter [ [[a e i o u] "true] [else "false] ]
     end

cond
----

     COND clauses						(library procedure)

command or operation.  The input is a list of lists (clauses); each clause is a list whose first element is either an expression whose value is TRUE or FALSE, or the word ELSE, and whose butfirst is a Logo expression or instruction.  COND examines the clauses in order.  If a clause begins with the word ELSE (upper or lower case), then the butfirst of that clause is evaluated and CASE outputs its value, if any.  Otherwise, the first element of the clause is evaluated; the resulting value must be TRUE or FALSE.  If it's TRUE, then the butfirst of that clause is evaluated and COND outputs its value, if any.  If the value is FALSE, then COND goes on to the next clause.  If no clause is satisfied, COND does nothing.  Example:

     to evens :numbers	; select even numbers from a list
     op cond [ [[emptyp :numbers] []]
               [[evenp first :numbers]	; assuming EVENP is defined
                fput first :numbers evens butfirst :numbers]
               [else evens butfirst :numbers] ]
     end

Template-based Iteration
========================

The procedures in this section are iteration tools based on the idea of a template.  This is a generalization of an instruction list or an expression list in which slots are provided for the tool to insert varying data.  Four different forms of template can be used.

The most commonly used form for a template is explicit-slot form, or question mark form.  Example:

     ? show map [? * ?] [2 3 4 5]
     [4 9 16 25]
     ?

In this example, the MAP tool evaluated the template [? * ?] repeatedly, with each of the members of the data list [2 3 4 5] substituted in turn for the question marks.  The same value was used for every question mark in a given evaluation.  Some tools allow for more than one datum to be substituted in parallel; in these cases the slots are indicated by ?1 for the first datum, ?2 for the second, and so on:

     ? show (map [(word ?1 ?2 ?1)] [a b c] [d e f])
     [ada beb cfc]
     ?

If the template wishes to compute the datum number, the form (? 1) is equivalent to ?1, so (? ?1) means the datum whose number is given in datum number 1.  Some tools allow additional slot designations, as shown in the individual descriptions.

The second form of template is the named-procedure form.  If the template is a word rather than a list, it is taken as the name of a procedure.  That procedure must accept a number of inputs equal to the number of parallel data slots provided by the tool; the procedure is applied to all of the available data in order.  That is, if data ?1 through ?3 are available, the template "PROC is equivalent to [PROC ?1 ?2 ?3].

     ? show (map "word [a b c] [d e f])
     [ad be cf]
     ?

     to dotprod :a :b	; vector dot product
     op apply "sum (map "product :a :b)
     end

The third form of template is named-slot or lambda form.  This form is indicated by a template list containing more than one member, whose first member is itself a list.  The first member is taken as a list of names; local variables are created with those names and given the available data in order as their values.  The number of names must equal the number of available data.  This form is needed primarily when one iteration tool must be used within the template list of another, and the ? notation would be ambiguous in the inner template.  Example:

     to matmul :m1 :m2 [:tm2 transpose :m2]	; multiply two matrices
     output map [[row] map [[col] dotprod :row :col] :tm2] :m1
     end

The fourth form is procedure text form, a variant of lambda form.  In this form, the template list contains at least two members, all of which are lists.  This is the form used by the DEFINE and TEXT primitives, and APPLY accepts it so that the text of a defined procedure can be used as a template.

Note:  The fourth form of template is interpreted differently from the others, in that Logo considers it to be an independent defined procedure for the purposes of OUTPUT and STOP.  For example, the following two instructions are identical:

     ? print apply [[x] :x+3] [5]
     8
     ? print apply [[x] [output :x+3]] [5]
     8

although the first instruction is in named-slot form and the second is in procedure-text form.  The named-slot form can be understood as telling Logo to evaluate the expression :x+3 in place of the entire invocation of apply, with the variable x temporarily given the value 5.  The procedure-text form can be understood as invoking the procedure

     to foo :x
     output :x+3
     end

with input 5, but without actually giving the procedure a name.  If the use of OUTPUT were interchanged in these two examples, we'd get errors:

     ? print apply [[x] output :x+3] [5]
     Can only use output inside a procedure
     ? print apply [[x] [:x+3]] [5]
     You don't say what to do with 8

The named-slot form can be used with STOP or OUTPUT inside a procedure, to stop the enclosing procedure.

The following iteration tools are extended versions of the ones in Appendix B of the book _Computer_Science_Logo_Style,_Volume_3: _Advanced_Topics_ by Brian Harvey [MIT Press, 1987].  The extensions are primarily to allow for variable numbers of inputs.

apply
-----

     APPLY template inputlist

command or operation.  Runs the "template," filling its slots with the members of inputlist.  The number of members in inputlist must be an acceptable number of slots for template.  It is illegal to apply the primitive TO as a template, but anything else is okay.  APPLY outputs what template outputs, if anything.

See TO .

invoke
------

     INVOKE template input				(library procedure)
     (INVOKE template input1 input2 ...)

command or operation.  Exactly like APPLY except that the inputs are provided as separate expressions rather than in a list.

foreach
-------

     FOREACH data template				(library procedure)
     (FOREACH data1 data2 ... template)

command.  Evaluates the template list repeatedly, once for each member of the data list.  If more than one data list are given, each of them must be the same length.  (The data inputs can be words, in which case the template is evaluated once for each character.

In a template, the symbol ?REST represents the portion of the data input to the right of the member currently being used as the ? slot-filler. That is, if the data input is [A B C D E] and the template is being evaluated with ? replaced by B, then ?REST would be replaced by [C D E]. If multiple parallel slots are used, then (?REST 1) goes with ?1, etc.

In a template, the symbol # represents the position in the data input of the member currently being used as the ? slot-filler.  That is, if the data input is [A B C D E] and the template is being evaluated with ? replaced by B, then # would be replaced by 2.

map
---

     MAP template data				(library procedure)
     (MAP template data1 data2 ...)

outputs a word or list, depending on the type of the data input, of the same length as that data input.  (If more than one data input are given, the output is of the same type as data1.)  Each member of the output is the result of evaluating the template list, filling the slots with the corresponding member(s) of the data input(s).  (All data inputs must be the same length.)  In the case of a word output, the results of the template evaluation must be words, and they are concatenated with WORD.

In a template, the symbol ?REST represents the portion of the data input to the right of the member currently being used as the ? slot-filler. That is, if the data input is [A B C D E] and the template is being evaluated with ? replaced by B, then ?REST would be replaced by [C D E]. If multiple parallel slots are used, then (?REST 1) goes with ?1, etc.

In a template, the symbol # represents the position in the data input of the member currently being used as the ? slot-filler.  That is, if the data input is [A B C D E] and the template is being evaluated with ? replaced by B, then # would be replaced by 2.

See WORD .

map.se
------

     MAP.SE template data				(library procedure)
     (MAP.SE template data1 data2 ...)

outputs a list formed by evaluating the template list repeatedly and concatenating the results using SENTENCE.  That is, the members of the output are the members of the results of the evaluations.  The output list might, therefore, be of a different length from that of the data input(s).  (If the result of an evaluation is the empty list, it contributes nothing to the final output.)  The data inputs may be words or lists.

In a template, the symbol ?REST represents the portion of the data input to the right of the member currently being used as the ? slot-filler. That is, if the data input is [A B C D E] and the template is being evaluated with ? replaced by B, then ?REST would be replaced by [C D E]. If multiple parallel slots are used, then (?REST 1) goes with ?1, etc.

In a template, the symbol # represents the position in the data input of the member currently being used as the ? slot-filler.  That is, if the data input is [A B C D E] and the template is being evaluated with ? replaced by B, then # would be replaced by 2.

See SENTENCE .

filter
------

     FILTER tftemplate data				(library procedure)

outputs a word or list, depending on the type of the data input, containing a subset of the members (for a list) or characters (for a word) of the input.  The template is evaluated once for each member or character of the data, and it must produce a TRUE or FALSE value.  If the value is TRUE, then the corresponding input constituent is included in the output.

     ? print filter "vowelp "elephant
     eea
     ?

In a template, the symbol ?REST represents the portion of the data input to the right of the member currently being used as the ? slot-filler. That is, if the data input is [A B C D E] and the template is being evaluated with ? replaced by B, then ?REST would be replaced by [C D E].

In a template, the symbol # represents the position in the data input of the member currently being used as the ? slot-filler.  That is, if the data input is [A B C D E] and the template is being evaluated with ? replaced by B, then # would be replaced by 2.

find
----

     FIND tftemplate data				(library procedure)

outputs the first constituent of the data input (the first member of a list, or the first character of a word) for which the value produced by evaluating the template with that consituent in its slot is TRUE.  If there is no such constituent, the empty list is output.

In a template, the symbol ?REST represents the portion of the data input to the right of the member currently being used as the ? slot-filler. That is, if the data input is [A B C D E] and the template is being evaluated with ? replaced by B, then ?REST would be replaced by [C D E].

In a template, the symbol # represents the position in the data input of the member currently being used as the ? slot-filler.  That is, if the data input is [A B C D E] and the template is being evaluated with ? replaced by B, then # would be replaced by 2.

reduce
------

     REDUCE template data				(library procedure)

outputs the result of applying the template to accumulate the members of the data input.  The template must be a two-slot function.  Typically it is an associative function name like "SUM.  If the data input has only one constituent (member in a list or character in a word), the output is that consituent.  Otherwise, the template is first applied with ?1 filled with the next-to-last consitient and ?2 with the last constituent.  Then, if there are more constituents, the template is applied with ?1 filled with the next constituent to the left and ?2 with the result from the previous evaluation.  This process continues until all constituents have been used.  The data input may not be empty.

Note: If the template is, like SUM, the name of a procedure that is capable of accepting arbitrarily many inputs, it is more efficient to use APPLY instead of REDUCE.  The latter is good for associative procedures that have been written to accept exactly two inputs:

     to max :a :b
     output ifelse :a > :b [:a] [:b]
     end

     print reduce "max [...]

Alternatively, REDUCE can be used to write MAX as a procedure that accepts any number of inputs, as SUM does:

     to max [:inputs] 2
     if emptyp :inputs ~
        [(throw "error [not enough inputs to max])]
     output reduce [ifelse ?1 > ?2 [?1] [?2]] :inputs
     end

See SUM , APPLY .

crossmap
--------

     CROSSMAP template listlist			(library procedure)
     (CROSSMAP template data1 data2 ...)

outputs a list containing the results of template evaluations.  Each data list contributes to a slot in the template; the number of slots is equal to the number of data list inputs.  As a special case, if only one data list input is given, that list is taken as a list of data lists, and each of its members contributes values to a slot.  CROSSMAP differs from MAP in that instead of taking members from the data inputs in parallel, it takes all possible combinations of members of data inputs, which need not be the same length.

     ? show (crossmap [word ?1 ?2] [a b c] [1 2 3 4])
     [a1 a2 a3 a4 b1 b2 b3 b4 c1 c2 c3 c4]
     ?

For compatibility with the version in the first edition of CSLS  (Computer Science Logo Style), CROSSMAP templates may use the notation :1 instead of ?1 to indicate slots.

See MAP .

cascade
-------

     CASCADE endtest template startvalue		(library procedure)
     (CASCADE endtest tmp1 sv1 tmp2 sv2 ...)
     (CASCADE endtest tmp1 sv1 tmp2 sv2 ... finaltemplate)

outputs the result of applying a template (or several templates, as explained below) repeatedly, with a given value filling the slot the first time, and the result of each application filling the slot for the following application.

In the simplest case, CASCADE has three inputs.  The second input is a one-slot expression template.  That template is evaluated some number of times (perhaps zero).  On the first evaluation, the slot is filled with the third input; on subsequent evaluations, the slot is filled with the result of the previous evaluation.  The number of evaluations is determined by the first input.  This can be either a nonnegative integer, in which case the template is evaluated that many times, or a predicate expression template, in which case it is evaluated (with the same slot filler that will be used for the evaluation of the second input) repeatedly, and the CASCADE evaluation continues as long as the predicate value is FALSE.  (In other words, the predicate template indicates the condition for stopping.)

If the template is evaluated zero times, the output from CASCADE is the third (startvalue) input.  Otherwise, the output is the value produced by the last template evaluation.

CASCADE templates may include the symbol # to represent the number of times the template has been evaluated.  This slot is filled with 1 for the first evaluation, 2 for the second, and so on.

     ? show cascade 5 [lput # ?] []
     [1 2 3 4 5]
     ? show cascade [vowelp first ?] [bf ?] "spring
     ing
     ? show cascade 5 [# * ?] 1
     120
     ?

Several cascaded results can be computed in parallel by providing additional template-startvalue pairs as inputs to CASCADE.  In this case, all templates (including the endtest template, if used) are multi-slot, with the number of slots equal to the number of pairs of inputs.  In each round of evaluations, ?2 represents the result of evaluating the second template in the previous round.  If the total number of inputs (including the first endtest input) is odd, then the output from CASCADE is the final value of the first template.  If the total number of inputs is even, then the last input is a template that is evaluated once, after the end test is satisfied, to determine the output from CASCADE.

     to fibonacci :n
     output (cascade :n [?1 + ?2] 1 [?1] 0)
     end

     to piglatin :word
     output (cascade [vowelp first ?] ~
        [word bf ? first ?] ~
        :word ~
        [word ? "ay])
     end

cascade.2
---------

     CASCADE.2 endtest temp1 startval1 temp2 startval2  (library procedure)

outputs the result of invoking CASCADE with the same inputs.  The only difference is that the default number of inputs is five instead of three.

transfer
--------

     TRANSFER endtest template inbasket		(library procedure)

outputs the result of repeated evaluation of the template.  The template is evaluated once for each member of the list inbasket.  TRANSFER maintains an outbasket that is initially the empty list.  After each evaluation of the template, the resulting value becomes the new outbasket.

In the template, the symbol ?IN represents the current member from the inbasket; the symbol ?OUT represents the entire current outbasket. Other slot symbols should not be used.

If the first (endtest) input is an empty list, evaluation continues until all inbasket members have been used.  If not, the first input must be a predicate expression template, and evaluation continues until either that template's value is TRUE or the inbasket is used up.

Macros
******

.macro
------

     .MACRO procname :input1 :input2 ...			(special form)
     .DEFMACRO procname text

A macro is a special kind of procedure whose output is evaluated as Logo instructions in the context of the macro's caller.  .MACRO is exactly like TO except that the new procedure becomes a macro; .DEFMACRO is exactly like DEFINE with the same exception.

Macros are useful for inventing new control structures comparable to REPEAT, IF, and so on.  Such control structures can almost, but not quite, be duplicated by ordinary Logo procedures.  For example, here is an ordinary procedure version of REPEAT:

     to my.repeat :num :instructions
     if :num=0 [stop]
     run :instructions
     my.repeat :num-1 :instructions
     end

This version works fine for most purposes, e.g.,

     my.repeat 5 [print "hello]

But it doesn't work if the instructions to be carried out include OUTPUT, STOP, or LOCAL.  For example, consider this procedure:

     to example
     print [Guess my secret word.  You get three guesses.]
     repeat 3 [type "|?? | ~
        if readword = "secret [pr "Right! stop]]
     print [Sorry, the word was "secret"!]
     end

This procedure works as written, but if MY.REPEAT is used instead of REPEAT, it won't work because the STOP will stop MY.REPEAT instead of stopping EXAMPLE as desired.

The solution is to make MY.REPEAT a macro.  Instead of actually carrying out the computation, a macro must return a list containing Logo instructions.  The contents of that list are evaluated as if they appeared in place of the call to the macro.  Here's a macro version of REPEAT:

     .macro my.repeat :num :instructions
     if :num=0 [output []]
     output sentence :instructions ~
        (list "my.repeat :num-1 :instructions)
     end

Every macro is an operation --- it must always output something.  Even in the base case, MY.REPEAT outputs an empty instruction list.  To show how MY.REPEAT works, let's take the example

     my.repeat 5 [print "hello]

For this example, MY.REPEAT will output the instruction list

     		[print "hello my.repeat 4 [print "hello]]

Logo then executes these instructions in place of the original invocation of MY.REPEAT; this prints hello once and invokes another repetition.

The technique just shown, although fairly easy to understand, has the defect of slowness because each repetition has to construct an instruction list for evaluation.  Another approach is to make my.repeat a macro that works just like the non-macro version unless the instructions to be repeated include OUTPUT or STOP:

     .macro my.repeat :num :instructions
     catch "repeat.catchtag ~
        [op repeat.done runresult [repeat1 :num :instructions]]
     op []
     end

     to repeat1 :num :instructions
     if :num=0 [throw "repeat.catchtag]
     run :instructions
     .maybeoutput repeat1 :num-1 :instructions
     end

     to repeat.done :repeat.result
     if emptyp :repeat.result [op [stop]]
     op list "output quoted first :repeat.result
     end

If the instructions do not include STOP or OUTPUT, then REPEAT1 will reach its base case and invoke THROW.  As a result, my.repeat's last instruction line will output an empty list, so the second evaluation of the macro result will do nothing.  But if a STOP or OUTPUT happens, then REPEAT.DONE will output a STOP or OUTPUT instruction that will be re-executed in the caller's context.

The macro-defining commands have names starting with a dot because macros are an advanced feature of Logo; it's easy to get in trouble by defining a macro that doesn't terminate, or by failing to construct the instruction list properly.

Lisp users should note that Logo macros are NOT special forms.  That is, the inputs to the macro are evaluated normally, as they would be for any other Logo procedure.  It's only the output from the macro that's handled unusually.

Here's another example:

     .macro localmake :name :value
     output (list "local~
        word "" :name   ~
        "apply          ~
        ""make          ~
        (list :name :value))
     end

It's used this way:

     to try
     localmake "garply "hello
     print :garply
     end

LOCALMAKE outputs the list

     		[local "garply apply "make [garply hello]]

The reason for the use of APPLY is to avoid having to decide whether or not the second input to MAKE requires a quotation mark before it.  (In this case it would --- MAKE "GARPLY "HELLO --- but the quotation mark would be wrong if the value were a list.)

It's often convenient to use the ` function to construct the instruction list:

     .macro localmake :name :value
     op `[local ,[word "" :name] apply "make [,[:name] ,[:value]]]
     end

On the other hand, ` is pretty slow, since it's tree recursive and written in Logo.

See TO , DEFINE , APPLY , STOP , OUTPUT .

.defmacro
---------

See dMACRO .

macrop
------

     MACROP name
     MACRO? name

outputs TRUE if its input is the name of a macro.

macroexpand
-----------

     MACROEXPAND expr				(library procedure)

takes as its input a Logo expression that invokes a macro (that is, one that begins with the name of a macro) and outputs the the Logo expression into which the macro would translate the input expression.

     .macro localmake :name :value
     op `[local ,[word "" :name] apply "make [,[:name] ,[:value]]]
     end

     ? show macroexpand [localmake "pi 3.14159]
     [local "pi apply "make [pi 3.14159]]

Error Processing
****************

If an error occurs, Logo takes the following steps.  First, if there is an available variable named ERRACT, Logo takes its value as an instructionlist and runs the instructions.  The operation ERROR may be used within the instructions (once) to examine the error condition.  If the instructionlist invokes PAUSE, the error message is printed before the pause happens. Certain errors are "recoverable"; for one of those errors, if the instructionlist outputs a value, that value is used in place of the expression that caused the error.  (If ERRACT invokes PAUSE and the user then invokes CONTINUE with an input, that input becomes the output from PAUSE and therefore the output from the ERRACT instructionlist.)

It is possible for an ERRACT instructionlist to produce an inappropriate value or no value where one is needed.  As a result, the same error condition could recur forever because of this mechanism.  To avoid that danger, if the same error condition occurs twice in a row from an ERRACT instructionlist without user interaction, the message "Erract loop" is printed and control returns to toplevel.  "Without user interaction" means that if ERRACT invokes PAUSE and the user provides an incorrect value, this loop prevention mechanism does not take effect and the user gets to try again.

During the running of the ERRACT instructionlist, ERRACT is locally unbound, so an error in the ERRACT instructions themselves will not cause a loop.  In particular, an error during a pause will not cause a pause-within-a-pause unless the user reassigns the value [PAUSE] to ERRACT during the pause.  But such an error will not return to toplevel; it will remain within the original pause loop.

If there is no available ERRACT value, Logo handles the error by generating an internal THROW "ERROR.  (A user program can also generate an error condition deliberately by invoking THROW.)  If this throw is not caught by a CATCH "ERROR in the user program, it is eventually caught either by the toplevel instruction loop or by a pause loop, which prints the error message. An invocation of CATCH "ERROR in a user program locally unbinds ERRACT, so the effect is that whichever of ERRACT and CATCH "ERROR is more local will take precedence.

If a floating point overflow occurs during an arithmetic operation, or a two-input mathematical function (like POWER) is invoked with an illegal combination of inputs, the doesn't like message refers to the second operand, but should be taken as meaning the combination.

See ERRACT , THROW , ERROR , CATCH , PAUSE , CONTINUE .

Error Codes
===========

Here are the numeric codes that appear as the first member of the list output by ERROR when an error is caught, with the corresponding messages. Some messages may have two different codes depending on whether or not the error is recoverable (that is, a substitute value can be provided through the ERRACT mechanism) in the specific context.  Some messages are warnings rather than errors; these will not be caught.  Errors 0 and 32 are so bad that Logo exits immediately.

       0	Fatal internal error (can't be caught)
       1	Out of memory
       2	Stack overflow
       3	Turtle out of bounds
       4	PROC doesn't like DATUM as input (not recoverable)
       5	PROC didn't output to PROC
       6	Not enough inputs to PROC
       7	PROC doesn't like DATUM as input (recoverable)
       8	Too much inside ()'s
       9 	You don't say what to do with DATUM
      10	')' not found
      11	VAR has no value
      12	Unexpected ')'
      13	I don't know how to PROC (recoverable)
      14	Can't find catch tag for THROWTAG
      15	PROC is already defined
      16	Stopped
      17	Already dribbling
      18	File system error
      19	Assuming you mean IFELSE, not IF (warning only)
      20	VAR shadowed by local in procedure call (warning only)
      21	Throw "Error
      22	PROC is a primitive
      23	Can't use TO inside a procedure
      24	I don't know how to PROC (not recoverable)
      25	IFTRUE/IFFALSE without TEST
      26	Unexpected ']'
      27	Unexpected '}'
      28	Couldn't initialize graphics
      29	Macro returned VALUE instead of a list
      30	You don't say what to do with VALUE
      31	Can only use STOP or OUTPUT inside a procedure
      32	APPLY doesn't like BADTHING as input
      33	END inside multi-line instruction
      34	Really out of memory (can't be caught)

Special Variables
*****************

Logo takes special action if any of the following variable names exists. They follow the normal scoping rules, so a procedure can locally set one of them to limit the scope of its effect.  Initially, no variables exist except for ALLOWGETSET, CASEIGNOREDP, and UNBURYONEDIT, which are TRUE and buried.

allowgetset
-----------

     ALLOWGETSET                           (variable)

if TRUE, indicates that an attempt to use a procedure that doesn't exist should be taken as an implicit getter or setter procedure (setter if the first three letters of the name are SET) for a variable of the same name (without the SET if appropriate).

caseignoredp
------------

     CASEIGNOREDP                                (variable)

if TRUE, indicates that lower case and upper case letters should be considered equal by EQUALP, BEFOREP, MEMBERP, etc.  Logo initially makes this variable TRUE, and buries it.

See EQUALP , BEFOREP , MEMBERP .

erract
------

     ERRACT                                      (variable)

an instructionlist that will be run in the event of an error.  Typically has the value [PAUSE] to allow interactive debugging.

See PAUSE .

fullprintp
----------

     FULLPRINTP

if TRUE, then words that were created using backslash or vertical bar (to include characters that would otherwise not be treated as part of a word) are printed with the backslashes or vertical bars shown, so that the printed result could be re-read by Logo to produce the same value. If FULLPRINTP is TRUE then the empty word (however it was created) prints as ||.  (Otherwise it prints as nothing at all.)

loadnoisily
-----------

     LOADNOISILY                                 (variable)

if TRUE, prints the names of procedures defined when loading from a file (including the temporary file made by EDIT).

See EDIT .

printdepthlimit
---------------

     PRINTDEPTHLIMIT                             (variable)

if a nonnegative integer, indicates the maximum depth of sublist structure that will be printed by PRINT, etc.

See PRINT .

printwidthlimit
---------------

     PRINTWIDTHLIMIT                             (variable)

if a nonnegative integer, indicates the maximum number of members in any one list that will be printed by PRINT, etc.

See PRINT .

redefp
------

     REDEFP                                      (variable)

if TRUE, allows primitives to be erased (ERASE) or redefined (COPYDEF).

See ERASE , COPYDEF .

startup
-------

     STARTUP                                     (variable)

if assigned a list value in a file loaded by LOAD, that value is run as an instructionlist after the loading.

See LOAD .

unburyonedit
------------

     UNBURYONEDIT                            (variable)

if TRUE, causes any procedure defined during EDIT or LOAD to be unburied, so that it will be saved by a later SAVE.  Files that want to define and bury procedures must do it in that order.

See EDIT , see LOAD , see SAVE .

usealternatenames
-----------------

     USEALTERNATENAMES					(variable)

if TRUE, causes Logo to generate non-English words (from the Messages file) instead of TRUE, FALSE, END, etc.

Internationalization
********************

Berkeley Logo has limited support for non-English-speaking users. Alas, there is no Unicode support, and high-bit-on ASCII codes work in some contexts but not others.

If you want to translate Berkeley Logo for use with another language, there are three main things you have to do:

     	1. Primitive names
     	2. Error (and other) messages
     	3. Documentation

For primitive names, the easiest thing is to provide a startup file that defines aliases for the English primitive names, using COPYDEF:

     COPYDEF "AVANT "FORWARD

This should take care of it, unless your language's name for one primitive is spelled like the English name of a different primitive.  In that case you have to turn REDEFP on and be sure to copy the non-conflicting name before overwriting the conflicting one!

"Primitives" that are actually in the Logo library, of course, can just be replaced or augmented with native-language-named Logo procedures and filenames.

Of course Logo programs will still not look like your native language if the word order is dramatically different, especially if you don't put verbs before their objects.

For error messages, there is a file named Messages in the logolib directory with texts of messages, one per line.  You can replace this with a file for your own language.  Do not add, delete, or reorder lines; Logo finds messages by line number.  The sequences %p, %s, and %t in these messages represent variable parts of the message and should not be translated.  (%p PRINTs the variable part, while %s SHOWs it -- that is, the difference is about whether or not brackets are shown surrounding a list.  %t means that the variable part is a C text string rather than a Logo object.)  If you want to change the order of two variable parts (no reorderable message has more than two), you would for example replace the line

     %p doesn't like %s as input

with

     %+s is a lousy input to %p

The plus sign tells the message printer to reverse the order; you must reverse the order of %p and %s, if both are used, to match.  The plus sign goes just after the first percent sign in the message, which might not be at the beginning of the line.  The sequence \n in a message represents a newline; don't be fooled into thinking that the "n" is part of the following word.

Some messages appear twice in the file; this isn't a mistake.  The two spaces before "to" in "I don't know how  to" aren't a mistake either. The message containing just "%p" is for user-provided error messages in THROW "ERROR.  The message "  in %s\n%s" is the part of all error messages that indicates where the error occurred if it was inside a procedure; you might want to change the word "in" to your language. "%s defined\n" is what LOAD prints for each procedure defined if the variable LOADNOISILY is TRUE.  "to %p\nend\n\n" is what EDIT puts in the temporary file if you ask to edit a procedure that isn't already defined.

Also in the Messages file are lines containing only one word each; the first of these is the word "true".  Some of these words are recognized by Logo in user input; some are generated by Logo; some are both.  For example, the words TRUE and FALSE are recognized as Boolean values by IF and IFELSE, and are also generated by Logo as outputs from the primitive predicates such as EQUALP.  The word END is recognized as the end of a procedure definition, and may be generated when Logo reconstructs a procedure body for PO or EDIT.  I've used capital letters in this paragraph for easier reading, but the words in the Messages file should be in lower case.

If you replace these with non-English words, Logo will *recognize* both the English names and your alternate names.  For example, if you replace the word "true" with "vrai" then Logo will understand both of these:

     IF "TRUE [PRINT "YES]
     IF "VRAI [PRINT "YES]

The variable UseAlternateNames determines whether Logo will *generate* other-language names -- for example, whether predicate functions return the other-language alternates for TRUE and FALSE.  This variable is FALSE by default, meaning that the English words will be generated.

You might wish to have English-named predicate functions generate English TRUE and FALSE, while other-language-named predicates generate the alternate words.  This can be done by leaving UseAlternateNames false, and instead of defining the other-language predicates with COPYDEF, do it this way:

     to french.boolean :bool
     if equalp :bool "true [output "vrai]
     if equalp :bool "false [output "faux]
     output :bool	; shouldn't happen
     end

     to make.french.predicate :french :english :arity
     define :french `[[[inputs] ,[:arity]]
                      [output french.boolean
     			    apply ,[word "" :english] :inputs]]
     end

     ? make.french.predicate "egal? "equal? 2
     ? pr egal? 3 4
     faux
     ? pr egal? 4 4
     vrai
     ? pr equal? 3 4
     false
     ? pr equal? 4 4
     true

The third input to make.french.predicate is the number of inputs that the predicate expects.  This solution isn't quite perfect because the infix predicates (=, <, >) will still output in English.  If you want them to generate alternate-language words, set UseAlternateNames to TRUE instead.

Some of the words in this section of the Messages file are names of Logo primitives (OUTPUT, STOP, GOTO, TAG, IF, IFELSE, TO, .MACRO).  To translate these names, you must use COPYDEF as described earlier, in addition to changing the names in Messages.  You should be consistent in these two steps. Don't forget the period in ".macro"!

For documentation, there are two kinds: this manual and the help files. The latter are generated automatically from this manual if you have a Unix system, so in that case you need only translate this manual, maintaining the format.  (The automatic helpfile generator notices things like capital letters, tabs, hyphens, and equal signs at the beginnings of lines.)  The program makefile.c may require modification because a few of the primitive names are special cases (e.g., LOG10 is the only name with digits included).

If you don't have Unix tools, you can just translate each helpfile individually.  A period in a primitive name is represented as a D in the filename; there are no files for question marks because the HELP command looks for the file named after the corresponding primitive that ends in P.