Loading source files

The Glasgow Haskell Compiler User's Guide, Version 5.04
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# 3.2. Loading source files

Suppose we have the following Haskell source code, which we place in a file `Main.hs` in the current directory:

    main = print (fac 20)

    fac 0 = 1
    fac n = n * fac (n-1)

To load a Haskell source file into GHCi, use the `:load` command:

    Prelude> :load Main
    Compiling Main             ( Main.hs, interpreted )
    Ok, modules loaded: Main.
    *Main>

GHCi has loaded the `Main` module, and the prompt has changed to “`*Main>`” to indicate that the current context for expressions typed at the prompt is the `Main` module we just loaded (we'll explain what the `*` means later in Section 3.4.1). So we can now type expressions involving the functions from `Main.hs`:

    *Main> fac 17
    355687428096000

Loading a multi-module program is just as straightforward; just give the name of the “topmost” module to the `:load` command (hint: `:load` can be abbreviated to `:l`). The topmost module will normally be `Main`, but it doesn't have to be. GHCi will discover which modules are required, directly or indirectly, by the topmost module, and load them all in dependency order.

## 3.2.1. Modules vs. filenames

Question: How does GHC find the filename which contains module `M`? Answer: it looks for the file ``M`.hs`, or ``M`.lhs`. This means that for most modules, the module name must match the filename. If it doesn't, GHCi won't be able to find it.

There is one exception to this general rule: when you load a program with `:load`, or specify it when you invoke `ghci`, you can give a filename rather than a module name. This filename is loaded if it exists, and it may contain any module you like. This is particularly convenient if you have several `Main` modules in the same directory and you can't call them all `Main.hs`.

The search path for finding source files is specified with the `-i` option on the GHCi command line, like so:

    ghci -idir1:...:dirn

or it can be set using the `:set` command from within GHCi (see Section 3.7.2)[1]

One consequence of the way that GHCi follows dependencies to find modules to load is that every module must have a source file. The only exception to the rule is modules that come from a package, including the `Prelude` and standard libraries such as `IO` and `Complex`. If you attempt to load a module for which GHCi can't find a source file, even if there are object and interface files for the module, you'll get an error message.

One final note: if you load a module called Main, it must contain a `main` function, just like in GHC.

## 3.2.2. Making changes and recompilation

If you make some changes to the source code and want GHCi to recompile the program, give the `:reload` command. The program will be recompiled as necessary, with GHCi doing its best to avoid actually recompiling modules if their external dependencies haven't changed. This is the same mechanism we use to avoid re-compiling modules in the batch compilation setting (see Section 4.9.5).

### Notes

[1] | Note that in GHCi, and `––make` mode, the `-i` option is used to specify the search path for source files, whereas in standard batch-compilation mode the `-i` option is used to specify the search path for interface files, see Section 4.9.2.

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