R: Binding and Environment Adjustments

bindenv {base} | R Documentation

## Binding and Environment Adjustments

### Description

These functions represent an experimental interface for adjustments to environments and bindings within environments. They allow for locking environments as well as individual bindings, and for linking a variable to a function.

### Usage

    lockEnvironment(env, bindings = FALSE)
    environmentIsLocked(env)
    lockBinding(sym, env)
    bindingIsLocked(sym, env)
    makeActiveBinding(sym, fun, env)
    bindingIsActive(sym, env)

### Arguments

`env` | an environment.
`bindings` | logical specifying whether bindings should be locked.
`sym` | a name object or character string
`fun` | a function taking zero or one arguments

### Details

The function `lockEnvironment` locks its environment argument, which must be a proper environment, not NULL. Locking the NULL (base) environment may be supported later. Locking the environment prevents adding or removing variable bindings from the environment. Changing the value of a variable is still possible unless the binding has been locked.

`lockBinding` locks individual bindings in the specified environment. The value of a locked binding cannot be changed. Locked bindings may be removed from an environment unless the environment is locked.

`makeActiveBinding` installs `fun` so that getting the value of `sym` calls `fun` with no arguments, and assigning to `sym` calls `fun` with one argument, the value to be assigned. This allows things like C variables linked to R variables and variables linked to data bases to be implemented. It may also be useful for making thread-safe versions of some system globals.

### Author(s)

Luke Tierney

### Examples

    # locking environments
    e<-new.env()
    assign("x",1, env=e)
    get("x",env=e)
    lockEnvironment(e)
    get("x",env=e)
    assign("x",2, env=e)
    try(assign("y",2, env=e)) # error

    # locking bindings
    e<-new.env()
    assign("x",1, env=e)
    get("x",env=e)
    lockBinding("x", e)
    try(assign("x",2, env=e)) # error

    # active bindings
    f<-local({
        x <- 1
        function(v) {
           if (missing(v))
               cat("get\n")
           else {
               cat("set\n")
               x <<- v
           }
           x
        }
    })
    makeActiveBinding("fred", f, .GlobalEnv)
    bindingIsActive("fred", .GlobalEnv)
    fred
    fred<-2
    fred

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