delayedAssign {base}

R Documentation

Delay Evaluation

Description

delayedAssign creates a promise to evaluate the given expression if its value is requested. This provides direct access to the lazy evaluation mechanism used by R for the evaluation of (interpreted) functions.

Usage

delayedAssign(x, value, eval.env = parent.frame(1), 
              assign.env = parent.frame(1))

Arguments

x	a variable name (given as a quoted string in the function call)
value	an expression to be assigned to x
eval.env	an environment in which to evaluate value
assign.env	an environment in which to assign x

Details

Both eval.env and assign.env default to the currently active environment.

The expression assigned to a promise by delayedAssign will not be evaluated until it is eventually ‘forced’. This happens when the variable is first accessed.

When the promise is eventually forced, it is evaluated within the environment specified by eval.env (whose contents may have changed in the meantime). After that, the value is fixed and the expression will not be evaluated again.

Value

This function is invoked for its side effect, which is assigning a promise to evaluate value to the variable x.

See Also

substitute, to see the expression associated with a promise.

Examples

msg <- "old"
delayedAssign("x", msg)
msg <- "new!"
x #- new!
substitute(x) #- msg

delayedAssign("x", {
    for(i in 1:3)
        cat("yippee!\n")
    10
})

x^2 #- yippee
x^2 #- simple number

e <- (function(x, y = 1, z) environment())(1+2, "y", {cat(" HO! "); pi+2})
(le <- as.list(e)) # evaluates the promises

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