| delayedAssign {base} | R Documentation |
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.
delayedAssign(x, value, eval.env = parent.frame(1),
assign.env = parent.frame(1))
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 |
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.
This function is meant to replace the delay() function, to make it
more difficult for R code to see ``naked'' promises.
This function is invoked for its side effect, which is assigning
a promise to evaluate value to the variable x.
substitute, to see the expression associated with a promise.
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