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))a variable name (given as a quoted string in the function call)
an expression to be assigned to x
an environment in which to evaluate value
an environment in which to assign x
This function is invoked for its side effect, which is assigning
a promise to evaluate value to the variable 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.
substitute, to see the expression associated with a
promise, if assign.env is not the .GlobalEnv.
# NOT RUN {
msg <- "old"
delayedAssign("x", msg)
substitute(x) # shows only 'x', as it is in the global env.
msg <- "new!"
x # new!
delayedAssign("x", {
for(i in 1:3)
cat("yippee!\n")
10
})
x^2 #- yippee
x^2 #- simple number
ne <- new.env()
delayedAssign("x", pi + 2, assign.env = ne)
## See the promise {without "forcing" (i.e. evaluating) it}:
substitute(x, ne) # 'pi + 2'
# }
# NOT RUN {
### Promises in an environment [for advanced users]: ---------------------
e <- (function(x, y = 1, z) environment())(cos, "y", {cat(" HO!\n"); pi+2})
## How can we look at all promises in an env (w/o forcing them)?
gete <- function(e_)
lapply(lapply(ls(e_), as.name),
function(n) eval(substitute(substitute(X, e_), list(X=n))))
(exps <- gete(e))
sapply(exps, typeof)
(le <- as.list(e)) # evaluates ("force"s) the promises
stopifnot(identical(unname(le), lapply(exps, eval))) # and another "Ho!"
# }