On 13-05-16 11:17 PM, Peter Meilstrup wrote:
On Thu, May 16, 2013 at 6:06 AM, McGehee, Robert <
robert.mcge...@geodecapital.com> wrote:
Duncan, Thank you for the clarification on how delayedAssign works. Should
R-level interfaces to promise objects ever become available, I expect they
would at time come in handy.
On the subject of substitute and delayedAssign, I do have a follow-up
question for the list. I'm trying to convert a named list of expression
objects into an environment of promise objects. After conversion, each
expression in the list will be automatically evaluated when the variable
with the same name is accessed in the environment. Effectively, I'm trying
to create a hash table of promise objects.
Populating a new environment with promises happens to be what "calling a
function" in R does anyway, so an elegant way to accomplish this goal is:
makePromiseEnv <- function(expressions, parent=parent.frame()) {
f <- function() environment()
formals(f) <- as.pairlist(expressions)
environment(f) <- parent
f()
}
e <- makePromiseEnv(alist(a = {print("hello"); 4}, b = {print("again");
6}))
e$a
[1] "hello"
[1] 4
e$a
[1] 4
e$b
[1] "again"
[1] 6
e$b
[1] 6
I like that solution, except for one thing: I don't see an easy way to
control the environment where those expressions will be executed. Since
you've set them as defaults on the arguments, they will be evaluated in
the evaluation frame of f(), and that might not be what we want. An
obvious example of the problem would be
e <- makePromiseEnv(alist(a = ls()))
I don't know what Robert would want
e$a
to print, but one somewhat natural version would be to have it evaluate
the ls() in the environment from which makePromiseEnv was called, i.e.
the global environment in this case. Neither your solution nor mine do
this, but I can see how to modify mine, since it makes the evaluation
environment of the expression explicit. Can you see a modification that
would do that with your approach?
Duncan Murdoch
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