Understanding eval
On Dec 22, 2007, at 4:44 PM, Duncan Murdoch wrote:
5) eval then creates the environment where this evaluation will take place. It does that by creating an environment containing the frame "a=5", and with enclosing environment the parent frame of foo, which is bar's environment. 6) So, as I understand it, the symbol "er" is going to now be evaluated in an environment where a is set to 5 and er is set to a, along with whatever is in the user's workspace.
I think this part is wrong. A better description is: er is going to be evaluated in an environment where a is set to 5. The parent of that environment is the bar evaluation frame, where er is set to be a promise to evaluate a in the global environment.
7) So the first step now is looking up a definition for er. Nothing is found in the current frame, so the evaluation proceeds to bar's environment, where the association "er=a" is found, so er is replaced by a.
No, at this point an attempt is made to force the promise. Promises have their own associated environments, and that's where the evaluation takes place. In the case of the er object, the associated environment is the one where bar(a) was called, i.e. the global environment.
8) Now, and perhaps this is where I misunderstand things, the lookup for a will take place. My thinking was that the lookup would start from the evaluation environment that eval created, and hence would locate the a=5 value. But this is clearly not what happens. Anyway, hope someone will correct me where I'm wrong, and explain to me what I am doing wrong, and ideally how to diagnose such things.
Diagnosing things like this is hard. Promises are very difficult things to look at: as soon as you try to do anything with them they get evaluated, and there's no way in R code to display them without that. You can use substitute() to extract the expression part, but there's no way to extract the environment part. Maybe there should be, but it's tricky to get the semantics right. If the function environment() worked to extract the environment of a promise, then all sorts of code would fail where I really wanted to evaluate the arg before extracting the environment.
Thank you Duncan, for the very clear explanation.
Ok, so the substitute "breaks through" the promise of expr, returning
as a language object the promise of er, and there's no easy way to
break through that. I ended up with the following, somewhat uglier
than I wanted, code, which seems to do what I need in this case, and
hopefully will still work in the more general case I want it to. The
idea was to break through the er promise in bar, before sending it
over to foo. Then foo receives simply an expression, which it can
then evaluate. Though I seem to have had to work a bit harder on that
part than I expected to. Perhaps there's an easier way? Or things
that can go seriously wrong with this way?
foo <- function(fr, expr) {
..obj <- list(.=fr)
..expr <- substitute(expr)
..txt <- parse( text=paste("substitute(",..expr,")") )
..expr <- eval(..txt, ..obj, parent.frame())
..expr <- eval(..expr, parent.frame())
eval(..expr, ..obj)
}
bar <- function(parent, er, ...) {
.fr=parent
g <- substitute(er)
foo(.fr, g)
}
> foo(5,.)
[1] 5
> bar(5,.)
[1] 5
Duncan Murdoch
Haris Skiadas Department of Mathematics and Computer Science Hanover College