[R] Understanding eval

[Charilaos Skiadas]

That's a very good idea. I'll have to see if and how it applies to  
the more general case I'm dealing with.  ( Essentially I am trying to  
create "tkcbind" and "tkrbind" kind of functions, that group together  
tcltk widgets under a new frame they create, and the frame has to be  
created before the widgets are and the widgets should be created with  
the frame as their parent  etc. ... ). Anyway, that's another story  
for another day ;).

Haris Skiadas
Department of Mathematics and Computer Science
Hanover College

On Dec 22, 2007, at 8:51 PM, Gabor Grothendieck wrote:

> Duncan has already pointed out that consideration of promises is
> what is missing in the description but in addition the way lm and
> other functions in R get around it is to use match.call like this:
>
> bar2 <- function(er) {
> 	mc <- match.call()
> 	mc[[1]] <- as.name("foo")
> 	names(mc)[[2]] <- "expr"
> 	eval.parent(mc)
> }
> bar2(a)
>
> On Dec 22, 2007 3:30 PM, Charilaos Skiadas <cskiadas at gmail.com> wrote:
>> After many hours of debugging code, I came to the conclusion that I
>> have a fundamental misunderstanding regarding eval, and hope that
>> someone here can explain to me, why the following code acts as it  
>> does:
>>
>> foo <- function(expr) {
>>   eval(substitute(expr), envir=list(a=5), enclos=parent.frame())
>> }
>> bar <- function(er) {
>>   foo(er)
>> }
>>
>>> foo(a)
>> [1] 5
>>> bar(a)
>> Error in eval(expr, envir, enclos) : object "a" not found
>>
>>
>> Now, regarding the "bar(a)" call, this is my understanding of what
>> happens, hoping someone will correct me where I'm wrong.
>>
>> 1) bar is called. Its evaluation frame contains the association  
>> "er=a".
>> 2) bar calls foo. So foo is called, and its evaluation frame contains
>> the association "expr=er", with enclosing environment the local
>> environment of bar.
>> 3) foo calls eval.
>> 4) eval starts by evaluating "substitute(expr)" in foo's environment.
>> "substitute" then locates expr in foo's environment, and replaces it
>> with er. So the result of this process is the symbol er, which is
>> what will now be evaluated by eval.
>> 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.
>> 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.
>> 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.
>>
>> Thanks,
>> Haris Skiadas
>> Department of Mathematics and Computer Science
>> Hanover College
>>