[R] Understanding eval

[Charilaos Skiadas]

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