[R] Summary: do.call and environments

[Liaw, Andy]

Gabor,

> From: Gabor Grothendieck
> 
> Note that R and S are fundamentally different when it comes to
> scoping.  
> 
> R uses lexical scoping, i.e. the parent environment of a function
> is the environment at the point where it is *defined* whereas
> S uses dynamic scoping, i.e. the parent environment in a function 
> is the environment at the point where the function is *called*.

I don't think that's quite right.  S does not use dynamic scope.  This
simple example fails in S-PLUS (6.1):

> g <- function() x + y
> f <- function() {
+   x <- 2
+   y <- 3
+   g()
+ }
> f()
Problem in g(): Object "y" not found 
Use traceback() to see the call stack

It would have worked if S has dynamic scope: variables defined in the
caller's (f in this case) frame is visible to the function being called (g
in this case).  The S scoping rule is described in the S-PLUS programmer's
guide, as well as V&R's `S Programming' (which also describes R's lexical
scope).

And of course, the example doesn't work in R, either:

> g <- function() x + y
> f <- function() {
+   x <- 3
+   y <- 2
+   g()
+ }
> f()
Error in g() : Object "x" not found

Cheers,
Andy
 
> Thus, anything regarding scoping will be different in the 
> two systems.
> 
> Note that in S, the question is easy since all you need is:
> 
> x <- 7
> fx <- function(y) print(x*y)
> f <- function(fx, x) do.call(fx,list(3))
> f("fx",2)
> 
> This gives 6 in S but 21 in R.  (Better check this since I
> don't have access to S and am going by my understanding.)
> 
> In fact, this entire exercise can be regarded as simulating
> S-style dynamic scoping in R.
> 
> ---
> Date:   Thu, 11 Mar 2004 09:45:32 +0100 
> From:   Thomas Petzoldt <petzoldt at rcs.urz.tu-dresden.de>
> To:   Spencer Graves <spencer.graves at pdf.com> 
> Cc:   <ggrothendieck at myway.com>, <r-help at stat.math.ethz.ch> 
> Subject:   Re: [R] Summary: do.call and environments 
> 
>  
> Hello,
> 
> > > f("fx",2)
> > [1] 6
> > 
> > I would have naively expected 14. From whence cometh "6"?
> > Also, I prefer to use transportable code wherever feasible. The 
> 
> 2*3=6, which was the intention. It is in fact only a proof of 
> correctness, that "7" is not used here. The proposal of Gabor does 
> exactly, what I want, but if it does not work on S-PLUS, it's 
> a serious 
> disadvantage and I should check this too.
> 
> Thomas P.
> 
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