[R] do.call and environments

[Tony Plate]

Andy, I think you're correct that Thomas Petzoldt wants dynamic scoping, but
isn't your argument here about strong typing vs weak typing?  To 
paraphrase, one
could say: For example, if weak typing were allowed I might write a 
function `g' that
passes argument `x' to fx() as a character, or a data frame, or a list, or 
an `lm' object,
or a connection, ....  How would you write fx() to deal with that nightmare 
if you have
weak typing? (Actually, the S language does have weak typing, and some 
functions
deal with the "nightmare" by checking the types of their arguments.  Other 
functions
just pass their arguments along without type checking, resulting in cryptic 
error
messages).

I think the issues around lexical vs dynamic scope are more to do with dynamic
scope making programming accidents more likely, because of the risk that
a variable in a higher frame is masked unintentionally by another variable of
the same name in an intervening frame, and more difficult to track down,
because the error can depend on the exact calling sequence when it occurred.

cheers,

Tony Plate

At Wednesday 11:03 AM 3/10/2004, Liaw, Andy wrote:
>Seems to me what you want is dynamic scoping: `x' is not defined in `fx'.
>You want `x' to be found in the scope of the function(s) that calls `fx',
>rather than the environment where `fx' is defined.  I was told (thanks,
>Robert!) that that is a very bad idea:  as the author of `fx', you want some
>assurance of what `x' might be.  This is done via R's lexical scope.  With
>dynamic scope, there is absolutely no way to do that.  For example, I might
>write a function `g' that define `x' as a character, or a data frame, or a
>list, or an `lm' object, or a connection, ....  How would you write `fx' to
>deal with that nightmare if you have dynamic scope?
>
>Andy
>
> > From: Thomas Petzoldt
> >
> > Hello,
> >
> > I want to call a function "fx" given by name, where some "global"
> > variables (in the environment of fx) are passed to the function. For
> > compatibility reasons I cannot modify the parameter list of fx and I
> > want to avoid setting variables in the global environment
> > (e.g. via <<-)
> >
> > Is there a way, how to do this?
> >
> > Thomas P.
> >
> > The example:
> >
> > fx <- function(y) print(x*y)
> >
> > f <- function(fun, xx) {
> >    fxx <- function() {do.call(fun, list(y=3))}
> >    x <- x
> >    fxx()
> > }
> >
> > f("fx", 13)
> >
> > ## does not work, because fx does not find x
> >
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