[R] R 'function' as "subroutine"

[Gabor Grothendieck]

On 10/3/07, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
> On 10/3/2007 7:56 AM, (Ted Harding) wrote:
> > Hi Folks,
> >
> > The question I'm asking, regarding the use of function
> > definitions in the context described below, is whether
> > there are subtle traps or obscure limitations I should
> > watch out for. It is probably a rather naive question...
> >
> > Quite often, one has occasion to execute interactively
> > a lot of R commands in which, from time to time, one has
> > occasion to repeat exactly a sequence of commands which
> > one has entered earlier. These commands would only refer
> > to variables which have been created at the "top level" of
> > the program and which exist at the time the sequence of
> > commands is entered.
> >
> > So it would be convenient to refer to such a sequence of
> > commands as a "named block" -- just give its name, and
> > they are executed.
> >
> > In my experiments, wrapping the first occurrence of such
> > a sequence in a function definition seems to work, e.g.
> > the first time they are needed:
> >
> > block1 <- function(){
> >   sequence of commands that you would have enetered
> >   for execution at this point
> > }
> > block1()
> >
> > This first call to block1() seems to work OK, in my tests,
> > PROVIDED, of course,
> > a) The variables it uses and assigns to exist already;
> > b) all internal "<-" assignments are written "<<-".
> > Then, of course, the next time that block is needed,
> > you can call block1() again.
> >
> > But can this usage of function definition give rise
> > to problems? R scoping can be a bit tricky! And I
> > think I am perhaps being naive ...
>
> Doing this is a bad idea in the long run.  Once you have several of
> these little snippets defined, you'll forget the details of what each of
> them does, and then the "side effects" of writing global variables will
> come back and bite you really badly.
>

There is a version of defmacro in the source code of the Rcmdr
package that supports local variables.  Any variable beginning with ..
(two dots) is regarded to be local.