[R] static vs. lexical scope

William Michels
2019-09-26T16:14:45Z

The best summary I've read on the subject of R's scoping rules (in
particular how they compare to scoping rules in S-PLUS) is Dr. John
Fox's "Frames, Environments, and Scope in R and S-PLUS", written
as an
Appendix to the first edition of his book, An R and S-PLUS Companion
to Applied Regression (2002).

In this document Dr. Fox refers to "lexical" scoping primarily,
however where "static" scoping is mentioned, it is defined as
equivalent to "lexical" scoping. The Appendix is available as a PDF
from:

https://socialsciences.mcmaster.ca/jfox/Books/Companion-1E/appendix-scope.pdf

HTH, Bill.

W. Michels, Ph.D.



On Thu, Sep 26, 2019 at 7:59 AM Duncan Murdoch <murdoch.duncan at
gmail.com> wrote:
>
> On 26/09/2019 9:44 a.m., Richard O'Keefe wrote:
> > Actually, R's scope rules are seriously weird.
> > I set out to write an R compiler, wow, >20 years ago.
> > Figured out how to handle optional and keyword parameters efficiently,
> > figured out a lot of other things, but choked on the scope rules.
> > Consider
> >
> >> x <- 1
> >> f <- function () {
> > +   a <- x
> > +   x <- 2
> > +   b <- x
> > +   c(a=a, b=b)
> > + }
> >> f()
> > a b
> > 1 2
> >> x
> > [1] 1
> >
> > It's really not clear what is going on here.
>
> This is all pretty clear:  in the first assignment, x is found in the
> global environment, because it does not exist in the evaluation frame.
> In the second assignment, a new variable is created in the evaluation
> frame.  In the third assignment, that new variable is used to set the
> value of b.
>
> > However, ?assign can introduce new variables into an environment,
> > and from something like
> >    with(df, x*2-y)
> > it is impossible for a compiler to tell which, if either, of x and y
is to
> > be obtained from df and which from outside.  And of course ?with
> > is just a function:
> >
> >> df <- data.frame(y=24)
> >> w <- with
> >> w(df, x*2-y)
> > [1] -22
> >
> > So you cannot in general tell *which* function can twist the
environment
> > in which its arguments will be evaluated.
>
> It's definitely hard to compile R because of the scoping rules, but
that
> doesn't make the scoping rules unclear.
>
> > I got very tired of trying to explore a twisty maze of documentation
and
> > trying to infer a specification from examples.  I would come up with
an
> > ingenious mechanism for making the common case tolerable and the
> > rare cases possible, and then I'd discover a bear trap I
hadn't seen.
> > I love R, but I try really hard not to be clever with it.
>
> I think the specification is really pretty simple.  I'm not sure it is
> well documented anywhere, but I think I understand it pretty well, and
> it doesn't seem overly complicated to me.
>
> > So while R's scoping is *like* lexical scoping, it is *dynamic*
lexical
> > scoping, to coin a phrase.
>
> I'd say it is regular lexical scoping but with dynamic variable
> creation. Call that dynamic lexical scoping if you want, but it's not
> really a mystery.
>
> Duncan Murdoch
>
> >
> > On Thu, 26 Sep 2019 at 23:56, Martin M?ller Skarbiniks Pedersen
> > <traxplayer at gmail.com> wrote:
> >>
> >> On Wed, 25 Sep 2019 at 11:03, Francesco Ariis <fa-ml at
ariis.it> wrote:
> >>>
> >>> Dear R users/developers,
> >>> while ploughing through "An Introduction to R" [1],
I found the
> >>> expression "static scope" (in contraposition to
"lexical scope").
> >>>
> >>> I was a bit puzzled by the difference (since e.g. Wikipedia
conflates the
> >>> two) until I found this document [2].
> >>
> >>
> >> I sometimes teach a little R, and they might ask about
static/lexical scope.
> >> My short answer is normally that S uses static scoping and R uses
> >> lexical scoping.
> >> And most all modern languages uses lexical scoping.
> >> So if they know Java, C, C# etc. then the scoping rules for R are
the same.
> >>
> >> I finally says that it is not a full answer but enough for most.
> >>
> >> Regards
> >> Martin
> >>
> >> ______________________________________________
> >> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more,
see
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >
> > ______________________________________________
> > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.