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 Møller Skarbiniks Pedersen
<traxpla...@gmail.com> wrote:
On Wed, 25 Sep 2019 at 11:03, Francesco Ariis <fa...@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
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