[R] static vs. lexical scope

[Jeff Newmiller]

I found this confusing until I learned about environments. The current state of the environment that was active at the time the function was defined is searched, not a frozen copy of the enclosing environment as it existed at the time the function was defined.

x <- 1 # as it was when f was created
f <- function () {
  a <- x # x not yet in current environment
  x <- 2 # always modifies current environment
  b <- x # finds the definition nearest along the enclosing environments list
  c(a=a, b=b)
}
x <- 3 # change after f was created
f()
## a b
## 3 2
x # changes within function don't affect enclosing or calling environments
## [1] 3

Duncan refers to "evaluation frame" (a term described in the R Language Definition) but I tend to think of a "current environment" and two linked lists of supplementary environments: the search path of `parent.env`()s (a.k.a. enclosing environments built from the current environments that were active when functions were defined) and the call chain of `parent.frame`()s (function environments that were active at the point functions were called). You can almost forget about the parent.frame chain unless you are creating your own non-standard evaluation functions (like `with` or `subset`)... 

Environments mutate over time. Specifically, the list of enclosing environments doesn't change but the variables in them do.

(Credit to Hadley Wickham's Advanced R, criticisms to me.)

On September 26, 2019 7:58:57 AM PDT, Duncan Murdoch <murdoch.duncan using 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 Møller Skarbiniks Pedersen
>> <traxplayer using gmail.com> wrote:
>>>
>>> On Wed, 25 Sep 2019 at 11:03, Francesco Ariis <fa-ml using 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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>> 
>> ______________________________________________
>> R-help using 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 using 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.

-- 
Sent from my phone. Please excuse my brevity.