[R] on lexical scoping....

[akshay kulkarni]

Dear Ducan,
                        Very informative! THanks a lot!

THanking you,
Yours sincerely,
AKSHAY M KULKARNI
________________________________
From: Duncan Murdoch <murdoch.duncan using gmail.com>
Sent: Tuesday, April 4, 2023 8:14 PM
To: akshay kulkarni <akshay_e4 using hotmail.com>; R help Mailing list <r-help using r-project.org>
Subject: Re: [R] on lexical scoping....

On 04/04/2023 10:35 a.m., akshay kulkarni wrote:
> Dear Duncan,
>                           THanks for the reply.
>
> I am looking at the technical point. The behavior you just described, as
> far as I know, is only for functions right?

No, not at all.  Every function you write in R has an associated
environment (which you can find using environment(fn)).  That only
applies to searches taking place while evaluating the function.  The
starting point is a temporary environment to hold local variables,
called the "evaluation frame".  The parent of that environment is the
environment given by environment(fn).

When you type something at the top level, evaluation is done very
similarly, except that the starting point for the search is the global
environment.

THre is no documentation
> ever, which says that the code looks for x in the search path. Could you
> please point me to some resources where I can find further information
> on lexical scoping for the code "typed" in the console (but not in a
> function)?

See section 3.5.1 in the R Language Definition for a description of the
relation between the global environment and the search list.  See
section 2.1.10 in that manual for how R looks up variables.

Duncan Murdoch

>
> THanking you,
> Yours sincerely,
> AKSHAY M KULKARNI
>
> ------------------------------------------------------------------------
> *From:* Duncan Murdoch <murdoch.duncan using gmail.com>
> *Sent:* Tuesday, April 4, 2023 7:48 PM
> *To:* akshay kulkarni <akshay_e4 using hotmail.com>; R help Mailing list
> <r-help using r-project.org>
> *Subject:* Re: [R] on lexical scoping....
> On 04/04/2023 9:56 a.m., akshay kulkarni wrote:
>> Dear Members,
>>                               I have the following code typed at the console prompt:
>>
>> y   <-   x*10
>>
>> X has not been defined and the above code throws an object not found error. That is, the global environment does not contain x. Why doesn't it look further in the environment stack, like that of packages? There are thousands of packages that contain the variable  named  x. Of course, that happens if the above code is in a function
> (or does it?).
>>
>> What concept of R is at work in this dichotomy?
>>
>
> First, some background:
>
> Packages are associated with multiple environments.  There is the
> internal one that the package sees, and the external one that contains
> just the exports.
>
> These are sometimes called the "package" environment and the "namespace"
> environment, but I don't think those names are used consistently.
> (There's another one containing the imports, but for these purposes,
> it's indistinguishable from the internal one.)
>
> When a package is loaded by loadNamespace("pkg"), nothing happens in the
> global environment:  no new variables are visible.
>
> When it is attached by library("pkg"), a lot more happens.  First, it is
> loaded as above, then the search list is modified.  The global
> environment is entry 1 in the search list; it stays there, but its
> "parent" is set to a copy of the external environment from the new
> package, and the parent of that environment is set to the previous
> parent, the second entry in the search list.
>
> Okay, so now you search for "x" in the global environment, and it's not
> there.  It then goes to the other entries in the search list, which are
> typically external environments from various packages.  None of those
> packages export "x", so it is not found.
>
> It doesn't matter if those packages use "x" without exporting it,
> because R won't look at internal environments in this kind of search.
>
> And it doesn't matter what happens in other packages that are not on the
> search list (i.e. not "attached" because you never called library() or
> require() on them), because they just aren't in the chain of
> environments where R looks.
>
> Duncan Murdoch


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