[racket-dev] question, issue(?) with the scope of identifiers passed into define-syntax-rule

From: Thomas Lynch (thomas.lynch at reasoningtechnology.com)
Date: Thu Jan 15 22:27:50 EST 2015

Yes, I see some of this at the bottom of the chapter 16 in the guide,  and
the recursion supported with
 (require
<http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29>
 (for-meta
<http://docs.racket-lang.org/reference/require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._for-meta%29%29>
 n racket/base)).


On Fri, Jan 16, 2015 at 11:21 AM, Thomas Lynch <
thomas.lynch at reasoningtechnology.com> wrote:

> Thank you for that nice explanation.  I'm reminded of the scope variables
> carried in Ruby.  In Mathematica the renaming of module variables is
> explicit.  There do not appear to be run phases there.  Also thanks for the
> working example using syntax parse.  Thus far I have working examples using
> define-syntax via datum and define-syntax with case (thanks to Matthais).
>
> The following questions might best be answered by pointing me to some
> other docs, haven't run across yet:
>
> Are there more than two phases?  There is the syntax phase, i.e. compile
> time phase, and then a run phase.  Lexical scope at the syntax phase would
> be static based textual layout of the program - i.e. lexical scope.  Then
> the macros (syntax transformers) are gone (as the syntax has been
> transformed) after the syntax phase.  At run time then, we have no access
> to the syntax object information?
>
> Academic question, but can syntax information, or other values be shared
> across phases?  As an example, could one store the file name, line, and
> column number for an identifier, the make use of that later?
>
> How is it that we do work inside of a macro, for example by making use of
> require-for-syntax?  Does that imply that the routines that we make use of
> in the macro for doing work had their own syntax phase, and now they are in
> run time phase during the macro's syntax phase?   .. then if those worker
> routines make use of macros ...   seems this recursion could be arbitrarily
> deep.
>
> Can the syntax analysis be explicitly invoked?   Sort of like (apply ...)
>  or (eval ..) ?  Come to think of it, eval must do the syntax phase, then
> the run phase, so perhaps it is calls to that which causes the recursion.
>
> On Fri, Jan 16, 2015 at 11:01 AM, Alexander D. Knauth <
> alexander at knauth.org> wrote:
>
>> But I think it’s important that it doesn’t use gensym or something like
>> that, it uses syntax-marks, which means you can break these lexical scoping
>> rules if you want/need to by using either syntax-local-introduce or
>> datum->syntax:
>>
>> #lang racket
>> (require syntax/parse/define)
>> (define-simple-macro (with-tables stem body ...)
>>   #:with table-author-id (syntax-local-introduce #'table-author)
>>   (let([table-publication (string-append stem "_publication")]
>>        [table-author-id (string-append stem "_author")]
>>        [table-bridge-publication-author (string-append stem
>> "_bridge_publication_author")]
>>        [table-unique-counters (string-append stem "_unique_counters")]
>>        )
>>     body ...
>>     ))
>> (with-tables "x" table-author) ;”x_author"
>>
>>
>> On Jan 15, 2015, at 9:23 PM, Alexander McLin <alex.mclin at gmail.com>
>> wrote:
>>
>> Warning I am still a Racket intermediate user but I've been studying
>> syntactic extensions a lot the past several months.
>>
>> The problem here is macros in Racket have lexical scope just like
>> procedures, they are hygienic macros. The identifiers you introduced in the
>> with-tables macro only exist or refer to other bindings in the same lexical
>> scope as where you originally wrote the macro.
>>
>> When you invoke the macro and pass in table-author, even though it is
>> spelled the same as the identifier you wrote in the macro definition, they
>> are not the same. When the macro expands, hygiene is implemented by
>> renaming all identifiers in the macro to unique non-clashing symbols that
>> don't conflict with others existing in the scope the macro is expanding in.
>>
>> The table-author identifier in the macro in the let form is renamed to
>> something different like g6271 or something along those lines.
>>
>> Furthermore, you need to be careful about what you mean by evaluation. In
>> the presence of macros, you have the concept of syntax phase(or
>> compile-time or expand-time) evaluation versus run-time evaluation. When
>> the macro is expanding, it does it thing, processing the original syntax
>> into the new piece of syntax that replaces what was there previously such
>> as (with-tables "x" table-author) which is then finally evaluated during
>> run-time.
>>
>> (with-tables "x" table-author) will expand into something looking similar
>> to the following, just to give you an idea of what macro expansion looks
>> like:
>>
>> (let ((g6191 (string-append "x" "_publication"))
>>        (g6271 (string-append "x" "_author"))
>>        (g6369 (string-append "x" "_bridge_publication_author"))
>>        (g6445 (string-append "x" "_unique_counters")))
>>    table-author)
>>
>> Note that the original table-author identifier has been replaced by a
>> different identifier that still has the same binding you originally defined.
>>
>> The table-author identifier you passed to the macro gets inserted in the
>> body position and then the expanded code is evaluated at run-time and of
>> course gives you a run-time error since table-author does not refer to
>> anything and thus when it's evaluated, it is recognized as an undefined
>> identifier.
>>
>> (with-tables "x" "hello") works because what you get in return is:
>>
>> (let ((g6191 (string-append "x" "_publication"))
>>        (g6271 (string-append "x" "_author"))
>>        (g6369 (string-append "x" "_bridge_publication_author"))
>>        (g6445 (string-append "x" "_unique_counters")))
>>    "hello")
>>
>> "hello" is just a self-evaluating string giving you back "hello" from
>> within the let form.
>>
>> On Thu, Jan 15, 2015 at 12:12 AM, Thomas Lynch <
>> thomas.lynch at reasoningtechnology.com> wrote:
>>
>>> I have a simple syntax rule:
>>>
>>>   Welcome to Racket v5.2.1.
>>>   [email protected]> (define-syntax-rule (with-tables stem body ...)
>>>     (let(
>>>           [table-publication (string-append stem "_publication")]
>>>           [table-author (string-append stem "_author")]
>>>           [table-bridge-publication-author (string-append stem
>>> "_bridge_publication_author")]
>>>           [table-unique-counters (string-append stem "_unique_counters")]
>>>           )
>>>       body ...
>>>       ))
>>>
>>> Which works fine when I don't reference the environment defined by the
>>> let:
>>>
>>>
>>>   [email protected]>
>>>   [email protected]> (with-tables "x" "hello")
>>>
>>>   "hello"
>>>
>>>
>>> However when I pass it an identifier corresponding to one of the
>>> variables defined in the let:
>>>
>>>   [email protected]> (with-tables "x" table-author)
>>>   reference to undefined identifier: table-author
>>>   stdin::1167: table-author
>>>
>>> The identifier passed in doesn't seem to be part of the local let
>>> context, but carried in a different context, or perhaps it was evaluated as
>>> an operand.  I didn't expect either of those.  Can someone point me at a
>>> description of the expected behavior, or give me a tip here on what is
>>> happening and why.
>>>
>>> ... in Wolfram language there is a 'Hold' operator for situations like
>>> this.  Apparently inside the macro we have to do some evaluation to handle
>>> the work of the macro,  is that why the operand is evaluated?
>>>
>>> Thanks in advance for explaining the evaluation/context model here.
>>>
>>> Thomas
>>>
>>> _________________________
>>>   Racket Developers list:
>>>   http://lists.racket-lang.org/dev
>>>
>>>
>> _________________________
>>  Racket Developers list:
>>  http://lists.racket-lang.org/dev
>>
>>
>>
>
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