[plt-scheme] scope of module exports

No, Jon just forgot that he needed stx not #'stx. [I communicated  
with him directly.] -- Matthias


On Dec 11, 2006, at 2:27 PM, Jens Axel Søgaard wrote:

> Jon Rafkind skrev:
>> I came across a, what I thought, strange situation with macros and  
>> modules. If I provide an identifier that was created with datum- 
>> >syntax-object with macro A and define that identifier with macro  
>> B then the module system will complain that the identifier is not  
>> provided by the module. Basically this doesn't work:
>> (module x1 mzscheme
>> (define-syntax (b1 stx)
>>  (syntax-case stx ()
>>    ((_) (with-syntax ((f (datum->syntax-object #'stx 'foo)))
>>       #'(define f 2)))))
>> (define-syntax (b2 stx)
>>  (syntax-case stx ()
>>    ((_) (with-syntax ((f (datum->syntax-object #'stx 'foo)))
>>       #'(provide f)))))
>> (b1)
>> (b2)
>> )
>> x.ss:1:0: module: provided identifier not defined or imported at:  
>> foo in: (#%plain-module-begin (require-for-syntax mzscheme)  
>> (define-syntax (b1 stx) (syntax-case stx () (...
>> I suppose because the provide statement in b2 doesn't see the  
>> variable that was defined in b1. But if the syntax expansion phase  
>> happens before the module evaluation phase then shouldn't this be  
>> ok? I guess I'm missing something.
>
> When is two identifiers "the same"?
>
> In this case, I believe, you need the two foos to be
> binding-identifier=?. Operationally this means that besides
> having the same symbolic name, they must also have the same marks.
> Thus they need to be introduced be the same macro use.
>
> The section on provide in "PLT MzScheme: Language Manual" refers
> to section 12.3.5:
>
>   When a top-level definition binds an identifier that originates  
> from a
>   macro expansion, the definition captures only uses of the identifier
>   that are generated by the same expansion. This behavior is  
> consistent
>   with internal definitions (see section 2.8.5), where the defined
>   identifier turns into a fresh lexical binding.
>
> Throwing hygiene overboard, you can use syntax-local-introduce to
> avoid the expansion to add a mark to the identifier foo:
>
> (module x1 mzscheme
>
>   (define-syntax (b1 stx)
>     (syntax-case stx ()
>       ((_) (with-syntax
>             ((f (syntax-local-introduce
>                    (datum->syntax-object #'stx 'foo))))
>              #'(define f 2)))))
>
>   (define-syntax (b2 stx)
>     (syntax-case stx ()
>       ((_) (with-syntax
>             ((f (syntax-local-introduce
>                    (datum->syntax-object #'stx 'foo))))
>              #'(provide f)))))
>
>   (b1)
>   (b2))
>
> > (require x1)
> > foo
> 2
>
>
> -- 
> Jens Axel Søgaard
>
>
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