[plt-dev] Symptom of REPL's hopelessness?

syntax-local-context, I think.

Robby

On Fri, Oct 2, 2009 at 6:17 AM, Casey Klein
<clklein at eecs.northwestern.edu> wrote:
> On Thu, Oct 1, 2009 at 3:35 PM, Matthew Flatt <mflatt at cs.utah.edu> wrote:
>> At Thu, 1 Oct 2009 15:01:56 -0500, Casey Klein wrote:
>>> Can someone explain why this macro expands without error in a module
>>> but not in the REPL?
>>>
>>> (define-syntax (m stx)
>>>  (syntax-case stx ()
>>>    [(_ x)
>>>     (with-syntax ([(y) (generate-temporaries (syntax (x)))])
>>>       (syntax (define (y) y)))]))
>>>
>>> > (m q)
>>> compile: unbound identifier (and no #%top syntax transformer is bound) in: q1
>>
>> Yes, it's a hopeless-top-level sort of problem. At the point where the
>> expander is trying to figure out where `y' comes from, there is no
>> suitable `y', yet, and there won't be until the `define' is actually
>> evaluated.
>>
>> There's a special hack to solve this problem: when a top-level
>> `define-syntaxes' gets zero results from the RHS expression, it just
>> makes the identifiers exist (in a suitable way) without defining
>> anything. So, this works:
>>
>>  (define-syntax (m stx)
>>  (syntax-case stx ()
>>    [(_ x)
>>     (with-syntax ([(y) (generate-temporaries (syntax (x)))])
>>       (syntax
>>        (begin
>>         (define-syntaxes (y) (values))
>>         (define (y) y))))]))
>>
>
> I see that this works only at the top-level. Is there a way to check
> whether this application of m is at the top-level, so that I can
> conditionally include the define-syntaxes trick?
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