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

From: Casey Klein (clklein at eecs.northwestern.edu)
Date: Fri Oct 2 07:17:04 EDT 2009

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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