[racket] Strange macro behavior with set! and syntax-local-introduce

Then use gensym instead of g and "~a" instead. 



On May 15, 2014, at 5:38 PM, Matthias Felleisen <matthias at ccs.neu.edu> wrote:

> 
> Did you want this: 
> 
> #lang racket
> 
> (require (for-syntax syntax/parse racket/syntax))
> 
> (define-for-syntax funny #f)
> 
> (define-syntax (make-funny-set! stx)
>  (syntax-parse stx
>    [(_ v) #`(define #,(begin (set! funny (format-id stx "g")) funny) v)]))
> 
> (define-syntax (funny-ref stx)
>  (syntax-parse stx
>    [(_) funny]))
> 
> (define-syntax (funny-set! stx)
>  (syntax-parse stx
>    [(_ v) #`(set! #,funny v)]))
> 
> (make-funny-set! 2)
> (void (void (void (funny-set! 3))))
> (funny-ref)
> 
> [I had to write such a macro a while back, and the above is roughly what I remember doing. Note the lexical context]
> 
> 
> On May 15, 2014, at 5:25 PM, Spencer Florence <spencer at florence.io> wrote:
> 
>> I'm attempting to write a macro which introduces a new id, then another macro that set!s that id. 
>> Example:
>> 
>> #lang racket
>> (require (for-syntax syntax/parse racket/syntax))
>> (define-for-syntax funny #f)
>> (define-syntax (make-funny-set! stx)
>>  (syntax-parse stx
>>    [(_ v)
>>     (define unmarked (generate-temporary))
>>     (set! funny (syntax-local-introduce unmarked))
>>     #`(define #,unmarked v)]))
>> (define-syntax (funny-ref stx)
>>  (syntax-parse stx
>>    [(_)
>>     funny]))
>> (define-syntax (funny-set! stx)
>>  (syntax-parse stx
>>    [(_ v)
>>     #`(set! #,(syntax-local-introduce funny) v)]))
>> 
>> (make-funny-set! 2)
>> (funny-set! 3)
>> (funny-ref)
>> 
>> This program works as I expect, evaluating to 3. However if I change (funny-set! 3) to (void (funny-set! 3)) I get the error: "set!: unbound identifier in module in: g1"
>> 
>> I do not get this error if I change (funny-ref) to (void (funny-ref)).
>> 
>> If I look at the expansion of the (void (funny-set! 3)) program in drracket's macro stepper the the g1 in (define g1 2) and the g1 in (void (set! g1 3)) have the same color.
>> 
>> To keep on with the strange, if I change the #,(syntax-local-introduce funny) inside of funny-set! to #,funny inside the behavior of all programs remains the same.
>> 
>> Could someone explain whats going on?
>> 
>> --Spencer
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> 
> 
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