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

How does this help us understand the original, strange program?

Robby

On Thu, May 15, 2014 at 4:51 PM, Matthias Felleisen
<matthias at ccs.neu.edu> wrote:
>
> 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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