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

On May 15, 2014, at 5:56 PM, Robby Findler wrote:

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


I am providing a work-around because I recognize the idiom. 
I can't explain the original. 



On May 15, 2014, at 6:23 PM, Spencer Florence wrote:

> I'm not sure how gensym fixes this:


Not #f, stx: 

(define-syntax (make-funny-set! stx)
  (syntax-parse stx
    [(_ v) #`(define #,(begin (set! funny (datum->syntax stx (gensym))) funny) v)]))












> 
> #lang racket
> 
> (require (for-syntax syntax/parse racket/syntax racket/format))
> 
> (define-for-syntax funny #f)
> 
> (define-syntax (make-funny-set! stx)
>   (syntax-parse stx
>     [(_ v) #`(define #,(begin (set! funny (datum->syntax #f (gensym))) 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)
> 
> errors with:
> 
>   g27591: unbound identifier;
>    also, no #%top syntax transformer is bound in: g27591
> 
> regardless of how many calls to void there are (inclusive of 0). This makes sense, as the syntax marks don't add up. The calls to "syntax-local-introduce" in the original program aught to solve this, but as we saw... `/me points to robby's email`
> 
> 
> 
> And removing the datum->syntax errors with:
> 
>   funny-ref: received value from syntax expander was not syntax
>     received: 'g27876
> 
> Which also makes sense: a symbol is not Syntax.
> 
> 
> 
> 
> On Thu, May 15, 2014 at 4:56 PM, Robby Findler <robby at eecs.northwestern.edu> wrote:
> 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
> >>> ____________________
> >>> Racket Users list:
> >>> http://lists.racket-lang.org/users
> >>
> >>
> >> ____________________
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> >
> >
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