[racket] define-syntax vs let-syntax

The behavior of `set!' in the right-hand side of a `let-syntax' binding
to mutate a `define-for-syntax'ed variable was completely broken.
(Apparently, no one has ever tried that combination before.) I've
pushed a fix.

At Tue, 16 Nov 2010 12:54:17 -0700, Jon Rafkind wrote:
> On 11/16/2010 12:45 PM, Jon Rafkind wrote:
> > `define-syntax' has different behavior from `let-syntax' when the
> > transformer operates on top-level bindings in the transformer environment.
> >
> > (define-for-syntax foo 1)
> >
> > (define-syntax (x1 stx)
> >   (printf "z: foo is ~a\n" foo)
> >   (set! foo (add1 foo))
> >   (printf "z: foo is ~a\n" foo)
> >   #'1)
> >
> > (x1)
> >
> > #;
> > (let-syntax ([x1 (lambda (stx)
> >                    (printf "z: foo is ~a\n" foo)
> >                    (set! foo (add1 foo))
> >                    (printf "z: foo is ~a\n" foo)
> >                    #'1)])
> >   (x1))
> >
> > With `define-syntax' I see
> > z: foo is 1
> > z: foo is 2
> >
> > But with `let-syntax' I see
> > z: foo is 1
> > z: foo is 1
> >
> > Is this a bug or am I missing something?
> I guess it has to do with expansion order somehow. If I try to
> interleave calls to a macro defined by `define-syntax' and the
> `let-syntax' expression I see the `define-syntax' expansions happen first.
> 
> (define-for-syntax foo 1)
> 
> (define-syntax (x2 stx)
>   (printf "z: foo is ~a\n" foo)
>   (set! foo (add1 foo))
>   (printf "z: foo is ~a\n" foo)
>   #'1)
> (x2)
> (let-syntax ([x1 (lambda (stx)
>                    (printf "let z: foo is ~a\n" foo)
>                    (set! foo (add1 foo))
>                    (printf "let z: foo is ~a\n" foo)
>                    #'1)])
>   (x1))
> 
> (x2)
> 
> z: foo is 1
> z: foo is 2
> z: foo is 2
> z: foo is 3
> let z: foo is 3
> let z: foo is 3
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