Thanks , I think I understand it now , so roughly following steps
happens(apology for incorrect terminology) :
1. bind the identifier 'phases' to some val(a syntax transformer) in phase 1
i) during the above step id 'phase2' is bound to some val i.e syntax
transformer in phase 2
ii) (phase2) is expanded i.e it is looked up in phase 2 and it's syntax
transformer applied , which returns the syntax object which is basically
#'0
since cnt was 0 at the time of application of transformer.
iii) step 1 done.
2. encounters macro call (set-compile) and expansion takes place and set
cnt=1(phase 1) , cnt=2(phase 2).
3. encounters macro call (phases) looks up in phase 1 and its transformer
is applied and we get (list 1 0) , since
(p1 cnt=1) and (p2 0).
On Tue, Jan 17, 2012 at 12:51 PM, Jon Rafkind <rafk...@cs.utah.edu> wrote:
> I'm guessing a little bit but I think its because the `phases' macro is
> expanded at phase 2 before `set-compile' is called. If you put
> (set-compile) before the definition of `phases' then you get '(1 2).
>
>
> On 01/17/2012 12:06 AM, Veer Singh wrote:
>
>
> Why macro call (phases) produces '(1 0) and not '(1 2) ?
>
>
> (module mod1 racket
> (require (for-syntax syntax/parse))
> ;"mod-compile.rkt"))
> (require (for-syntax
> (for-syntax syntax/parse racket )))
> ;"mod-compile.rkt")))
> (provide set-compile phases)
>
> ;For phase 1
> (begin-for-syntax
> (define cnt 0)
>
> (define (store! v)
> (set! cnt v)))
>
> ;For phase 2
> (begin-for-syntax (begin-for-syntax
> (define cnt 0)
>
> (define (store! v)
> (set! cnt v))))
>
> ;;set the value of "cnt" in phase1 and phase2
> (define-syntax (set-compile stx)
> (syntax-parse stx
> [(_) #'(begin
>
> (begin-for-syntax
> ; in "cnt" in phase 1 cnt=1
> (store! 1)
> (begin-for-syntax
> ; in "cnt" in phase 2 cnt=2
> (store! 2)))
> 'done)]))
>
>
> ;;return the value of cnt in phase1 and phase2
> (define-syntax (phases stx)
>
> ;phase2 ???
> (define-syntax (phase2 stx)
> (syntax-parse stx
> [(_) (with-syntax ([p cnt])
> #'p)]))
>
>
> ;phase1
> (syntax-parse stx
> [(_) (with-syntax ([p1 cnt]
> [p2 (phase2)])
> #'(list p1 p2))]))
>
> (set-compile)
> (phases)
> ;produces (list 1 0), but why not (list 1 2)
>
> )
>
>
> Thanks
>
>
>
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