[racket] Help with phases

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 <rafkind at 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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