[plt-scheme] macro expension order

Chongkai Zhu skrev:
> Hi all,
> 
> I have the following program;
> 
>  (define-syntax NONE (syntax-id-rules () (NONE 'NONE)))
>  (define-syntax SOME (syntax-rules () ((SOME g57951) (list 'SOME g57951))))
>  (define expt
>    (match-lambda*
>      ((list NONE n) (expt (SOME 2) n))
>      ((list (list 'SOME b) 0) 1)
>      ((list (list 'SOME b) n) (if (= (mod n 2) 0) (expt (SOME (* b b)) 
> (/ n 2)) (* b (expt (SOME b) (- n 1)))))))
> 
> It is automatically translated from the following SML program:
> 
> datatype 'a option = NONE | SOME of 'a
> 
> fun expt (NONE, n) = expt (SOME 2, n)
>   | expt (SOME b, 0) = 1
>   | expt (SOME b, n) =
>     if n mod 2 = 0 then
>        expt (SOME (b*b), n div 2)
>     else
>        b * expt (SOME b, n-1)


Why not represent NONE and SOME as structures?

(define-struct NONE ())
(define-struct SOME (b))

(require (lib "plt-match.ss"))

(define expt
   (match-lambda*
     [(list (struct NONE ()))
      (expt (make-SOME 2) n)]
     [(list (struct SOME (b)) 0)
      1]
     [(list (struct SOME (b)) n)
      (if (= (modulo n 2) 0)
          (expt (make-SOME (* b b)) (quotient n 2))
          (* b (expt (make-SOME b) (- n 1))))]))

(expt (make-SOME 2) 0) ; => 1
(expt (make-SOME 2) 1) ; => 2
(expt (make-SOME 2) 2) ; => 4
(expt (make-SOME 2) 3) ; => 8


-- 
Jens Axel Søgaard