[plt-scheme] matching/dispatching a la Erlang

William Sprague wrote:
> I want to be able to do something like the following, where
> "case-on-steroids" is my desired operator:
> 
> (define (choose input)
>   (case-on-steroids (X Y)       ; define X and Y as free variables
>     ((X X Y) (+ X Y))           ; matches (input 1 1 2), bind X->1, Y->2
>     ((1 X (2 Y) X) (* X Y))     ; matches (input 1 100 '(2 300) 100)
>                                 ; X->100, Y->300
>     (_ 'whatever))))            ; matches anything no bindings

I believe you want Bigloo-style pattern matching.  I can't remember if there's
a PLT package that provides such matching, but in Bigloo it would look like:

(define (choose input)
  (match-case input
    (((kwote input) ?x ?x ?y)       (+ x y))
    (((kwote input) 1 ?x (2 ?y) ?x) (* x y))
    (?- 'whatever)))

(choose '(input 1 1 2)) ;=> 3
(choose '(input 1 100 (2 300) 100)) ;=> 3000

You could write it equivalently in PLT as:

(require (lib "match.ss"))
(define choose
  (match-lambda
    [('input x-1 x-2 y)
     (=> fail)
     (if (eq? x-1 x-2)
         (+ x-1 y)
         (fail))]
    [('input 1 x-1 (2 y) x-2)
     (=> fail)
     (if (eq? x-1 x-2)
         (* x-1 y)
         (fail))]
    [_ 'whatever]))

HTH

-d