[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

Here is my take on your example:

(define (choose input)
  (match input
    [(1 x (2 y))          (* x y)]
    [(x1 x2 y)  (=> fail) (if (not (= x1 x2))
                              (fail)
                              (+ x1 y))]
    [_                   'whatever]))

(choose '(3 3 5))
(choose '(1 3 5))
(choose '(1 3 (2 7)))

If there is an easier way to express that the two values
called x1 and x2 above must be equal - I'd like to see it.

-- 
Jens Axel Søgaard