[plt-scheme] Printing and Overloading

Paulo Jorge de Oliveira Cantante de Matos wrote:
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>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> 
> Hi all,
> 
> I've been thinking about this lately bout I've not had the time to try
> things out. However, as the result of some hours of research I found
> nothing in PLT Scheme that allows me to easily do one of the following
> things:
> 
> Imagine I have a structure/class tuple (x1, x2) and I want to:
> 
> 1. Overload primitive +, so that (+ (make-tuple 2 3) (make-tuple 2 3))
> gives me (4, 6).
> 

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

(define-struct tuple (x y) (make-inspector))

(define +
   (let ((old+ +))
     (match-lambda*
       ((list (and t (? tuple?)) ...)
        (make-tuple
         (apply + (map tuple-x t))
         (apply + (map tuple-y t))))
       (else (apply old+ else)))))

> 2. Print the tuple so that (write (make-tuple 2 3)) or even (format "~a"
> (make-tuple 2 3)) prints something like "(2, 3)".
> 

Similar to above.  Just capture the old function before you rename it.

Bruce