Re: another take on hackers and painters

[Guy Steele - Sun Microsystems Labs <address@hidden>]

   Date: Thu, 22 May 2003 18:27:30 -0500 (CDT)
   From: Jacob Matthews <jacobm@cs.uchicago.edu>
   To: Michael Vanier <mvanier@cs.caltech.edu>
   Cc: paul@prescod.net, "" <ll1-discuss@ai.mit.edu>
   Subject: Re: another take on hackers and painters
   
   On Thu, 22 May 2003, Michael Vanier wrote:
   
   > Of course, this isn't autoconversion, but something like this could be used
   > for autoconversion.
   
   Indeed. Here's a complete autoconverting + implemented in PLT Scheme:
   
   (define real+ +)
   (define (+ . args)
     (define (?->num x)
       (cond
         ((string? x) (or (string->number x)
                          (error '+ "expected a number string, got non-number
   string ~s" x)))
         ((number? x) x)
         (else (error '+ "expected number or string, got ~s" x))))
     (apply real+ (map ?->num args)))

Or, even better, and simpler:

(define real+ +)
(define (+ . args)
  (define (?->num x)
    (cond
      ((string? x) 1)
      ((number? x) x)
      (else 0)))
  (apply real+ (map ?->num args)))

And powerful: one can easily count the number of strings
in a list x of strings and other (non-number) items by saying

(apply + x)

--Guy





















(with tongue firmly in cheek)