Jeremy Smith <[EMAIL PROTECTED]> writes:
>> What about overloading operators like #'+?
>> Maybe also some reference to generic functions?
>
> Personally, I'd use optional parameters for this. Here's a piece of
> (probably awful) Lisp code to demonstrate:
>
> (defun my+(num1 num2 &optional (num3 0))
> (+ num1 num2 num3))
Ikes, no! This is how it is done:
;;; Step 1: shadow
(defpackage "MY+" (:use :cl) (:shadow "+"))
(in-package :my+)
;;; Step 2: define a variant with fixed number of arguments
(defgeneric +/2 (a b))
(defvar *swap* nil)
;;; Step 3: default method delegates to CL:+, and deals with alternative
;;; order or arguments, so that specializing on the first argument will
;;; be enough
(defmethod +/2 (a b)
(if *swap*
(cl:+ a b)
(let ((*swap* t))
(+/2 b a))))
;;; Step 4: oddball perlish method to show-off
(defmethod +/2 ((string string) x)
(+/2 (read-from-string string) x))
;;; Step 5: delegate from + to +/2
(defun + (&rest arguments)
(reduce #'+/2 arguments :initial-value 0))
;;; Done
(+ "1" 1 "1") ; => 4
Cheers,
-- Nikodemus Schemer: "Buddha is small, clean, and serious."
Lispnik: "Buddha is big, has hairy armpits, and laughs."
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