Re: Problem merging generics across modules

[David Pirotte <david@altosw.be>]

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Hello Andrew,

> I am trying to extend the definition of a few primitives,
> including equal?, in a project of mine. Let's say that a.scm contains:

> (define-module (project a)
>   #:use-module (oop goops)  
>   #:export (equal?)

When you extend a guile primitive, don't #:export it in your module definition,
because that creates a new binding, hiding guile's core primitive. You should use
#:re-export instead, though in this very specific case of extending a guile core
primitive it is not even necessary, because when guile 'encounter' your
define-method, it does this:

	it creates a generic function for the method name, if it does not already
	exists, by calling define-generic [1]:

		define-generic checks that the name was (or not) previously bound to
		a scheme procedure, in which case it incorporates it into the new
		generic function as its default procedure

	it adds your methods to the generic function...

David

[1]	unless you really know what you are doing, you don't want to call
	define-generic yourself, because if it existed already, then that previous
	generic function would discarded and replaced by a new, empty generic
	function. goops calls define-generic for you, as part ofthe define-method
	protocol, iff there is no generic function for that name, so you are always
	on the safe side to rely on goops here.

;;;
;;; Extending equal?
;;;

(define-module (extending-equal)
  #:use-module (oop goops)

  #:export (<color>
            !r
            !g
            !b))

(define-class <color> ()
  (r #:accessor !r #:init-keyword #:r #:init-value 0)
  (g #:accessor !g #:init-keyword #:g #:init-value 0)
  (b #:accessor !b #:init-keyword #:b #:init-value 0))

(define-method (equal? (c1 <color>) (c2 <color>))
  (pk "in extended equal? ...")
  (and (= (!r c1) (!r c2))
       (= (!g c1) (!g c2))
       (= (!b c1) (!b c2))))

;;;
;;; Let's try it
;;;

GNU Guile 2.2.2.3-0c102

scheme@(guile-user)> (add-to-load-path (getcwd))
scheme@(guile-user)> ,use (oop goops)
scheme@(guile-user)> ,use (extending-equal)
;;; note: source file /usr/alto/projects/guile/goops/extending-equal.scm
;;; ...
scheme@(guile-user)> (make <color>)
$2 = #<<color> 5610ca6b2ba0>
scheme@(guile-user)> (make <color>)
$3 = #<<color> 5610ca7480f0>
scheme@(guile-user)> (equal? $2 $3)

;;; ("in extended equal? ...")
$4 = #t

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