GNU bug report logs - #26026
Defining a method named zero? breaks primitive zero?

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Alejandro Sanchez <hiphish <at> openmailbox.org>
Subject: bug#26026: closed (Re: bug#26026: Defining a method named zero?
 breaks primitive zero?)
Date: Wed, 19 Apr 2017 15:13:02 +0000

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Your bug report

#26026: Defining a method named zero? breaks primitive zero?

which was filed against the guile package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 26026 <at> debbugs.gnu.org.

-- 
26026: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=26026
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Andy Wingo <wingo <at> igalia.com>
To: Alejandro Sanchez <hiphish <at> openmailbox.org>
Cc: 26026-done <at> debbugs.gnu.org
Subject: Re: bug#26026: Defining a method named zero? breaks primitive zero?
Date: Wed, 19 Apr 2017 17:12:12 +0200

On Wed 08 Mar 2017 12:07, Alejandro Sanchez <hiphish <at> openmailbox.org> writes:

> If I define a ‘zero?’ predicate method for a custom class the primitive ‘zero?’ is lost. Here is a simple vector module:
>
> 	;;; File vector2.scm
> 	(define-module (vector2)
> 	  #:use-module (oop goops)
> 	  #:export (<vector2> get-x get-y zero?))
> 	
> 	(define-class <vector2> ()
> 	  (x #:init-value 0 #:getter get-x #:init-keyword #:x)
> 	  (y #:init-value 0 #:getter get-y #:init-keyword #:y) )
>
> 	(define-generic zero?)
> 	(define-method (zero? (v <vector2>))
> 	  (and (zero? (get-x v))
> 	       (zero? (get-y v))))
>
> In the Guile REPL try executing the following code:
>
> 	scheme@(guile-user)> (use-modules (oop goops) (vector2))
> 	scheme@(guile-user)> (zero? (make <vector2>))
>
> This will display 
>
> 	WARNING: (guile-user): `zero?' imported from both (ice-9 r5rs) and (vector2)
> 	ERROR: In procedure scm-error:
> 	ERROR: No applicable method for #<<generic> zero? (1)> in call (zero? 0)
> 	
> 	Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
> 	scheme@(guile-user) [1]> ,bt
> 	In vector2.scm:
> 	     11:7  2 (_ #<<vector2> 105e87e00>)
> 	In oop/goops.scm:
> 	   1438:4  1 (cache-miss 0)
> 	In unknown file:
> 	           0 (scm-error goops-error #f "No applicable method for ~S in call ~S" (#<<gen…> …) …)
>
> Apparently the problem is that ‘zero?’ is defined in two modules and
> the vector2 definition overrides it. This isn’t the case with other
> primitives like ‘+’ or ‘*’, so this seems like a bug? I had built
> Guile from HEAD a few days ago, my package manager shows 6fff84d as
> the version number, so I guess that must be the hash of the commit
> HEAD was pointing to at that time.

Actually the (vector2) module makes a fresh definition for zero?.  You
can tell because zero? is in its export list.  So instead of extending
the primitive-generic that is zero?, you are making a new definition.
See:

  scheme@(guile-user)> (define-module (foo) #:export (zero?))
  $1 = #<directory (foo) 1203c80>
  scheme@(foo)> (zero? 0)
  <unnamed port>:4:0: <unnamed port>:4:0: Unbound variable: zero?

  Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.

If you want to extend a primitive-generic, then do that by not exporting
zero?.  In a way it's like mutating the primitive in place, giving it
additional powers.

Andy

[Message part 3 (message/rfc822, inline)]

From: Alejandro Sanchez <hiphish <at> openmailbox.org>
To: bug-guile <at> gnu.org
Subject: Defining a method named zero? breaks primitive zero?
Date: Wed, 8 Mar 2017 12:07:56 +0100

If I define a ‘zero?’ predicate method for a custom class the primitive ‘zero?’ is lost. Here is a simple vector module:

	;;; File vector2.scm
	(define-module (vector2)
	  #:use-module (oop goops)
	  #:export (<vector2> get-x get-y zero?))
	
	(define-class <vector2> ()
	  (x #:init-value 0 #:getter get-x #:init-keyword #:x)
	  (y #:init-value 0 #:getter get-y #:init-keyword #:y) )

	(define-generic zero?)
	(define-method (zero? (v <vector2>))
	  (and (zero? (get-x v))
	       (zero? (get-y v))))

In the Guile REPL try executing the following code:

	scheme@(guile-user)> (use-modules (oop goops) (vector2))
	scheme@(guile-user)> (zero? (make <vector2>))

This will display 

	WARNING: (guile-user): `zero?' imported from both (ice-9 r5rs) and (vector2)
	ERROR: In procedure scm-error:
	ERROR: No applicable method for #<<generic> zero? (1)> in call (zero? 0)
	
	Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
	scheme@(guile-user) [1]> ,bt
	In vector2.scm:
	     11:7  2 (_ #<<vector2> 105e87e00>)
	In oop/goops.scm:
	   1438:4  1 (cache-miss 0)
	In unknown file:
	           0 (scm-error goops-error #f "No applicable method for ~S in call ~S" (#<<gen…> …) …)

Apparently the problem is that ‘zero?’ is defined in two modules and the vector2 definition overrides it. This isn’t the case with other primitives like ‘+’ or ‘*’, so this seems like a bug? I had built Guile from HEAD a few days ago, my package manager shows 6fff84d as the version number, so I guess that must be the hash of the commit HEAD was pointing to at that time.

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