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

[Andy Wingo <wingo@igalia.com>]

On Wed 08 Mar 2017 12:07, Alejandro Sanchez <hiphish@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