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Subject: |
Defining a method named zero? breaks primitive zero? |
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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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--- Begin Message ---
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Subject: |
Re: bug#26026: Defining a method named zero? breaks primitive zero? |
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Date: |
Wed, 19 Apr 2017 17:12:12 +0200 |
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User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) |
On Wed 08 Mar 2017 12:07, Alejandro Sanchez <address@hidden> 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
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