From: Andy Wingo <wingo@igalia.com>
To: Alejandro Sanchez <hiphish@openmailbox.org>
Cc: 26106@debbugs.gnu.org
Subject: bug#26106: Defining a method named '-' with one parameter
Date: Wed, 19 Apr 2017 17:00:25 +0200 [thread overview]
Message-ID: <87vaq0l9x2.fsf@igalia.com> (raw)
In-Reply-To: <F7542921-8F39-4B4E-B8ED-B6E8F53DAD10@openmailbox.org> (Alejandro Sanchez's message of "Wed, 15 Mar 2017 14:35:18 +0100")
On Wed 15 Mar 2017 14:35, Alejandro Sanchez <hiphish@openmailbox.org> writes:
> If I define a method named ‘-‘ which only takes in one parameter, the expression ‘(- v)’ gets rewritten to ‘(- 0 v)’. Here is a minimal example:
>
> (use-modules (oop goops))
>
> (define-class <vector2> ()
> (x #:init-value 0 #:getter get-x #:init-keyword #:x)
> (y #:init-value 0 #:getter get-y #:init-keyword #:y))
>
> (define-method (* (n <number>) (v <vector2>))
> (make <vector2> #:x (* n (get-x v)) #:y (* n (get-y v))))
>
> (define-method (- (v <vector2>))
> (* -1 v))
>
> (define v (make <vector2> #:x 1 #:y 2))
> (* -1 v) ; Works fine
> (- v) ; Throws error
>
> Here is the error message:
>
> scheme@(guile-user)> (- v)
> ERROR: In procedure scm-error:
> ERROR: No applicable method for #<<generic> - (2)> in call (- 0 #<<vector2> 10a8e4020>)
>
> Entering a new prompt. Type `,bt' for a backtrace or `,q' to continue.
> scheme@(guile-user) [1]> ,bt
> In current input:
> 23:0 2 (_)
> In oop/goops.scm:
> 1438:4 1 (cache-miss 0 #<<vector2> 10a8e4020>)
> In unknown file:
> 0 (scm-error goops-error #f "No applicable method for ~S in call ~S" (#<<generic> - (2)> (- 0 #<<vec…>)) #)
Is (- x) equivalent to (* x -1) ?
Right now there are a few things happening. The "primitive expansion"
phase in an early part of the compiler turns (- x) to (- 0 x), where
obviously it should not be doing that. But can it turn it into (* x -1)
? Note that somewhat confusingly, a later part of the compiler that can
detect when X is a real number will undo that transformation, turning it
to (- 0 x) when X is real. So that sounds OK from an optimization point
of view but is the (* x -1) tranformation correct from the math point of
view?
Andy
prev parent reply other threads:[~2017-04-19 15:00 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-15 13:35 bug#26106: Defining a method named '-' with one parameter Alejandro Sanchez
2017-04-19 15:00 ` Andy Wingo [this message]
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