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Subject: bug#26106: Defining a method named '-' with one parameter
Resent-From: Alejandro Sanchez <hiphish@HIDDEN>
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Resent-Date: Wed, 15 Mar 2017 13:36:01 +0000
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From: Alejandro Sanchez <hiphish@HIDDEN>
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Date: Wed, 15 Mar 2017 14:35:18 +0100
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If I define a method named =E2=80=98-=E2=80=98 which only takes in one =
parameter, the expression =E2=80=98(- v)=E2=80=99 gets rewritten to =
=E2=80=98(- 0 v)=E2=80=99. Here is a minimal example:

	(use-modules (oop goops))
=09
	(define-class <vector2> ()
	  (x #:init-value 0 #:getter get-x #:init-keyword #:x)
	  (y #:init-value 0 #:getter get-y #:init-keyword #:y))
=09
	(define-method (* (n <number>) (v <vector2>))
	  (make <vector2> #:x (* n (get-x v)) #:y (* n (get-y v))))
=09
	(define-method (- (v <vector2>))
	  (* -1 v))
=09
	(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>)
=09
	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=E2=80=A6>)) #)

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Alejandro Sanchez <hiphish@HIDDEN>
Subject: bug#26106: Acknowledgement (Defining a method named '-' with one
 parameter)
Message-ID: <handler.26106.B.148958494114610.ack <at> debbugs.gnu.org>
References: <F7542921-8F39-4B4E-B8ED-B6E8F53DAD10@HIDDEN>
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Date: Wed, 15 Mar 2017 13:36:02 +0000

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Subject: bug#26106: Defining a method named '-' with one parameter
Resent-From: Andy Wingo <wingo@HIDDEN>
Original-Sender: "Debbugs-submit" <debbugs-submit-bounces <at> debbugs.gnu.org>
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To: Alejandro Sanchez <hiphish@HIDDEN>
Cc: 26106 <at> debbugs.gnu.org
From: Andy Wingo <wingo@HIDDEN>
References: <F7542921-8F39-4B4E-B8ED-B6E8F53DAD10@HIDDEN>
Date: Wed, 19 Apr 2017 17:00:25 +0200
In-Reply-To: <F7542921-8F39-4B4E-B8ED-B6E8F53DAD10@HIDDEN> (Alejandro
 Sanchez's message of "Wed, 15 Mar 2017 14:35:18 +0100")
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On Wed 15 Mar 2017 14:35, Alejandro Sanchez <hiphish@HIDDEN> write=
s:

> If I define a method named =E2=80=98-=E2=80=98 which only takes in one pa=
rameter, the expression =E2=80=98(- v)=E2=80=99 gets rewritten to =E2=80=98=
(- 0 v)=E2=80=99. Here is a minimal example:
>
> 	(use-modules (oop goops))
>=20=09
> 	(define-class <vector2> ()
> 	  (x #:init-value 0 #:getter get-x #:init-keyword #:x)
> 	  (y #:init-value 0 #:getter get-y #:init-keyword #:y))
>=20=09
> 	(define-method (* (n <number>) (v <vector2>))
> 	  (make <vector2> #:x (* n (get-x v)) #:y (* n (get-y v))))
>=20=09
> 	(define-method (- (v <vector2>))
> 	  (* -1 v))
>=20=09
> 	(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 #<<vecto=
r2> 10a8e4020>)
>=20=09
> 	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 c=
all ~S" (#<<generic> - (2)> (- 0 #<<vec=E2=80=A6>)) #)

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

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