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* Using '-1' in a method named '*'
@ 2017-02-27 10:06 Alejandro Sanchez
  2017-02-27 19:21 ` Andy Wingo
  0 siblings, 1 reply; 13+ messages in thread
From: Alejandro Sanchez @ 2017-02-27 10:06 UTC (permalink / raw)
  To: guile-user

Hello,

I have been trying to write a Guile library for 3D motion and I wrote a ‘vector3’ class and added the method ‘*’ for scaling a vector. This works fine for any scalar, except for ‘-1’. Here is a minimal code example, I am using Guile 2.0.14 on OS X 10.11.6:

(use-modules (oop goops))

(define-class <vector3> ()
  ;; Only one slot, for simlicity
  (x #:init-value 0 #:getter get-x #:init-keyword #:x))

(define-method (*  (n <real>) (v <vector3>))
  ;; Return anything, doesn't matter
  3)


(define v (make <vector3> #:x 1))

(* -1 v)  ; Does not work
(* -2 v)  ; Works fine


The error message is:

<unnamed port>:14:0: In procedure #<procedure 104d7b8e0 at <current input>:14:0 ()>:
<unnamed port>:14:0: In procedure -: Wrong type argument in position 1: #<<vector3> 104ccdce0>

The backtrace is:

     16:0  0 (#<procedure 104db6240 at <current input>:16:0 ()>)

This issue only happens when the argument is -1, it doesn’t happen for other negative integers and it doesn’t happen for ‘-1.0’ either. And it’s not just the literal ‘-1’, if instead I use an expression that evaluates to ‘-1’ like ‘(- 2 3)’ the same happens. However, changing the order of arguments in the definition of the method does work.

The reason for this seems that the expression ‘(* -1 x)’ for any x is re-written by Guile to ‘(- x)’, because by adding the following method it gets working:

(define-method (- (v <vector3>))
  2)

However, since '(* -1 v)’ gets re-written to ‘(- v)’ the result is ‘2' instead of ‘3’. Granted, this is a contrived example and in any scenario I can think of '(* -1 v)’ and ‘(- v)’ would yield the same result, but is this intended behaviour? It was something that got me hung up over the last few days quite a lot (I was already in the process of writing this email when it occurred to me to define the ‘-‘ method).


^ permalink raw reply	[flat|nested] 13+ messages in thread
* Re: Using '-1' in a method named '*'
@ 2017-03-03  0:07 Sandic, Aleksandar
  0 siblings, 0 replies; 13+ messages in thread
From: Sandic, Aleksandar @ 2017-03-03  0:07 UTC (permalink / raw)
  To: guile-user@gnu.org

Thank you, everyone. It is working now, I built from HEAD and negating a vector works as expected. This will bridge me over until 2.2 is released.


^ permalink raw reply	[flat|nested] 13+ messages in thread
end of thread, other threads:[~2017-03-03  0:07 UTC | newest]

Thread overview: 13+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2017-02-27 10:06 Using '-1' in a method named '*' Alejandro Sanchez
2017-02-27 19:21 ` Andy Wingo
2017-02-27 22:26   ` Alejandro Sanchez
2017-02-28  8:00   ` tomas
2017-02-28  8:54   ` Andy Wingo
2017-02-28  9:19     ` Andy Wingo
2017-02-28 14:24       ` Alejandro Sanchez
2017-02-28 17:53         ` Amirouche
2017-02-28 18:32           ` Ralf Mattes
2017-03-01 17:25         ` Andy Wingo
2017-03-01 23:46           ` Alejandro Sanchez
2017-03-02  0:18             ` Thomas Morley
  -- strict thread matches above, loose matches on Subject: below --
2017-03-03  0:07 Sandic, Aleksandar

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