Using '-1' in a method named '*'

[Alejandro Sanchez]

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).