Subject: bug#26106: Defining a method named '-' with one parameter

From: Alejandro Sanchez <[email-masked]>
Date: Wed, 15 Mar 2017 14:35:18 +0100

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…>)) #)