On Wed 15 Mar 2017 14:35, Alejandro Sanchez <hiph...@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
