The following code demonstrates that when byte-compiling (but not when interpreting), the call (- x) is transformed into (- 0 x). This results in failure of custom - methods defined via GOOPS. I think this says it all: $ guile3.0 GNU Guile 3.0.7 [...] scheme@(guile-user)> ,optimize (- x) $1 = (- 0 x) The longer version: $ cat minus.scm (use-modules (oop goops)) (define-class <Foo> () (bar #:init-keyword #:bar)) (define-method (- (x <Foo>)) (make <Foo> #:bar (- (slot-ref x 'bar)))) (display (slot-ref (- (make <Foo> #:bar 5)) 'bar)) (newline) $ guile3.0 --no-auto-compile minus.scm -5 $ guile3.0 minus.scm ;;; note: auto-compilation is enabled, set GUILE_AUTO_COMPILE=0 ;;; or pass the --no-auto-compile argument to disable. ;;; compiling /home/jean/tmp/minus.scm ;;; compiled /home/jean/.cache/guile/ccache/3.0-LE-8- 4.5/home/jean/tmp/minus.scm.go Backtrace: In ice-9/boot-9.scm: 1752:10 8 (with-exception-handler _ _ #:unwind? _ # _) In unknown file: 7 (apply-smob/0 #<thunk 7f569a4a0f60>) In ice-9/boot-9.scm: 724:2 6 (call-with-prompt _ _ #<procedure default-prompt-handle…>) In ice-9/eval.scm: 619:8 5 (_ #(#(#<directory (guile-user) 7f569a499c80>))) In ice-9/boot-9.scm: 2835:4 4 (save-module-excursion _) 4380:12 3 (_) In /home/jean/tmp/minus.scm: 9:19 2 (_) In oop/goops.scm: 1567:11 1 (cache-miss 0 #<<Foo> 7f56944e70b0>) 1585:2 0 (_ _ _) oop/goops.scm:1585:2: No applicable method for #<<generic> - (1)> in call (- 0 #<<Foo> 7f56944e70b0>)