Re: How to shadow a function temporarily? (flet and cl-flet)

[Emanuel Berg]

Alex Kost <alezost@gmail.com> writes:

> 1. The main question is: how can I override a
> function with another compatible function (with the
> same args) temporarily?

;;;; dynamic (Emacs wide)

;; first store the correct functions so we have them
;; (note: "subr" as in "subroutine", not "subtract")
(setq addition-fun    (symbol-function '+)) ; #<subr +>
(setq subtraction-fun (symbol-function '-)) ; #<subr ->

;; change + to do subtraction
(fset '+ subtraction-fun)
(+ 1 2 3) ; -4

;; revert
(fset '+ addition-fun)
(+ 1 2 3) ; 6

;;; lexical

(cl-labels ((+ (&rest args) (apply '- args)))
  (+ 1 2 3) ) ; -4
(+ 1 2 3)     ;  still 6

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