Question

subj.
Something like:

(lexical-let (oldf #'original-func)
   (flet ((original-func (arg)
             do-something
             (funcall oldf arg))) 
      do-something))

don’t work 🙁

Answer 1

Hopefully this will help you with the syntax, calling swap-function calls foo1 but executes foo2.

You could write this as a useful macro with-replace-function which binds the old function with the new function while executing a body you pass in.

(defun foo1()
  (insert "hi foo1"))

(defun foo2()
  (insert "hi foo2"))

(defun swap-function(old new)
  (let ((save-func (symbol-function old)))
    (fset old (symbol-function new))
    (funcall old)
    (fset old save-func)))

(swap-function #'foo1 #'foo2)