Should `flet` be replaced with `cl-flet` or `cl-letf` ?

Question

Some elisp functions that I have installed generate warnings:

`flet\' is an obsolete macro (as of 24.3); use either `cl-flet\' or `cl-letf\'.

Answer 1

flet isn't the same as either cl-flet or cl-letf. It's more dangerous (and maybe more powerful). That's why it's being deprecated.

Since it's different (binds dynamically a function name), you have to think in each case if it's appropriate to replace it with cl-flet.

Small example when flet can't be replaced with cl-flet

(defun adder (a b)
  (+ a b))

(defun add-bunch (&rest lst)
  (reduce #'adder lst))

(add-bunch 1 2 3 4)
;; 10

(flet ((adder (a b) (* a b)))
  (add-bunch 1 2 3 4))
;; 24

(cl-flet ((adder (a b) (* a b)))
  (add-bunch 1 2 3 4))
;; 10

Note that cl-flet does lexical binding, so the behavior of adder isn't changed, while flet does dynamic binding, which makes add-bunch temporarily produce a factorial.