Why use #' before function arguments in emacs-lisp?

Question

I\'m familiar with Emacs Lisp, but not Common (or any other) Lisp. Some Lisp programmers suggest (e.g. A basic function for emacs) that it\'s good to use #\' in fr

Answer 1

The quote character in #'foo has nothing to do with the one in 'foo.

#'foo is replaced at read time by (function foo). When that is compiled and executed, it looks up the functional definition named by foo (by defun, flet, labels or similar).

'foo is replaced at read time by (quote foo). When that is compiled and executed, it is simply replaced by the symbol foo.

Funcall and apply (and thus generally higher order functions) take function designators as arguments. A function designator can be a function or a symbol naming a function, so both #'foo and 'foo are function designators.

Therefore, the forms 'foo and #'foo seem interchangable at first glance. However, the lookup of the real function is done at different times—#'foo is looked up where it is invoked, while the function named by 'foo would only be looked up when it is finally applied.

If you use a function designator many times over, it is much more efficient to do the lookup just once, or even just at compile time. That can be a really big timesaver, and translates to a smoother user experience in an editor.