' versus #' what are the differences and when should I choose one over the other?

Question

Both seem to work equally well when mapping a function over a list, for example.

(mapcar #'1+ (list 1 2 3))
;; => (2 3 4)
(mapcar '1+ (list 1 2 3))
;; => (2 3 4)

Answer 1

#' (= function) is more verbose than ' (= quote), and it provides three practical benefits.

1. Byte compiler will check if the function is defined

Byte compile a file which contains the following code

(mapc #'2+ nil)
(mapc  '3+ nil)

The byte compiler will warns us that 2+ is not defined:

foo.el:3:1:Warning: the function `2+' is not known to be defined.

Although 3+ is also not defined, we won't get a warning because we're using '.

2. The code will be more precise to us

For example, when I see this two function calls, I will get more precise information

(foo #'bar) ;; => foo accepts a function and bar is a function
(foo  'bar) ;; => foo accepts a symbol

3. The code completion will be more precise

#' completes only symbols with a function definition while ' completes any symbols (functions/variables/faces etc).

Answer 2

quote (') returns its argument sexp - any Lisp object (thing).

function (#') returns its argument sexp as a function object. This also means that in code you byte-compile it tells the byte compiler that that thing is a function.

'foobar just evaluates to the symbol foobar. #'foobar evaluates to a function.

'(lambda) () (something)) evaluates to the list whose car is the symbol lambda, whose cadr is nil, and whose caddr is the list (something). There is no way for the byte-compiler or any code that uses this return value to know that that list is to be interpreted/used as a function (except code that in fact uses it that way). #'(lambda () (something)) evaluates to a function that accepts no arguments and invokes function something (passing it no args). ((lambda () (something) evaluates to the same function - unquoted lambda forms are self-evaluating.)

In Lisp, data and code (programs) have the same syntax. (lambda () (something) is both a list (when not evaluated) and a function (when evaluated). When you quote such a list, '(lambda () (something)), you get just that list, not a function.

If you then use that list as a function, it works, because when evaluated it is interpreted as a lambda form, which returns an anonymous function (which can have the same form as the lambda form itself).

When Emacs - in particular the byte compiler - can know that some bit of code is a function it can act accordingly, error-check, optimize, etc. If all Emacs knows is that there is a list whose car is the symbol lambda etc., there's no way it can or should make any assumption about it representing a function. The code in which it occurs could use it in some other way.

See the Elisp manual, nodes Anonymous Functions and Quoting.