Pass function symbol as argument via function or single quote?

Question

Are 'function-name and (function function-name) completely equivalent, or are there conditions under which I must use the more verbose (function function-name)?

(apply 'message ( list "call func via quote"))
(apply (function message) (list "call func via function keyword"))

Both of them yield the same effect. But in many elisp libraries, i am seeing people using (function func) to pass function symbols like in this example.

(defun dired-add-file (filename &optional marker-char)
  (dired-fun-in-all-buffers
   (file-name-directory filename) (file-name-nondirectory filename)
   (function dired-add-entry) filename marker-char))

Why do people use more verbose way, when they can use just a quote?

Answer 1

'foo and (quote foo) are identical: they prevent evaluation of foo.

#'foo and (function foo) are also identical: they extract the function binding of foo, including at compile time, which means that the function gets compiled.

So, if you want a symbol, use 'foo:

(make-variable-buffer-local 'my-local-var)

if you want a function, use #'foo:

(setq my-after-function #'my-function)

or

(apply #'my-function arg1 arg2 others)

See also

1. When to use 'quote in Lisp
2. Why sharp quote lambda expressions?