How to declare a function before its definition in common lisp?

Question

I will get an undefined function warning about f2 in SBCL with following code sample. Is it possible that I can declare f2 first, like in C, to avoid the warning. I Googled, without any clue.

(defun f ()
  (print (f2)))

(defun f2 ()
  (print "f2"))

Answer 1

If you use the function before you define it in a single compilation unit, e.g., the same file, then the compiler will not complain about the undefined function (plain load still may, so compile your code first!)

Otherwise, you can declaim ftype:

(declaim (ftype (function () t) f2)

meaning that f2 accepts no arguments and returns a single value of type t.

However, it makes much more sense to compile the file where you use the function while the definition is already loaded. You can (and should!) use asdf as a Lisp-specific make(1): specifying dependencies so that the compiler has the definitions of all the functions while it compiles their users.

Answer 2

If the functions are in the same file, a compiler won't give a warning.

Example SBCL:

bash-3.2$ sbcl
This is SBCL 1.3.10, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
* (compile-file "/tmp/order.lisp")

; compiling file "/private/tmp/order.lisp" (written 28 NOV 2016 12:14:37 PM):
; compiling (DEFUN F ...)
; compiling (DEFUN F2 ...)

; /tmp/order.fasl written
; compilation finished in 0:00:00.178
#P"/private/tmp/order.fasl"
NIL
NIL
* (load *)

T
*