Editing Common Lisp (section)

====Defining functions====
The [[Defun|macro <code>defun</code>]] defines functions where a function definition gives the name of the function, the names of any arguments, and a function body:

<syntaxhighlight lang="lisp">
 (defun square (x)
   (* x x))
</syntaxhighlight>

Function definitions may include compiler [[Directive (programming)|directives]], known as ''declarations'', which provide hints to the compiler about optimization settings or the data types of arguments. They may also include ''documentation strings'' (docstrings), which the Lisp system may use to provide interactive documentation:

<syntaxhighlight lang="lisp">
 (defun square (x)
   "Calculates the square of the single-float x."
   (declare (single-float x) (optimize (speed 3) (debug 0) (safety 1)))
   (the single-float (* x x)))
</syntaxhighlight>

Anonymous functions ([[function literal]]s) are defined using <code>lambda</code> expressions, e.g. <code>(lambda&nbsp;(x)&nbsp;(*&nbsp;x&nbsp;x))</code> for a function that squares its argument. Lisp programming style frequently uses higher-order functions for which it is useful to provide anonymous functions as arguments.

Local functions can be defined with <code>flet</code> and <code>labels</code>.

<syntaxhighlight lang="lisp">
 (flet ((square (x)
          (* x x)))
   (square 3))
</syntaxhighlight>

There are several other operators related to the definition and manipulation of functions. For instance, a function may be compiled with the <code>compile</code> operator. (Some Lisp systems run functions using an interpreter by default unless instructed to compile; others compile every function).