Editing Lisp (programming language) (section)

===Lambda expressions and function definition===
Another special operator, {{Lisp2|lambda}}, is used to bind variables to values which are then evaluated within an expression. This operator is also used to create functions: the arguments to {{Lisp2|lambda}} are a list of arguments, and the expression or expressions to which the function evaluates (the returned value is the value of the last expression that is evaluated). The expression
<syntaxhighlight lang="Lisp">
 (lambda (arg) (+ arg 1))
</syntaxhighlight>
evaluates to a function that, when applied, takes one argument, binds it to {{Lisp2|arg}} and returns the number one greater than that argument. Lambda expressions are treated no differently from named functions; they are invoked the same way. Therefore, the expression
<syntaxhighlight lang="Lisp">
 ((lambda (arg) (+ arg 1)) 5)
</syntaxhighlight>
evaluates to {{Lisp2|6}}. Here, we're doing a function application: we execute the [[anonymous function]] by passing to it the value 5.

Named functions are created by storing a lambda expression in a symbol using the defun macro.
<syntaxhighlight lang="Lisp">
 (defun foo (a b c d) (+ a b c d))
</syntaxhighlight>

{{Lisp2|(defun f (a) b...)}} defines a new function named {{Lisp2|f}} in the global environment. It is conceptually similar to the expression:

<syntaxhighlight lang="Lisp">
 (setf (fdefinition 'f) #'(lambda (a) (block f b...)))
</syntaxhighlight>

where {{Lisp2|setf}} is a macro used to set the value of the first argument {{Lisp2|fdefinition 'f}} to a new function object. {{Lisp2|fdefinition}} is a global function definition for the function named {{Lisp2|f}}. {{Lisp2|#'}} is an abbreviation for {{Lisp2|function}} special operator, returning a function object.