defun

(defun name (params...) body...)

Defines a function named name in the function namespace, with the given parameter list and body, and returns the name symbol. The body is not evaluated at definition time; it runs on each call, returning the value of the last body form. Per Lisp-2 the definition lives in the function namespace, so the name is reachable in call position (and via #'name) without colliding with any like-named variable.

Lambda list keywords

The parameter list supports the Common Lisp lambda-list keywords &optional, &rest, &key, &allow-other-keys, and &aux (in that order). A default form is evaluated only when the argument is absent, and it can reference parameters bound to its left. An optional or keyword parameter may declare a supplied-p variable that is t when the caller passed the argument.

An unknown keyword argument signals an error unless the lambda list declares &allow-other-keys or the caller passes :allow-other-keys t. &aux introduces auxiliary variables bound like a trailing let*. &whole is not supported.

Calling a function with too few required arguments (or too many, for a fixed-arity function) signals an error in the interpreter and is a compile error on the JVM/WASM backends.

> (defun f (a b) (+ a b))
> (f 1)
Function expects 2 arguments, got 1

setf-function names

The name may be a (setf name) list instead of a plain symbol. This defines a setf-function: the writer invoked when name is used as a setf place. The new value is passed as the first argument (it is the last required parameter of the setf lambda list, per the Common Lisp convention), so (setf (name arg...) value) calls the writer with value followed by arg.... The function is also first-class through #'(setf name).

Only the (setf name) form is supported (a two-element list); symbol-function/fboundp of a (setf ...) name is not.