Defining Functions

We usually give a name to a function when it is first created. This is called defining a function, and it is done with the defun macro.

— Macro: defun name args [doc] [declare] [interactive] body...

defun is the usual way to define new Lisp functions. It defines the symbol name as a function with argument list args and body forms given by body. Neither name nor args should be quoted.
doc, if present, should be a string specifying the function's documentation string (see Function Documentation). declare, if present, should be a declare form specifying function metadata (see Declare Form). interactive, if present, should be an interactive form specifying how the function is to be called interactively (see Interactive Call).
The return value of defun is undefined.
Here are some examples:
          (defun foo () 5)
          (foo)
               ⇒ 5
          
          (defun bar (a &optional b &rest c)
              (list a b c))
          (bar 1 2 3 4 5)
               ⇒ (1 2 (3 4 5))
          (bar 1)
               ⇒ (1 nil nil)
          (bar)
          error--> Wrong number of arguments.
          
          (defun capitalize-backwards ()
            "Upcase the last letter of the word at point."
            (interactive)
            (backward-word 1)
            (forward-word 1)
            (backward-char 1)
            (capitalize-word 1))
Be careful not to redefine existing functions unintentionally. defun redefines even primitive functions such as car without any hesitation or notification. Emacs does not prevent you from doing this, because redefining a function is sometimes done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition.

— Function: defalias name definition &optional doc

This function defines the symbol name as a function, with definition definition (which can be any valid Lisp function). Its return value is undefined.
If doc is non-nil, it becomes the function documentation of name. Otherwise, any documentation provided by definition is used.
Internally, defalias normally uses fset to set the definition. If name has a defalias-fset-function property, however, the associated value is used as a function to call in place of fset.
The proper place to use defalias is where a specific function name is being defined—especially where that name appears explicitly in the source file being loaded. This is because defalias records which file defined the function, just like defun (see Unloading).
By contrast, in programs that manipulate function definitions for other purposes, it is better to use fset, which does not keep such records. See Function Cells.

You cannot create a new primitive function with defun or defalias, but you can use them to change the function definition of any symbol, even one such as car or x-popup-menu whose normal definition is a primitive. However, this is risky: for instance, it is next to impossible to redefine car without breaking Lisp completely. Redefining an obscure function such as x-popup-menu is less dangerous, but it still may not work as you expect. If there are calls to the primitive from C code, they call the primitive's C definition directly, so changing the symbol's definition will have no effect on them.

See also defsubst, which defines a function like defun and tells the Lisp compiler to perform inline expansion on it. See Inline Functions.

Alternatively, you can define a function by providing the code which will inline it as a compiler macro. The following macros make this possible.

— Macro: define-inline name args [doc] [declare] body...

Define a function name by providing code that does its inlining, as a compiler macro. The function will accept the argument list args and will have the specified body.
If present, doc should be the function's documentation string (see Function Documentation); declare, if present, should be a declare form (see Declare Form) specifying the function's metadata.

Functions defined via define-inline have several advantages with respect to macros defined by defsubst or defmacro:

Like defmacro, a function inlined with define-inline inherits the scoping rules, either dynamic or lexical, from the call site. See Variable Scoping.

The following macros should be used in the body of a function defined by define-inline.

— Macro: inline-quote expression

Quote expression for define-inline. This is similar to the backquote (see Backquote), but quotes code and accepts only ,, not ,@.

— Macro: inline-letevals (bindings...) body...

This is similar to let (see Local Variables): it sets up local variables as specified by bindings, and then evaluates body with those bindings in effect. Each element of bindings should be either a symbol or a list of the form (var expr); the result is to evaluate expr and bind var to the result. The tail of bindings can be either nil or a symbol which should hold a list of arguments, in which case each argument is evaluated, and the symbol is bound to the resulting list.

13.4 Defining Functions