What Is a Function

13.1 What Is a Function?

In a general sense, a function is a rule for carrying out a computation given input values called arguments. The result of the computation is called the value or return value of the function. The computation can also have side effects, such as lasting changes in the values of variables or the contents of data structures (see Definition of side effect). A pure function is a function which, in addition to having no side effects, always returns the same value for the same combination of arguments, regardless of external factors such as machine type or system state.

In most computer languages, every function has a name. But in Lisp, a function in the strictest sense has no name: it is an object which can optionally be associated with a symbol (e.g., car) that serves as the function name. See Function Names. When a function has been given a name, we usually also refer to that symbol as a “function” (e.g., we refer to “the function car”). In this manual, the distinction between a function name and the function object itself is usually unimportant, but we will take note wherever it is relevant.

Certain function-like objects, called special forms and macros, also accept arguments to carry out computations. However, as explained below, these are not considered functions in Emacs Lisp.

Here are important terms for functions and function-like objects:

lambda expression: A function (in the strict sense, i.e., a function object) which is written in Lisp. These are described in the following section. See Lambda Expressions.
primitive: A function which is callable from Lisp but is actually written in C. Primitives are also called built-in functions, or subrs. Examples include functions like car and append. In addition, all special forms (see below) are also considered primitives.
Usually, a function is implemented as a primitive because it is a fundamental part of Lisp (e.g., car), or because it provides a low-level interface to operating system services, or because it needs to run fast. Unlike functions defined in Lisp, primitives can be modified or added only by changing the C sources and recompiling Emacs. See Writing Emacs Primitives.
special form: A primitive that is like a function but does not evaluate all of its arguments in the usual way. It may evaluate only some of the arguments, or may evaluate them in an unusual order, or several times. Examples include if, and, and while. See Special Forms.
macro: A construct defined in Lisp, which differs from a function in that it translates a Lisp expression into another expression which is to be evaluated instead of the original expression. Macros enable Lisp programmers to do the sorts of things that special forms can do. See Macros.
command: An object which can be invoked via the command-execute primitive, usually due to the user typing in a key sequence bound to that command. See Interactive Call. A command is usually a function; if the function is written in Lisp, it is made into a command by an interactive form in the function definition (see Defining Commands). Commands that are functions can also be called from Lisp expressions, just like other functions.
Keyboard macros (strings and vectors) are commands also, even though they are not functions. See Keyboard Macros. We say that a symbol is a command if its function cell contains a command (see Symbol Components); such a named command can be invoked with M-x.
closure: A function object that is much like a lambda expression, except that it also encloses an environment of lexical variable bindings. See Closures.
byte-code function: A function that has been compiled by the byte compiler. See Byte-Code Type.
autoload object: A place-holder for a real function. If the autoload object is called, Emacs loads the file containing the definition of the real function, and then calls the real function. See Autoload.

You can use the function functionp to test if an object is a function:

— Function: functionp object

This function returns t if object is any kind of function, i.e., can be passed to funcall. Note that functionp returns t for symbols that are function names, and returns nil for special forms.

It is also possible to find out how many arguments an arbitrary function expects:

— Function: func-arity function

This function provides information about the argument list of the specified function. The returned value is a cons cell of the form (min . max), where min is the minimum number of arguments, and max is either the maximum number of arguments, or the symbol many for functions with &rest arguments, or the symbol unevalled if function is a special form.
Note that this function might return inaccurate results in some situations, such as the following:

Functions defined using apply-partially (see apply-partially).
Functions that are advised using advice-add (see Advising Named Functions).
Functions that determine the argument list dynamically, as part of their code.

Unlike functionp, the next three functions do not treat a symbol as its function definition.

— Function: subrp object

This function returns t if object is a built-in function (i.e., a Lisp primitive).

          (subrp 'message)            ; message is a symbol,
               ⇒ nil                 ;   not a subr object.
          (subrp (symbol-function 'message))
               ⇒ t

— Function: byte-code-function-p object

This function returns t if object is a byte-code function. For example:
          (byte-code-function-p (symbol-function 'next-line))
               ⇒ t

— Function: subr-arity subr

This works like func-arity, but only for built-in functions and without symbol indirection. It signals an error for non-built-in functions. We recommend to use func-arity instead.