(lispref.info)What Is a Function
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What Is a Function?
In a general sense, a function is a rule for carrying on a
computation given several values called "arguments". The result of the
computation is called the value of the function. The computation can
also have side effects: lasting changes in the values of variables or
the contents of data structures.
Here are important terms for functions in Emacs Lisp and for other
In Emacs Lisp, a "function" is anything that can be applied to
arguments in a Lisp program. In some cases, we use it more
specifically to mean a function written in Lisp. Special forms and
macros are not functions.
A "primitive" is a function callable from Lisp that is written in
C, such as `car' or `append'. These functions are also called
"built-in" functions or "subrs". (Special forms are also
Usually the reason that a function is a primitives is because it is
fundamental, or provides a low-level interface to operating system
services, or because it needs to run fast. Primitives can be
modified or added only by changing the C sources and recompiling
the editor. See Note: Writing Emacs Primitives.
A "lambda expression" is a function written in Lisp. These are
described in the following section.
Note: Lambda Expressions.
A "special form" is 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. Many special forms are described
in Note: Control Structures.
A "macro" is a construct defined in Lisp by the programmer. It
differs from a function in that it translates a Lisp expression
that you write into an equivalent expression to be evaluated
instead of the original expression. Note: Macros, for how to
define and use macros.
A "command" is an object that `command-execute' can invoke; it is
a possible definition for a key sequence. Some functions are
commands; a function written in Lisp is a command if it contains an
interactive declaration (Note: Defining Commands.). Such a
function can be called from Lisp expressions like other functions;
in this case, the fact that the function is a command makes no
Strings are commands also, even though they are not functions. A
symbol is a command if its function definition is a command; such
symbols can be invoked with `M-x'. The symbol is a function as
well if the definition is a function. Note: Command Overview.
A "keystroke command" is a command that is bound to a key sequence
(typically one to three keystrokes). The distinction is made here
merely to avoid confusion with the meaning of "command" in
non-Emacs editors; for programmers, the distinction is normally
A "byte-code function" is a function that has been compiled by the
byte compiler. Note: Byte-Code Type.
- Function: subrp OBJECT
This function returns `t' if OBJECT is a built-in function (i.e. a
(subrp 'message) ; `message' is a symbol,
=> nil ; not a subr object.
(subrp (symbol-function 'message))
- Function: byte-code-function-p OBJECT
This function returns `t' if OBJECT is a byte-code function. For
(byte-code-function-p (symbol-function 'next-line))
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