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(elisp)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
function-like objects.

"function"
     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.

"primitive"
     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
     considered primitives.)

     Usually the reason we implement a function as a primitive is either
     because it is fundamental, because it 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.

"lambda expression"
     A "lambda expression" is a function written in Lisp.  These are
     described in the following section.  Note: Lambda Expressions.

"special form"
     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.

"macro"
     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.  Macros enable Lisp
     programmers to do the sorts of things that special forms can do.
     Note: Macros, for how to define and use macros.

"command"
     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
     difference.

     Keyboard macros (strings and vectors) 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.

"keystroke command"
     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 Lisp programs, the distinction is normally
     unimportant.

"byte-code function"
     A "byte-code function" is a function that has been compiled by the
     byte compiler.  Note: Byte-Code Type.

 - Function: functionp object
     This function returns `t' if OBJECT is any kind of function, or a
     special form or macro.

 - 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 function provides information about the argument list of a
     primitive, SUBR.  The returned value is a pair `(MIN . MAX)'.  MIN
     is the minimum number of args.  MAX is the maximum number or the
     symbol `many', for a function with `&rest' arguments, or the
     symbol `unevalled' if SUBR is a special form.