(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
function-like objects.

     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
     considered primitives.)

     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.

"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.

     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.

"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 programmers, the distinction is normally

"byte-code function"
     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
     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

          (byte-code-function-p (symbol-function 'next-line))
               => t

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