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(elisp)Function Names

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Naming a Function
=================

   In most computer languages, every function has a name; the idea of a
function without a name is nonsensical.  In Lisp, a function in the
strictest sense has no name.  It is simply a list whose first element is
`lambda', a byte-code function object, or a primitive subr-object.

   However, a symbol can serve as the name of a function.  This happens
when you put the function in the symbol's "function cell" (Note: Symbol
Components).  Then the symbol itself becomes a valid, callable
function, equivalent to the list or subr-object that its function cell
refers to.  The contents of the function cell are also called the
symbol's "function definition".  The procedure of using a symbol's
function definition in place of the symbol is called "symbol function
indirection"; see Note: Function Indirection.

   In practice, nearly all functions are given names in this way and
referred to through their names.  For example, the symbol `car' works
as a function and does what it does because the primitive subr-object
`#<subr car>' is stored in its function cell.

   We give functions names because it is convenient to refer to them by
their names in Lisp expressions.  For primitive subr-objects such as
`#<subr car>', names are the only way you can refer to them: there is
no read syntax for such objects.  For functions written in Lisp, the
name is more convenient to use in a call than an explicit lambda
expression.  Also, a function with a name can refer to itself--it can
be recursive.  Writing the function's name in its own definition is much
more convenient than making the function definition point to itself
(something that is not impossible but that has various disadvantages in
practice).

   We often identify functions with the symbols used to name them.  For
example, we often speak of "the function `car'", not distinguishing
between the symbol `car' and the primitive subr-object that is its
function definition.  For most purposes, there is no need to
distinguish.

   Even so, keep in mind that a function need not have a unique name.
While a given function object _usually_ appears in the function cell of
only one symbol, this is just a matter of convenience.  It is easy to
store it in several symbols using `fset'; then each of the symbols is
equally well a name for the same function.

   A symbol used as a function name may also be used as a variable;
these two uses of a symbol are independent and do not conflict.  (Some
Lisp dialects, such as Scheme, do not distinguish between a symbol's
value and its function definition; a symbol's value as a variable is
also its function definition.)  If you have not given a symbol a
function definition, you cannot use it as a function; whether the
symbol has a value as a variable makes no difference to this.


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