A symbol can serve as the name of a function. This happens when the symbol’s function cell (see Symbol Components) contains a function object (e.g., a lambda expression). Then the symbol itself becomes a valid, callable function, equivalent to the function object in its function cell.
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 Symbol Function Indirection. If you have not given a symbol a function definition, its function cell is said to be void, and it cannot be used as a function.
In practice, nearly all functions have names, and are referred to by
their names. You can create a named Lisp function by defining a
lambda expression and putting it in a function cell (see Accessing Function Cell Contents). However, it is more common to use the defun macro,
described in the next section.
See Defining Functions.
We give functions names because it is convenient to refer to them by their names in Lisp expressions. Also, a named Lisp function can easily refer to itself—it can be recursive. Furthermore, primitives can only be referred to textually by their names, since primitive function objects (see Primitive Function Type) have no read syntax.
A function need not have a unique name. A given function object
usually appears in the function cell of only one symbol, but
this is just a convention. It is easy to store it in several symbols
using fset; then each of the symbols is a valid name for the
same function.
Note that 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. (This is not the case in some dialects of Lisp, like Scheme.)
By convention, if a function’s symbol consists of two names
separated by ‘--’, the function is intended for internal use and
the first part names the file defining the function. For example, a
function named vc-git--rev-parse is an internal function
defined in vc-git.el. Internal-use functions written in C have
names ending in ‘-internal’, e.g., bury-buffer-internal.
Emacs code contributed before 2018 may follow other internal-use
naming conventions, which are being phased out.