12.3 Naming a Function

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 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 Function Cells). However, it is more common to use the defun special form, 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.)