2.3.13 Function Type

Just as functions in other programming languages are executable, Lisp function objects are pieces of executable code. However, functions in Lisp are primarily Lisp objects, and only secondarily the text which represents them. These Lisp objects are lambda expressions: lists whose first element is the symbol lambda (see section 12.2 Lambda Expressions).

In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression is also called an anonymous function (see section 12.7 Anonymous Functions). A named function in Lisp is actually a symbol with a valid function in its function cell (see section 12.4 Defining Functions).

Most of the time, functions are called when their names are written in Lisp expressions in Lisp programs. However, you can construct or obtain a function object at run time and then call it with the primitive functions funcall and apply. See section 12.5 Calling Functions.