— Special Form: flet (bindings...) forms...

This form establishes let-style bindings on the function cells of symbols rather than on the value cells. Each binding must be a list of the form ‘(name arglist forms...)’, which defines a function exactly as if it were a defun* form. The function name is defined accordingly for the duration of the body of the flet; then the old function definition, or lack thereof, is restored.

While flet in Common Lisp establishes a lexical binding of name, Emacs Lisp flet makes a dynamic binding. The result is that flet affects indirect calls to a function as well as calls directly inside the flet form itself.

You can use flet to disable or modify the behavior of a function in a temporary fashion. This will even work on Emacs primitives, although note that some calls to primitive functions internal to Emacs are made without going through the symbol's function cell, and so will not be affected by flet. For example,

          (flet ((message (&rest args) (push args saved-msgs)))
            (do-something))

This code attempts to replace the built-in function message with a function that simply saves the messages in a list rather than displaying them. The original definition of message will be restored after do-something exits. This code will work fine on messages generated by other Lisp code, but messages generated directly inside Emacs will not be caught since they make direct C-language calls to the message routines rather than going through the Lisp message function.

Also note that many primitives (e.g. +) have special byte-compile handling. Attempts to redefine such functions using flet will fail if byte-compiled. In such cases, use labels instead.

Functions defined by flet may use the full Common Lisp argument notation supported by defun*; also, the function body is enclosed in an implicit block as if by defun*. See Program Structure.

— Special Form: labels (bindings...) forms...

The labels form is like flet, except that it makes lexical bindings of the function names rather than dynamic bindings. (In true Common Lisp, both flet and labels make lexical bindings of slightly different sorts; since Emacs Lisp is dynamically bound by default, it seemed more appropriate for flet also to use dynamic binding. The labels form, with its lexical binding, is fully compatible with Common Lisp.)

Lexical scoping means that all references to the named functions must appear physically within the body of the labels form. References may appear both in the body forms of labels itself, and in the bodies of the functions themselves. Thus, labels can define local recursive functions, or mutually-recursive sets of functions.

A “reference” to a function name is either a call to that function, or a use of its name quoted by quote or function to be passed on to, say, mapcar.