D.2 Obsolete Macros

The following macros are obsolete, and are replaced by versions with a ‘cl-’ prefix that do not behave in exactly the same way. Consequently, the cl.el versions are not simply aliases to the cl-lib.el versions.

Macro: flet (bindings…) forms… ¶

This macro is replaced by cl-flet (see Function Bindings), which behaves the same way as Common Lisp’s flet. This flet takes the same arguments as cl-flet, but does not behave in precisely the same way.

While flet in Common Lisp establishes a lexical function binding, this flet makes a dynamic binding (it dates from a time before Emacs had lexical binding). The result is that flet affects indirect calls to a function as well as calls directly inside the flet form itself.

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.

For those cases where the dynamic scoping of flet is desired, cl-flet is clearly not a substitute. The most direct replacement would be instead to use cl-letf to temporarily rebind (symbol-function 'fun). But in most cases, a better substitute is to use advice, such as:

(defvar my-fun-advice-enable nil)
(add-advice 'fun :around
            (lambda (orig &rest args)
              (if my-fun-advice-enable (do-something)
                (apply orig args))))

so that you can then replace the flet with a simple dynamically scoped binding of my-fun-advice-enable.

Note that many primitives (e.g., +) have special byte-compile handling. Attempts to redefine such functions using flet, cl-letf, or advice will fail when byte-compiled.

Macro: labels (bindings…) forms… ¶

This macro is replaced by cl-labels (see Function Bindings), which behaves the same way as Common Lisp’s labels. This labels takes the same arguments as cl-labels, but does not behave in precisely the same way.

This version of labels uses the obsolete lexical-let form (see Obsolete Lexical Binding), rather than the true lexical binding that cl-labels uses.