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1.2 Organization

The Common Lisp package is organized into four main files:

cl-lib.el
This is the main file, which contains basic functions and information about the package. This file is relatively compact.
cl-extra.el
This file contains the larger, more complex or unusual functions. It is kept separate so that packages which only want to use Common Lisp fundamentals like the cl-incf function won't need to pay the overhead of loading the more advanced functions.
cl-seq.el
This file contains most of the advanced functions for operating on sequences or lists, such as cl-delete-if and cl-assoc.
cl-macs.el
This file contains the features that are macros instead of functions. Macros expand when the caller is compiled, not when it is run, so the macros generally only need to be present when the byte-compiler is running (or when the macros are used in uncompiled code). Most of the macros of this package are isolated in cl-macs.el so that they won't take up memory unless you are compiling.

The file cl-lib.el includes all necessary autoload commands for the functions and macros in the other three files. All you have to do is (require 'cl-lib), and cl-lib.el will take care of pulling in the other files when they are needed.

There is another file, cl.el, which was the main entry point to this package prior to Emacs 24.3. Nowadays, it is replaced by cl-lib.el. The two provide the same features (in most cases), but use different function names (in fact, cl.el mainly just defines aliases to the cl-lib.el definitions). Where cl-lib.el defines a function called, for example, cl-incf, cl.el uses the same name but without the ‘cl-’ prefix, e.g., incf in this example. There are a few exceptions to this. First, functions such as cl-defun where the unprefixed version was already used for a standard Emacs Lisp function. In such cases, the cl.el version adds a ‘*’ suffix, e.g., defun*. Second, there are some obsolete features that are only implemented in cl.el, not in cl-lib.el, because they are replaced by other standard Emacs Lisp features. Finally, in a very few cases the old cl.el versions do not behave in exactly the same way as the cl-lib.el versions. See Obsolete Features.

Since the old cl.el does not use a clean namespace, Emacs has a policy that packages distributed with Emacs must not load cl at run time. (It is ok for them to load cl at compile time, with eval-when-compile, and use the macros it provides.) There is no such restriction on the use of cl-lib. New code should use cl-lib rather than cl.

There is one more file, cl-compat.el, which defines some routines from the older Quiroz cl.el package that are not otherwise present in the new package. This file is obsolete and should not be used in new code.