The Common Lisp Cookbook – Packages

See: The Complete Idiot’s Guide to Common Lisp Packages

List all Symbols in a Package

Common Lisp provides some macros to iterate through the symbols of a package. The two most interesting are: DO-SYMBOLS and DO-EXTERNAL-SYMBOLS. DO-SYMBOLS iterates over the symbols accessible in the package and DO-EXTERNAL-SYMBOLS only iterates over the external symbols (you can see them as the real package API).

To print all exported symbols of a package named “PACKAGE”, you can write:

(do-external-symbols (s (find-package "PACKAGE"))
  (print s))

You can also collect all these symbols in a list by writing:

(let (symbols)
  (do-external-symbols (s (find-package "PACKAGE"))
    (push s symbols))
  symbols)

Or you can do it with LOOP.

(loop for s being the external-symbols of (find-package "PACKAGE")
  collect s)

Package nickname

Nickname Provided by Packages

When defining a package, it is trivial to give it a nickname for better user experience. The following example is a snippet of PROVE package:

(defpackage prove
  (:nicknames :cl-test-more :test-more)
  (:export :run
           :is
           :ok)

Afterwards, a user may use nickname instead of the package name to refer to this package. For example:

(prove:run)
(cl-test-more:is)
(test-more:ok)

Please note that although Common Lisp allows defining multiple nicknames for one package, too many nicknames may bring maintenance complexity to the users. Thus the nicknames shall be meaningful and straightforward. For example:

(defpackage #:iterate
  (:nicknames #:iter))

(defpackage :cl-ppcre
  (:nicknames :ppcre)

Give a Package a Local Nickname

Sometimes it is handy to give a local name to an imported package to save some typing, especially when the imported package does not provide nice nicknames.

This can be achieved by using RENAME-PACKAGE. For example:

(asdf:load-system :cl-ppcre)

(defpackage :mypackage
  (:use :cl))
(in-package :mypackage)

;; Add nickname :RE to package :CL-PPCRE.
(rename-package :cl-ppcre :cl-ppcre '(re))

;; You can use :RE instead of :CL-PPCRE now.
(re:scan "a" "abc")

The function head of RENAME-PACKAGE is as follows:

(rename-package package-designator name &optional (nicknames nil))

As you may guess, this function can be used to rename a package by giving it a new name instead of a new nickname.

However, it is highly recommended not to do so. Because otherwise the package will be modified and the original name (:CL-PPCRE in the example above) will not be available any more after renaming. For example:

;; DO NOT DO THIS!
(rename-package :cl-ppcre :re)

;; ERROR!
(cl-ppcre:scan "a" "abc")

This might cause problems if you want to load another package that relies on the modified package.

Package locks

The package common-lisp and SBCL internal implementation packages are locked by default, including sb-ext.

In addition, any user-defined package can be declared to be locked so that it cannot be modified by the user. Attempts to change its symbol table or redefine functions which its symbols name result in an error.

More detailed information can be obtained from documents of SBCL and CLisp.

For example, if you try the following code:

(asdf:load-system :alexandria)
(rename-package :alexandria :alex)

You will get the following error (on SBCL):

Lock on package ALEXANDRIA violated when renaming as ALEX while
in package COMMON-LISP-USER.
   [Condition of type PACKAGE-LOCKED-ERROR]
See also:
  SBCL Manual, Package Locks [:node]

Restarts:
 0: [CONTINUE] Ignore the package lock.
 1: [IGNORE-ALL] Ignore all package locks in the context of this operation.
 2: [UNLOCK-PACKAGE] Unlock the package.
 3: [RETRY] Retry SLIME REPL evaluation request.
 4: [*ABORT] Return to SLIME's top level.
 5: [ABORT] abort thread (#<THREAD "repl-thread" RUNNING {10047A8433}>)

...

If a modification is required anyway, a package named cl-package-lock can be used to ignore package locks. For example:

(cl-package-locks:without-package-locks
  (rename-package :alexandria :alex))

Page source: packages.md