A Wafer Thin Common LISP Package Primer

Common LISP packages tend to trip up noobs presumably only familiar with packages as found in other languages; in those languages you usually have a file that contains a package and that package is only defined in that one file. We might call this a file-package. This is obviously a simplification; usually other languages have various ways to break the file-package rule, e.g. code to import things into or from somewhere else, dynamic loading, monkey patching, LD_PRELOAD, and worse.

Common LISP is, for some reason, rather REPL oriented--while you can create systems with Another System Definition Facility (ASDF) that contain something like the file-package as found in other languages, you can also type up a new package in the REPL, "change directory" into it, and wrangle various symbols therein. Packages thus can be built up or changed incrementally:

    $ sbcl --noinform
    * (defpackage :foo (:use :cl))
    #<PACKAGE "FOO">
    * (in-package :foo)
    #<PACKAGE "FOO">
    * (defparameter *foo* 42)
    *FOO*
    * (defun foo () *foo*)
    FOO
    * (in-package :common-lisp-user)
    #<PACKAGE "COMMON-LISP-USER">
    * (foo::foo)
    42
    * (defpackage :bar)
    #<PACKAGE "BAR">
    * (in-package :bar)
    #<COMMON-LISP:PACKAGE "BAR">
    * (cl:defparameter *bar* 640)
    *BAR*
    * (cl:defun bar () (cl:values (foo::foo) *bar*))
    BAR
    * (bar)
    42
    640
    * (in-package :cl-user)
    #<PACKAGE "COMMON-LISP-USER">
    * *package*
    #<PACKAGE "COMMON-LISP-USER">
    * (quit)

Now, there are good reasons to go with a file-package setup: files can be lobbed into version control, and files may integrate better with Integrated Development Environments than text scrolling to infinity in a REPL buffer. (I'm somewhat guessing here about IDE; the last IDE I used was Xcode, and that was a long time ago. Apparently there is M-. in emacs to find where code lives? But I am a barbarian unix sysadmin who uses vi to edit things.) But if you just want to mess around without the file-package formalism, LISP won't get in your way.

ASDF Home Page

11.1.1 Introduction to Packages

Details vary, but the HyperSpec usually can be found in an operating system port or package for offline use, e.g. `doas pkg_add clisp-hyperspec` on OpenBSD.

tags #lisp

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