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))
    #
    * (in-package :foo)
    #
    * (defparameter *foo* 42)
    *FOO*
    * (defun foo () *foo*)
    FOO
    * (in-package :common-lisp-user)
    #
    * (foo::foo)
    42
    * (defpackage :bar)
    #
    * (in-package :bar)
    #
    * (cl:defparameter *bar* 640)
    *BAR*
    * (cl:defun bar () (cl:values (foo::foo) *bar*))
    BAR
    * (bar)
    42
    640
    * (in-package :cl-user)
    #
    * *package*
    #
    * (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.

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