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2.4.1 Introduction ¶

The Common Lisp package system, designed and standardized several years ago, is not hierarchical. Since Common Lisp was standardized, other languages, including Java and Perl, have evolved namespaces which are hierarchical. This document describes a hierarchical package naming scheme for Common Lisp. The scheme was proposed by Franz Inc and implemented in their Allegro Common Lisp product; a compatible implementation of the naming scheme is implemented in CMUCL. This documentation is based on the Franz Inc. documentation, and is included with permission.

The goals of hierarchical packages in Common Lisp are:

• Reduce collisions with user-defined packages: it is a well-known problem that package names used by the Lisp implementation and those defined by users can easily conflict. The intent of hierarchical packages is to reduce such conflicts to a minimum.
• Improve modularity: the current organization of packages in various implementations has grown over the years and appears somewhat random. Organizing future packages into a hierarchy will help make the intention of the implementation more clear.
• Foster growth in Common Lisp programs, or modules, available to the CL community: the Perl and Java communities are able to contribute code to repositories, with minimal fear of collision, because of the hierarchical nature of the name spaces used by the contributed code. We want the Lisp community to benefit from shared modules in the same way.

In a nutshell, a dot (.) is used to separate levels in package names, and a leading dot signifies a relative package name. The choice of dot follows Java. Perl, another language with hierarchical packages, uses a colon (:) as a delimiter, but the colon is already reserved in Common Lisp. Absolute package names require no modifications to the underlying Common Lisp implementation. Relative package names require only small and simple modifications.