Testing the Practical Common Lisp code on Medley

When the Medley Interlisp Project began reviving the system around 2020, its Common Lisp implementation was in the state it had when commercial development petered off in the 1990s, mostly prior to the ANSI standard.

Back then Medley Common Lisp mostly supported CLtL1 plus CLOS and the condition system. Some patches submitted several years later to bring the language closer to CLtL2 needed review and integration.

Aside from these general areas there was no detailed information on what Medley missed or differed from ANSI Common Lisp.

In late 2021 Larry Masinter proposed to evaluate the ANSI compatibility of Medley Common Lisp by running the code of popular Common Lisp books and documenting any divergences. In March of 2024 I set to work to test the code of the book Practical Common Lisp by Peter Seibel.

I went over the book chapter by chapter and completed a first pass, documenting the effort in a GitHub issue and a series of discussion posts. In addition I updated a running list of divergences from ANSI Common Lisp.


Part of the code of the book is contained in the examples in the text and the rest in the downloadable source files, which constitute some more substantial projects.

To test the code on Medley I evaluated the definitions and expressions at a Xerox Common Lisp Exec, noting any errors or differences from the expected outcomes. When relevant source files were available I loaded them prior to evaluating the test expressions so that any required definitions and dependencies were present. ASDF hasn't been ported to Medley, so I loaded the files manually.

Adapting the code

Before running the code I had to apply a number of changes. I filled in any missing function and class definitions the book leaves out as incidental to the exposition. This also involved adding appropriate function calls and object instantiations to exercise the definitions or produce the expected output.

The source files of the book needed adaptation too due to the way Medley handles pure Common Lisp files.

Skipped code

The text and source files contain also code I couldn't run because some features are known to be missing from Medley, or key dependencies can't be fulfilled. For example, a few chapters rely on the AllegroServe HTTP server which doesn't run on Medley. Although Medley does have a XNS network stack, providing the TCP/IP network functions AllegroServe assumes would be a major project.

Some chapters depend on code in earlier chapters that uses features not available in Medley Common Lisp, so I had to skip those too.


Having completed the first pass over Practical Common Lisp, my initial impression is Medley's implementation of Common Lisp is capable and extensive. It can run with minor or no changes code that uses most basic and intermediate Common Lisp features.

The majority of the code I tried ran as expected. However, this work did reveal significant gaps and divergences from ANSI.

To account for the residential environment and other peculiarities of Medley, packages need to be defined in a specific way. For example, some common defpackage keyword arguments differ from ANSI. Also, uppercase strings seem to work better than keywords as package designators.

As for the gaps the loop iteration macro, symbol-macrolet, the #p reader macro, and other features turned out to be missing or not work.

While the incompatibilities with ANSI Common Lisp are relativaly easy to address or work around, what new users may find more difficult is understanding and using the residential environment of Medley.

Bringing Medley closer to ANSI Common Lisp

To plug the gaps this project uncovered Larry ported or implemented some of the missing features and fixed a few issues.

He ported a loop implementation which he's enhancing to add missing functionality like iterating over hash tables. Iterating over packages, which loop lacks at this time, is trickier. More work went into adding #p and an experimental symbol-macrolet.

Reviewing and merging the CLtL2 patches is still an open issue, a major project that involves substantial effort.

Future work and conclusion

When the new features are ready I'll do a second pass to check if more of the skipped code runs. Another outcome of the work may be the beginning of a test suite for Medley Common Lisp.

Regardless of the limitations, what the project highlighted is Medley is ready as a development environment for writing new Common Lisp code, or porting libraries and applications of small to medium complexity.

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