I am releasing CPSCM, a new Scheme compiler based on classic CPS conversion and trampolines. It will eventually support multiple backends (Javascript and Java are in the works), but currently it supports Scheme to Common Lisp translation. You can see it work right from your browser on the online demo page (no large jobs, please), or you can download and run it by following the instructions on the CPSCM homepage.

Macro-expansion is delegated to Al Petrofsky’s alexpander, which means that CPSCM has full syntax-rules support from the start. I’ll probably add define-macro support at some point. I don’t feel up to integrating syntax-case, but if anyone wants to contribute, it would be greatly appreciated.

Other than this, CPSCM supports full continuations, including correct call/cc + dynamic-wind interaction, and SRFI-0. It still lacks eval, error protection in dynamic-wind, streams, load, and multiple-file source facilities. An interesting point is that as soon as CPSCM is able to compile itself, eval can be added in (though the environment functions other than interaction-environment will be problematic.)

As with scsh-regexp, I will use Google Code Project Hosting. Some people have questioned this choice (and Google Code has earned mixed reviews); compared to Sourceforge, Google Code has a big advantage: they don’t make you fill out multi-page forms (and wait for approval!) for the “privillege” of uploading an open-source project.

In my last post, I mentioned generating the R5RS identifier list by scraping the HTML version of the R5RS standard. I decided to use Scheme for the job, and quickly learned that Chicken and SISC lack adequate regexp support (SISC has no support at all, apart from letting you interface with the underlying JVM). Eventually, I settled upon SCSH, as it has a powerful regexp API, as well as good shell integration.

The resulting SCSH script took forever to run (to be fair, I added code to separate procedure names from macro names, and didn’t bother optimizing beyond the naive O(n²) algorithm). I started to miss Chicken’s speed. The SCSH regexp API looked reasonably easy to port. I ended up writing both a Chicken and a SISC emulation layer (the latter based on java.util.regex). I am planning to add a pregexp backend as well, which would extend regexp support to any R5RS system.

Have a look at the scsh-regexp project for details, examples and news.

When interacting with a REPL, readline history and tab completion support are major productivity boosters. This is true in general, but especially so given Scheme’s long names (e.g. call-with-input-file). Some Schemes have integrated readline support, but the one I use most, namely SISC, does not.

The next best thing is to use something like rlwrap (as SISC actually does). «rlwrap COMMAND» adds history support out of the box by intercepting COMMAND’s standard input and output. Furthermore, I’ve just learned that, when configured properly, rlwrap can also autocomplete a predefined set of identifiers (and optionally learn new identifiers from standard input/output). SISC does not enable completion by default, but we can easily fix it. The relevant command-line arguments are

-b DELIMITER-CHARS

A list of word-separating delimiters. Whitespace is included by default; for Scheme, use “\”()[]’`”.

-H HISTORY-file

The history file.

-f IDENTIFIER-FILE

A file listing the identifiers to be completed by default, one per line (this option can occur multiple times). I wrote a script to parse the R5RS index HTML page; I’m putting the output online, so you can simply download the resulting R5RS identifier list. It makes sense to also add a -f HISTORY-FILE argument.

These are the basics; the man page documents a few more interesting options (in particular, see -r). Using rlwrap, you can enable readline history and completion for any interaction-challenged Scheme system.