First report on the Pre-Scheme Restoration — Pre-Scheme
It's been over 3 months since kicking off the Pre-Scheme Restoration project, so it's well and truly time for a progress update! I'm pleased to report that the bulk of the port to R7RS has been completed, with approximately 75% of the codebase successfully loading in 3 different R7RS-compatible Scheme implementations (Chibi, Sagittarius, and Guile) and 100% of the codebase running via a new R7RS compatibility layer for Scheme 48. The libraries which haven't yet been ported all directly interface with the Scheme expander front-end, and replacing that with a portable expander is the next major focus of the project. In the rest of this article I'll discuss the work that's been done to get to this point, and briefly outline the upcoming work.
One challenge with porting software is ensuring that errors aren't introduced during the porting process. The original Pre-Scheme compiler doesn't have an established test suite, so the only real test (aside from just loading the code) is to check that it continues to translate the Scheme 48 VM to identical C code. Being an end-to-end test, this requires the entire compiler to remain functioning throughout the porting process to verify that errors haven't been introduced. Any change breaking compatibility with the original platform would prevent the test from being run, and leave us in the painful situation of only being able to detect and debug errors after everything has been ported.
