For Dylan to be useful, it must interface seamlessly with C. Most interesting libraries are written in C, including all of the significant APIs for Unix systems. Ideally, it should be as easy to call a C function from a Dylan program as it is to call one from a C program.
We already have working callbacks from C to Dylan code, on both IA-32 and PowerPC—even with callbacks to closures.
The original Gwydion developers at CMU wrote a nifty tool called Melange. It parses C header files and automatically generates Dylan interface files. We’ve been slowly enhancing Melange to handle more complicated header files, and stomping out all the bugs which we find along the way.
Somewhere along we decided to rewrite parts of Melange, and a new project called Pidgin was started. Pidgin has a working C header parser, but is missing the code generation part. This project was later shelved.
Peter Housel is now working on a brand new C code representation library called CPR (for C Program Representation), inspired by SML/NJ’s ckit and George Necula’s CIL. This library will be able to parse C code and make its structure available to Dylan programs as an abstract syntax tree.
The current plan is that a new CPR-based FFI mechanism called
Collage will be replacing Melange in the hopefully near future.
Unlike Melange, Collage won’t usually generate interface files;
instead, it will be built into
d2c and DFMC.
For now, however, Melange is your best bet if you want to use a C library from a Gwydion Dylan program. If you are using Open Dylan, there is currently no really good alternative to Melange. The existing bindings are all generated by direct pattern-based translation of C header files into Dylan interface files. This is accomplished by a script written in an arguably arcane translation language called Gema. As stated above, the plan is for Collage to do this job better in the future.