BrianScheme

Remember back a year ago I tried my hand at a Lisp implementation called Wisp? Well, currently a co-worker of mine, Brian Taylor, is similarly working on his own Scheme implementation — but he knows more about what he's doing than I did, so it's more interesting. However, that expertise doesn't extend to inventing a clever name (Zing!): it's unsubtly called BrianScheme.

git clone git://github.com/netguy204/brianscheme.git

I've been hacking at it a little myself, cheering from the sidelines.

git remote add wellons git://github.com/skeeto/brianscheme.git

Like Wisp, it's written from scratch in C from the bottom up. Unlike Wisp, it has closures, lexical scoping, mark-and-sweep garbage collection, object system, and compiles to a bytecode (in memory). Continuations are still a ways off, but planned. One of the most powerful features so far is the foreign function interface (FFI). Now that he's implemented it with libffi he's barely had to touch the C code base. In fact, thanks to the FFI, the the C portion of BrianScheme will be shrinking.

For example, BrianScheme currently lacks floating point numbers, and its integers are currently just native fixnums. Sometime soon it will, like Wisp, use the GNU Multi-Precision Library (GMP) to provide bignums. Adding this will not require making any changes whatsoever to the C code. Using the object system (Tiny-CLOS), hooks in the reader and printer, and the FFI, this can be entirely implemented in the language itself.

Just-in-time compilation (JIT) has begun to be implemented without touching C. Again, done by pulling in in libjit with the FFI.

Because I wrote Wisp to be embeddable and a library, I was able to run Wisp in BrianScheme, via the FFI, and expose some bindings. For example, I can send it s-expressions to evaluate,

> (require 'wisp)
> (wisp:eval '(expt 6 56))
37711171281396032013366321198900157303750656

BrianScheme doesn't currently support threading, mainly because the garbage collector isn't ready for it. But remember how I mentioned GNU Pth last month? Again, I was able to load Pth with the FFI to add userspace threading, which is safe for the garbage collector because it's effectively an atomic operation. (Once continuations are implemented, this could actually be implemented without Pth, just by making good use of those continuations.) The current hangup is the REPL, which doesn't know about Pth and so it never yields. To take advantage of threading you have to suspend the REPL (with pth:join).

This REPL issue should be solved with the long term goal for BrianScheme. The C component of BrianScheme will merely exist for the purposes of bootstrapping the full system. During initialization, just about everything will be redefined in BrianScheme, with the original C definitions only living long enough to load what's needed. This includes reimplementing the reader itself in BrianScheme, which enables all sorts of possibilities, like the previously mentioned bignums implemented in the language itself, inline regular expressions, and proper yielding to the userspace thread scheduler.

So go ahead and clone Brian's repository (and add mine as a remote, too! :-D) and poke around at it. To compare to Wisp again, it's not quite as stable at the moment. It exits very easily from runtime errors, due to lacking error handling, so an instance generally doesn't live very long at the moment. This will probably be resolved sometime soon. Except for that, it does play well with Emacs as an inferior-lisp.

• lisp
• lang