Hey all,

Attached is a patch to add a "cooperative" REPL server to Guile.  This
new type of REPL server allows programs that run an event loop (like a
game or a simulation) to make use of a REPL server that doesn't present
a common pitfall of multithreaded programs: Crashing when 2 threads
write to the same resource at the same time.  The cooperative REPL
ensures that evaluation only happens within the context of a single
thread, and the user can control when evaluation is allowed to happen.

By cooperative, I mean that the client REPL's are run as coroutines
using prompts.  All of the REPL's run within the same thread, the thread
that calls (spawn-coop-server) and later (poll-coop-server).  Reading
user input is passed off to another thread and the REPL prompt is
aborted.  To actually evaluate code, the user must call
(poll-coop-server) periodically.  Only one REPL can evaluate code at a
time.

Things seem to be working well.  I did basic tests by connecting to the
server via telnet and later (when I was confident that I wouldn't crash
Emacs) via Geiser.

This patch is built on top of Mark Weaver's patch to add the (ice-9
mvars) module.

What do you think?

- Dave Thompson