GHC already garbage-collects threads that are blocked on an MVar
that is otherwise inaccessible (and hence cannot be updated). More precisely,
GHC sends the thread an asynchronous exception (ThreadBlocked or something), so
that it has a chance to clean up.
So perhaps the GC you want is already implemented?
The classic paper "The
Incremental Garbage Collection of Processes" (http://citeseer.ist.psu.edu/baker77incremental.html)
describes "futures" and how particularly garbage collecting them when
their pending result is no longer referenced. I've been playing with an
implementation of futures in Concurrent Haskell ( http://haskell.org/haskellwiki/Reactive),
using MVars, and I'm stumped about how to GC non-winning threads in a race
between futures ("parallel or"). I'm having winner kill loser,
which seems to work fine, though is potentially dangerous w.r.t locked
resources. Still, the elegance of a GC-based solution appeals to
me. Has anyone explored process GC ideas for Concurrent Haskell (or STM)?
Futures are implemented using Concurrent Haskell's MVars. I first tried
using STM and TVars, simply using orElse to implement mappend for
futures. However, I didn't see how to avoid nesting
"atomically", which yielded a run-time error. If anyone has
ideas about using STM & TVars for futures, I'd love to hear.
Thanks, - Conal