catch, throw, prompt, control, fluids, garbage collection

[Andy Wingo]

Hello all,

I am currently shaving a yak[0], trying to integrate nonlocal exits into
the VM. That's `catch', `throw', and all that. I have also been wanting
to integrate delimited continuations into Guile; that's a bigger topic.
Anyway it turns out that one can implement catch and throw using
delimited continuations. Flatt et al's paper at ICFP 07 [1] shows how.

Sitaram 93 shows why you want delimited continuations [2]. Felleisen 88
is the first appearance of delimited continuations that I'm aware of,
though I think it's behind a paywall; Sitaram and Felleisen 90 seems to
be an adequate substitute [3]. Dybvig et al 07 is a great summary of the
relationship between the various delimited continuation operators [4].

[0] http://projects.csail.mit.edu/gsb/old-archive/gsb-archive/gsb2000-02-11.html
[1] http://www.cs.utah.edu/plt/publications/icfp07-fyff.pdf
[2] http://www.ccs.neu.edu/scheme/pubs/pldi93-sitaram.pdf
[3] http://www.ccis.northeastern.edu/scheme/pubs/lasc1990-sf.pdf
[4] http://www.cs.indiana.edu/~dyb/pubs/monadicDC.pdf

So, the deal. The deal is that Flatt shows very clearly and elegantly
how to implement catch and throw using prompt, abort, and dynamic
binding. (Actually he uses the % flavor of prompt; and abort is like
Sitaram's control afaict. See Sitaram 93.) The dynamic binding is for
the current exception handler (corresponding to catch) and for the
current "abort handler", which is like a pre-unwind handler.

The problem: Well, you might want to set some exception handlers, and
then spawn a bunch of threads all with those handlers. But if a
pre-unwind handler throws an exception, that exception shouldn't be
handled by itself, or you get an infinite loop. Basically you need a bit
to say whether a given handler is running in a given thread or not -- a
fluid.

But you can't / shouldn't make a new fluid every time you enter a
`catch', because currently fluids are never garbage collected! We really
need to fix this. I think it's a 1.9 regression.

To do so effectively, I think you'd need to make fluid objects store
their values directly, so that the GC doesn't have to go through hoops
to know that they're collectable. Ideally they would get their values
via pthread_getspecific; but that would defeat some bits of ours about
"dynamic states" (not a very useful concept IMO), and the GC would need
help. Actually it would be nice if libgc supported thread-local
allocations. (Does it?)

Anyway, some disconnected thoughts here. I could have simply implemented
catch and throw using what we have now, but I think it's a great
opportunity to see if we can do delimited continations.

There's some half-complete prompt support in master. I thought I'd get
to the rest sooner, but it seems that it will take another couple weeks.
I haven't been able to have good hacktime over the last week or two, so
things are going slower than they usually do. Also, getting to
understand delimited continuations was a bit paralyzing.

Regarding the release... I have no thoughts. Perhaps I should prepare
the NEWS, and we get a release out the door this week, then the next one
will have catch/throw nicely integrated, and at that point I'll be
mostly happy for 2.0. Thoughts?

Cheers,

Andy
-- 
http://wingolog.org/