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Date: Thu, 13 Jan 2000 10:32:40 +0100
From: Xavier Leroy <Xavier.Leroy@inria.fr>
To: David McClain <dmcclain@azstarnet.com>, caml-list@inria.fr
Subject: Re: Threads Library
References: <000c01bf51e0$ef8295a0$250148bf@vega>
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In-Reply-To: <000c01bf51e0$ef8295a0$250148bf@vega>; from David McClain on Wed, Dec 29, 1999 at 02:41:49AM -0700
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> However, unlike CML, (on Windows/NT at least) the GC does not discard
> threads hung up on channels that are no longer in use by active threads.
> Hence the use of speculative spawning is not safe in WinNT/OCAML. (Early
> experiments generated thousands of threads before I had to kill off the
> program).

This is correct.  Currently, there is no automatic reclaimation of
threads whose identifier (the associated Thread.t value) is
unreachable, because those threads could be "daemon" threads that do
useful work in the background.

> Secondly, it would appear that the semantics of "with_abort" require that
> the wrapper function be called ahead of all "with" functions. But
> experiments where a with-function raises an exception bypass the actions of
> pending "with_abort" functions on non-selected channels. So evidently, one
> should not permit the use of uncaught exceptions inside of "with" functions.
> 
> Finally, where Reppy's CML uses continuations to effect tail calls, I wonder
> about the use of indefinite recursion triggered by a with-function. It would
> seem that one should treat composite event lists in much the same manner as
> "try-with" when it comes to recursion inside a try (resp. with) clause.
> Instead of tail calling from the "with" clause one should return a value to
> the outer level of the "sync" or "select", and then "match" on that value
> before recursing.

I haven't read Reppy's book yet, so I'm not absolutely sure I follow
you here, but generally speaking it is true that the "with_abort"
mechanism is more heavyweight in OCaml than in CML, because it is
implemented with "real" threads in OCaml while CML uses a much more
lightweight callcc-based implementation.

- Xavier Leroy