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From: Florian Weimer <fw@deneb.enyo.de>
To: Andreas Rossberg <rossberg@mpi-sws.org>
Cc: OCaml List <caml-list@inria.fr>
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Date: Thu, 10 Feb 2011 22:31:54 +0100
In-Reply-To: <8DECBDB1-84C6-4609-9C2A-8C9AEDE337A1@mpi-sws.org> (Andreas
	Rossberg's message of "Wed, 9 Feb 2011 22:12:49 +0100")
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Subject: Re: [Caml-list] Scoped Bound Resource Management just for C++?

* Andreas Rossberg:

>> If you have coroutines or threads with communication among them, you
>> can always turn type-enforced region-based handles into open handles
>> with an explicit close operation.
>
> I still don't know what you are talking about. Why should a suitable
> type system not be able to make such an operation ill-typed?

It's a general problem.  Once you've got message passing, you've got
mutable state and the ability to escape regions.  Basically, you do
something like this:

  val resource = openResource params
  fun loop()
    (case receiveMailbox () of
       READ count => (read resource count; sendMailbox ACK; loop ())
       WRITE buffer => (write resource buffer; sendMailbox ACK; loop ())
       CLOSE => ())
  end
  val _ = loop ()
  val _ = close resource

>> Like array bounds checking, integer division or other partial
>> functions. 8-)
>
> Right. But these are examples of collateral damage of useful features,
> while the one under discussion is just the damage, with no new
> expressiveness to justify it. ;)

It's not about expressiveness, it's about making writing correct (that
is, resource-aware) code easier.