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To: Alessandro Baretta <alex@baretta.com>
Cc: Xavier Leroy <xavier.leroy@inria.fr>, Ocaml <caml-list@inria.fr>
Subject: Re: [Caml-list] Has laziness changed type?
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	<20020909162320.D1781@pauillac.inria.fr>
	<3D7CB742.5030908@baretta.com>
From: John Prevost <j.prevost@cs.cmu.edu>
Date: 09 Sep 2002 11:00:31 -0400 (62.536 UMT)
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>>>>> "ab" == Alessandro Baretta <alex@baretta.com> writes:

    ab> This is a pity, in a way, but not really a big problem. I
    ab> often need to check whether a given lazy value corresponds
    ab> computes a meaningful value or raises an exception. To do this
    ab> I had code like the following

...

    ab> This is not terribly useful when you have to match against
    ab> only one lazy value, but the situation is different when you
    ab> have a tuple of lazy values, and need to perform different
    ab> actions depending on which subset of them computes a
    ab> meaningful value.

What's wrong with:

try (* do something with *) (Lazy.force foo) with _ ->
try (* do something with *) (Lazy.force bar) with _ ->
try (* do something with *) (Lazy.force doh) with _ ->
(* fallback code *)

or

let lf x = try Some (Lazy.force x) with _ -> None

match (lf a, lf b, lf c, lf d) with
  ...

or even

type 'a result = Value of 'a | Exception of 'a

let lf x = try Value (Lazy.force x) with e -> Exception e

...

The change to the lazy datatype means you have to do a little extra
effort if you want to maintain this kind of information.  But it's not
really a huge deal.

John.
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