At a minimum, this seems like a bug in the documentation. The documentation states very clearly that Undefined is called when a value is recursively forced.  Clearly, you get the same error when you force a lazy value that is in the process of being forced for the first time....

It does seem like fixing the behavior to match the current documentation would be superior to fixing the documentation to match the current behavior.

I'd suggest adding a bug report to the bug tracker.

y

On Tue, Feb 17, 2009 at 10:54 AM, <victor-caml-list@nicollet.net> wrote:
Hello,

I'm working with both lazy expressions and threads, and noticed that the
evaluation of lazy expressions is not thread-safe:

 let expr = lazy (Thread.delay 1.0)

 let _ =
   let thread = Thread.create (fun () -> Lazy.force expr) () in
     Lazy.force expr;
     Thread.join thread

The second thread to attempt an evaluation dies with Lazy.Undefined. The
source of this appears to be the standard Lazy.force behavior, which
replaces the closure in the lazy-block with a raise_undefined closure in
order to detect self-recursing lazy expressions.

Aside from handling a mutex myself (which I don't find very elegant for
a read operation in a pure functional program) is there a solution I can
use to manipulate lazy expressions in a pure functional multi-threaded
program?

--
Victor Nicollet

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