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Date: Tue, 17 Feb 2009 16:54:55 +0100
From: victor-caml-list@nicollet.net
To: caml-list@inria.fr
Subject: Lazy and Threads
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X-Spam: no; 0.00; expr:01 expr:01 1.0:98 threads:01 threads:01 undefined:01 undefined:01 closure:01 closure:01 lazy:02 lazy:02 functional:02 functional:02 mutex:03 let:03 

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