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