unexpected thread blocking

[Susumu Katayama]

Hello,

Actually I am not sure if this is really a bug, but anyway the following 
program
behaves in different manners between with and without -O.
Interestingly, ghci provides the same (expected) behavior as -O.

\begin{code}

import Control.Concurrent
import Control.Concurrent.MVar
import IO(hPutStrLn, stderr)

main = do
	  mvar <-newEmptyMVar	  hPutStrLn stderr "forking..."
	  thread <-forkIO (undef `seq` (putMVar mvar hoge))
	  isEmpty <-isEmptyMVar mvar
	  hPutStrLn stderr ("isEmpty = " ++ show isEmpty)

undef = undef
hoge  = "hoge"

\end{code}

Results:
  $ ghc testConcurrent.lhs -o tto.exe
  $ tto.exe
  forking...
  isEmpt<<<DOES NOT COME BACK>>>

  $ ghc testConcurrent.lhs -o tto.exe -O
  $ tto.exe
  forking...
  isEmpty = True

The difference also applies for more complicated programs.
Since both ghc-5.04-2 on WinXP and ghc-5.04-1 on Solaris 8 yield the same 
result,
it is quite likely that the problem is platform independent.

Thank you very much.

- Susumu

% BTW, what I want to do is something like
%   undef = length hoge
%   hoge  = somethingLazy -- result of auto-generated module that may not 
terminate.
% or I may well say that I need takeMVar with a timeout parameter.
% I would appreciate also if someone knows a good workaround other than 
giving -O/--interact.