Re: [Haskell-cafe] semantics of concurrent program depends on -O level, -f[no-]omit-yields

... try flushing stdout within the forked thread,

I did. The behaviour is still as described: depends on -O0/2, [no]omit-yield, and small changes in the source. While I agree with the general point - why would I need to hFlush after putStrLn? hGetBuffering stdout tells me it's LineBuffering, and putStrLn does write a line? - J.