On Wed, 2007-05-02 at 16:33 +0100, Neil Mitchell wrote:
Hi
Thanks to dcoutts, I have now come up with an answer. I don't understand why it works now, but not before.
main = p_System_IO_hGetChar 1 `seq` p_System_IO_hGetChar 2 `seq` putStrLn "done" This is fine (though note that p_System_IO_hGetChar 1 is a constant) the real problem is here: {-# NOINLINE p_System_IO_hGetChar #-} p_System_IO_hGetChar h = trace "i am here" $ unsafePerformIO $ getchar >>= \c -> print c >> return (if c == (-1) then 0 else chr_ c) You've tried to trick ghc into always calling this by passing a dummy 'h' parameter. Then 'h' is never used in the body. Note however that the whole body of this function is a constant and so ghc can (and at -O2 does) float it out as a CAF. This means you get the side effects of p_System_IO_hGetChar at most once. The solution of course is to use a full data dependency like IO or ST uses.
I do remember than browsing either Core or STG is not a fun thing to do...
So yeah, we see the above CAFs and let floating by looking at the core. We could do with a prettier pretty printer for core, I agree. Duncan