I wrote a program in Haskell that does real-time MIDI playback. This involves computing what are called MIDI "events" (things like notes and volume controls) and then using the PortMidi library to play back the events. PortMidi also provides a time reading function that is accurate to 1 millisecond which is useful for timing the playback accurately. I used 'evaluate' in the IO Monad to make sure that all events were fully computed before playback started. However I was still getting delays (timing errors). I added some code to the program that wrote the data to disk before playing it back, and the timing errors went away. Apparently 'evaluate' was not forcing a complete evaluation, while writing a disk file was. Or at least I think that's what happened. But writing to disk is a hack. What can I do to get 'evaluate' to work? A simplified version of the code is here: (the actual program is pretty long and complex so I posted something which is stripped down but I hope retains the essential features) http://lpaste.net/144762