Funny, I was trying the same Homework, implemented interleaveStreams using the same algorithm you did (taking one item from each stream per recursive call), and got the same result with the ruler function hanging. It was also fixed by changing interleaveStreams to take from just one stream per call and switch back and forth between streams. This happened even though I implemented my ruler function itself recursively: ruler' :: Integer -> Stream Integer ruler' n = interleaveStreams (streamRepeat n) (ruler' (n + 1)) ruler = ruler' 0 Sorry, but I also don't know why. I speculate that if one were to expand it out on pencil and paper, they'd find that the interleaved stream somehow expands at a different rate than the ruler (half or twice). -Curt