hi, it just took me an eternity to undestand a bug in my code *after* i had narrowed it down to eight lines or so. and as these bugs go, i feel very good about having found it and have to share it. although it's probably not that exciting to anybody except me. (-: here is the bug, narrowed to four lines. a function that only sometimes terminates. f = do n <- randomRIO (0,5) let l = replicate n '*' i = join $ repeat l -- infinite extension print (take 12 i) how many seconds does it take you to shout "you idiot!"? :-) spoiler below. (what would have been your fix?) cheers, matthias =================================================================== spoiler. the problem is that l is sometimes empty, and taking an element out of an infinite concatenation of empty lists takes even longer than understanding that that takes quite long. just deal with the empty-list case separately: g = do n <- randomRIO (0,5) let l = replicate n '*' i | null l = [] | otherwise = join $ repeat l print (take 12 i)