Using the function partition from the List module, a control stack overflow occurs when evaluating the following expression: List> head $ fst $ partition even [1..] (35929 reductions, 63867 cells) ERROR: Control stack overflow The definition of partition in both hugs and GHC libraries is: partition :: (a -> Bool) -> [a] -> ([a],[a]) partition p xs = foldr select ([],[]) xs where select x (ts,fs) | p x = (x:ts,fs) | otherwise = (ts,x:fs) The helper-function select is strict in its second argument because of the pattern match on the tuple (ts,fs). Making the pattern match lazy by adding a ~ solves the problem! partition :: (a -> Bool) -> [a] -> ([a],[a]) partition p xs = foldr select ([],[]) xs where select x ~(ts,fs) | p x = (x:ts,fs) | otherwise = (ts,x:fs) Cheers, Bastiaan