# List.partition should be more lazy

**Bastiaan Heeren
**
bastiaan@cs.uu.nl

*Fri, 23 Nov 2001 14:54:49 +0100 (MET)*

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