Once again, I find myself making code that I think may exist somewhere in the libraries: [[ -- list find and remove extract0 :: (a->Bool) -> [a] -> Maybe (a,[a]) extract0 p as = ex [] as where ex seen (a:more) | p a = Just (a,revcat seen more) | otherwise = ex (a:seen) more ex _ [] = Nothing extract :: (a->Bool) -> [a] -> Maybe (a,[a]) extract _ [] = Nothing extract p (a:as) | p a = Just (a,as) | otherwise = pref a (extract p as) where pref a1 (Just (a,as)) = Just (a,a1:as) pref _ Nothing = Nothing -- make list from reversed initial sublist and given trailing sublist revcat :: [a] -> [a] -> [a] revcat [] more = more revcat (a:as) more = revcat as (a:more) t1 = extract (==2) [1,2,3,4] == Just (2,[1,3,4]) t2 = extract (>2) [1,2,3,4] == Just (3,[1,2,4]) t3 = extract (>5) [1,2,3,4] == Nothing ts = and [t1,t2,t3] ]] (1) is there a function like 'extract'/'extract0' in the standard libraries? (2) if not, is there any significant difference in the efficiency of the above implementations? I think they both use linear time and space, but the 'extract0' performs a second traversal of (part of) the list to construct the result, where 'extract' has to pick apart and test the result from each recursive call. To my eye, the 'extract0' version looks better. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact