provide a Seq instance for the prelude list.

hs/DataStructures.hs

@@ -75,6 +75,20 @@ reverse s = rev s nil
                  Nothing -> nil
                  Just x -> rev (rest l) $ cons x r
instance Seq [] where
    first [] = Nothing
    first (x:_) = Just x
    cons x xs = x:xs
    nil = []
    rest [] = []
    rest (_:xs) = xs
    isEmpty [] = True
    isEmpty _ = False
data List a = Nil | Cons a (List a) deriving Show
-- first and cons are O(1), everything else is O(n)