view_patterns_example_with_data_sequence.hs

{-# LANGUAGE ViewPatterns #-}

import qualified Data.Sequence as S
import System.Environment
import Control.Monad

data List a = Nil | Cons a (List a)

mylast :: S.Seq a -> Maybe a
mylast (S.viewr -> xs S.:> x) = Just x
mylast (S.viewr -> S.EmptyR)  = Nothing

main = do
    x <- liftM (read. head) getArgs :: IO Int
    putStrLn . show $ mylast $ S.fromList [1..x]