{-# LANGUAGE ViewPatterns, PatternSynonyms #-}

import Control.Arrow
import Data.List (uncons)

unsnoc :: [a] -> Maybe ([a], a)
unsnoc [] = Nothing
unsnoc [x] = Just ([], x)
unsnoc (x:xs) = (fmap.first) (x:) (unsnoc xs)

pattern xs :< x <- (unsnoc -> Just (xs,x)) where
  xs :< x = xs ++ [x]

pattern x :> xs <- (uncons -> Just (x,xs)) where
  x :> xs = x : xs

palindrome [] = True
palindrome [x] = True
palindrome (x:>xs:<y) = x == y && palindrome xs