{-# OPTIONS_GHC -cpp -fglasgow-exts #-}

import Data.Array.Base
import Data.Array.IO
import Data.Array.IArray
import Data.Array.MArray


class ListLike c e where
    nullL :: c e -> Bool
    headL :: c e -> e
    tailL :: c e -> c e

instance ListLike [] e where
    nullL = null
    headL = head
    tailL = tail

instance ListLike Slice e where
    nullL = nullS
    headL = headS
    tailL = tailS

-- Instance of ListLike: slice over a regular array
data Slice e = Slice Int (Array Int e)
arraySlice arr | (0,_) <- bounds arr  =  Slice 0 arr
listSlice xs         =  arraySlice (listArray (0,length(xs)-1) xs)
nullS (Slice n arr)  =  n>hi  where (0,hi) = bounds arr
headS (Slice n arr)  =  arr!n
tailS (Slice n arr)  =  Slice (n+1) arr


-- Definitions of functions which use ListLike interface
-- to process any list or array
foldrL :: (ListLike c a) => (a -> b -> b) -> b -> c a -> b
foldrL k z xx | nullL xx  = z
              | otherwise = x `k` foldrL k z xs
                              where x  = headL xx
                                    xs = tailL xx

lengthL xs = foldrL (\_->(+1)) 0 xs
sumL  xs   = foldrL (+) 0 xs
prodL xs   = foldrL (*) 1 xs

main = do print$ lengthL [1..10]
          print$ lengthL (listSlice [1..10])
          print$ sumL [1..10]
          print$ sumL (listSlice [1..10])