Playing with "Pointers"

import Data.Array
import Control.Comonad



data Pointer a = Pointer !(Array Int a) !Int
  deriving (Show)



instance Functor Pointer where
  fmap f (Pointer a i) = Pointer (fmap f a) i



instance Comonad Pointer where
  extract  (Pointer a i) = a ! i
  extend f (Pointer a i) = Pointer a' i
    where
    a' = listArray (bounds a)
         [f (Pointer a j) | j <- range (bounds a)]



next :: Pointer a -> Maybe (Pointer a)
next (Pointer a i)
  | i < hi    = Just (Pointer a (i+1))
  | otherwise = Nothing
  where
  (_,hi) = bounds a



prev :: Pointer a -> Maybe (Pointer a)
prev (Pointer a i)
  | lo < i    = Just (Pointer a (i-1))
  | otherwise = Nothing
  where
  (lo,_) = bounds a



rule16 :: Pointer Bool -> Bool
rule16 p = case prev p of
  Nothing -> False
  Just p1 -> extract p1