http://stackoverflow.com/questions/40492860/strategies-to-deal-with-problems-when-downcasting-looks-like-the-best-fit

All.hs

module All where
import Life
import Grass
import Sheep
import Wolf

main = do
  putStrLn "Grass:"
  print $ eat Grass [Grass]
  print $ eat Grass [Sheep]
  print $ eat Grass [Wolf]
  putStrLn "Sheep:"
  print $ eat Sheep [Grass]
  print $ eat Sheep [Sheep]
  print $ eat Sheep [Wolf]
  putStrLn "Wolf:"
  print $ eat Wolf [Grass]
  print $ eat Wolf [Sheep]
  print $ eat Wolf [Wolf]

Grass.hs

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}
module Grass where
import Life

data Grass = Grass deriving Show
instance Life Grass where

Life.hs

{-# LANGUAGE Rank2Types, FlexibleInstances, MultiParamTypeClasses, FunctionalDependencies,OverlappingInstances #-}
module Life where

type World = (CanEat a b) => [a] -- or something more complex, cells, maps, ..

class Life a where

class (Life a, Life b)  => CanEat a b   where
  canEat :: a -> b -> Bool

instance (Life a, Life b) => CanEat a b where
  canEat _ _ = False

eat :: (CanEat a b) => a -> [b] -> [b]
eat _ [] = []
eat a (b:bs) = if (canEat a b) then bs else b : eat a bs

Sheep.hs

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}
module Sheep where
import Life
import Grass

data Sheep = Sheep deriving Show

instance Life Sheep where

instance CanEat Sheep Grass where
  canEat _ _ = True

Wolf.hs

{-# LANGUAGE FlexibleInstances, MultiParamTypeClasses #-}
module Wolf where
import Life
import Sheep

data Wolf = Wolf deriving Show

instance Life Wolf where

instance CanEat Wolf Sheep where
  canEat _ _ = True