gistfile1.hs

{-# LANGUAGE FlexibleContexts , ScopedTypeVariables , TypeFamilies #-}

import Data.Proxy

class Problem p where
  type State p :: *
  data Action p :: *

  actions :: State p -> [Action p]
  result :: State p -> Action p -> State p

  successor :: State p -> [(Action p,State p)]
  successor s = map (\a -> (a, (result s a))) $ actions s

  goal :: Proxy p -> State p
  goal = undefined

  isGoal :: (Problem p , Eq (State p)) => State p -> Bool
  isGoal s = s == goal (Proxy :: Proxy p)

data MovingRobot = MovingRobot

instance Problem MovingRobot where
  type State MovingRobot =  Int
  data Action MovingRobot = SLIDE_LEFT | SLIDE_RIGHT deriving (Show,Ord,Eq)

  actions x | x > goal (Proxy :: Proxy MovingRobot) = [SLIDE_LEFT]
              | x < goal (Proxy :: Proxy MovingRobot) = [SLIDE_RIGHT]
              | otherwise = []

  goal (Proxy :: Proxy MovingRobot) = 0

  result  x SLIDE_LEFT  = (x - 1)
  result  x SLIDE_RIGHT = (x + 1)