{-# OPTIONS_FRONTEND -F --pgmF=currypp #-}

import Control.SetFunctions

-- The implementation of coloring a map using a default rule
-- (by Sergio Antoy).

-- The states of the great north-west:
data State = WA | OR | ID | BC
 deriving (Eq,Show)

states :: [State]
states = [WA, OR, ID, BC]

-- Pair of states with a common border:
adjacent :: [(State, State)]
adjacent = [(WA,OR), (WA,ID), (WA,BC), (OR,ID), (ID,BC)]

data Color = Red | Green | Blue
 deriving (Eq,Show)

-- Some coloring for a state:
color :: State -> (State, Color)
color x = (x, Red ? Green ? Blue)

-- Checks a coloring w.r.t. adjacent states:
-- it is a failure if two states with the same color are adjacent:
check :: [(State, Color)] -> [(State,State)] -> [(State,Color)]
check (_ ++ [(s1,c)] ++ _ ++ [(s2,c)] ++ _) (_ ++ [(s1,s2)] ++ _) = failed
check'default x _ = x

main :: [(State, Color)]
main = check (map color states) adjacent