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

-- Map coloring:

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

-- The list of these states:
states :: [State]
states = [WA, OR, ID, BC]

-- List of state pairs with common border:
adjacent :: [(State,State)]
adjacent = [(WA,OR), (WA,ID), (WA,BC), (OR,ID), (ID,BC)]

-- The color to use:
data Color = Red | Green | Blue
 deriving (Eq, Show)

-- Color the list of states (possible solutions):
color :: State -> (State,Color)
--color s = (s, Red ? Green ? Blue)
color s = (s, aValue)

-- Correct coloring: it is failure if two states with a common border have
-- the same color:
check :: [(State,Color)] -> [(State,State)] -> [(State,Color)]
--       possible coloring  adjacent states    correct coloring
check (_ ++ [(s1,c)] ++ _ ++ [(s2,c)] ++ _) (_ ++ [(s1,s2)] ++ _) = failed
check'default scs _ = scs

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