From http://en.wikibooks.org/wiki/Haskell/YAHT/Type_basics#Multiple_Constructors

tuples.hs

data Tuple a b c d =
	OneTuple a |
	TwoTuple a b |
	ThreeTuple a b c |
	FourTuple a b c d

tuple1 :: Tuple a b c d -> Maybe a
tuple1 (OneTuple a) = Just a
tuple1 (TwoTuple a b) = Just a
tuple1 (ThreeTuple a b c) = Just a
tuple1 (FourTuple a b c d) = Just a

tuple2 :: Tuple a b c d -> Maybe b
tuple2 (TwoTuple a b) = Just b
tuple2 (ThreeTuple a b c) = Just b
tuple2 (FourTuple a b c d) = Just b
tuple2 _ = Nothing

tuple3 :: Tuple a b c d -> Maybe c
tuple3 (ThreeTuple a b c) = Just c
tuple3 (FourTuple a b c d) = Just c
tuple3 _ = Nothing

tuple4 :: Tuple a b c d -> Maybe d
tuple4 (FourTuple a b c d) = Just d
tuple4 _ = Nothing

tupletuple :: Tuple a b c d -> Either (Either a (a, b)) (Either (a, b, c) (a, b, c, d))
tupletuple (OneTuple a) = Left (Left a)
tupletuple (TwoTuple a b) = Left (Right (a, b))
tupletuple (ThreeTuple a b c) = Right (Left (a, b, c))
tupletuple (FourTuple a b c d) = Right (Right (a, b, c, d))