How to make a type with restrictions

Question

For example I want to make a type MyType of integer triples. But not just Cartesian product of three Integer, I want the type to represent all (x, y, z) such that x + y + z

Answer 1

Just elaborating on ivanm's answer:

data MyType = MT {x :: Int, y :: Int, z :: Int } deriving Show

createMyType :: Int -> Int -> Int -> Maybe MyType
createMyType a b c
    | a + b + c == 5 = Just MT { x = a, y = b, z = c }
    | otherwise      = Nothing