Question

I’m trying to put this data type in a Haskell Set, but I don’t want to give it a general instance of Ord. So I want to give the set an ordering on y-coördinate but without instance Ord Vector. Is this possible?

    data Vector = V 
    { x :: Double
    , y :: Double
    } deriving (Eq)

Answer 1

Set requires you to use the default Ord instance of the element type.

If you want to use a different Ord instance, the standard way to do that is to use a custom newtype wrapper and then write an Ord instance for that:

newtype Y = Y { unY :: Vector } deriving Eq
instance Ord Y where compare = comparing ((y . unY) &&& (x . unY))

But since this way of comparing is equivalent to the way binary tuples are compared, KennyTM’s solution is the simplest here.