Making a custom datatype ord-able

Question

Can someone explain to me how do I make a custom data type ord-able?

** I'm not allowed to make modifications to Suit itself, eg. deriving (Eq, Ord)

data Suit = Clubs | Diamonds | Hearts | Spades deriving (Eq)

My Attempt:

instance Ord Suit where
    compare suit1 suit2 = compare suit1 suit2

but that seems to go on a continuous loop and doesn't halt.

Answer 1

The definition of Ord looks something like (but not quite)

class Ord a where
    compare :: a -> a -> Ordering

and Ordering has three possible values: LT, EQ, GT .

So, you need to define what the result of each comparison should be. Something like:

instance Ord Suit where
    compare Clubs Diamonds    = LT
    compare Diamonds Clubs    = GT
    compare Diamonds Diamonds = EQ
    compare Diamonds _        = LT -- Diamonds are lower than everything besides Clubs and Diamonds

Your actual ordering may be different, but that should give you the basic idea.

Answer 2

You can use standalone deriving with the same effect.

deriving instance Enum Suit
deriving instance Ord Suit

Answer 3

A way to write a custom Ord instance where you don't have to spell out the result of each comparison is:

instance Ord Suit where
    compare a b = compare (relativeRank a) (relativeRank b) where
         relativeRank Diamonds = 1
         relativeRank Clubs = 2
         relativeRank Hearts = 3
         relativeRank Spades = 4

Here, you only need to mention each constructor once, and you can easily decide on a different ordering.

You can just as well use compare Data.Function.on relativeRank , but this is maybe easier to understand.