[Haskell-beginners] Ord and Eq instances for complex types

[Daniel Fischer]

On Tuesday 18 October 2011, 23:13:03, Mike Meyer wrote:
> I'm working on a problem dealing with poker hands, and need to rank
> them. The Ord instance seems like a natural for this, but the code came
> out with a lot of repetitions in it.

> 
> Is there some third alternative I've overlooked that combines the best
> features of both approaches?
> 
>   Thanks,
>   <mike

Well, you could use record syntax in the Hand type to avoid defining cards 
in the Eq instance,

data Hand
    = HighCard { cards :: [Card] }
    | PairOf { rank :: Rank, cards :: [Card }
...
    | FulHouse { rank, minorRank :: Rank, cards :: [Card] }
    | FourOfAKind { rank :: Rank, cards :: [Card] }
    | StraightFlush { cards :: [Card] }
      deriving Show

instance Eq Hand where  -- if your hands are well-formed, you
    h1 == h2 = cards h1 == cards h2  -- could also derive Eq

quality :: Hand -> Int
quality HighCard{} = 0
quality PairOf{} = 1
...
quality StraightFlush{} = 8

instance Ord Hand where
    compare h1 h2 = 
      case compare (quality h1) (quality h2) of
        EQ -> case h1 of
                StraightFlush c1 -> compare c1 (cards h2)
                FourOfAKind r1 c1 -> 
                  case compare r1 (rank h2) of
                    EQ -> compare c1 (cards h2)
                    other -> other
                ...
        other -> other