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

19 Oct 2011


      On Tue, Oct 18, 2011 at 2:47 PM, Daniel Fischer <
daniel.is.fischer@googlemail.com> wrote:
...
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
I was thinking of some kind of mixed solution over lunch. This looks better
than what I had in mind. But what does "well-formed" mean in this context?
...
quality :: Hand -> Int
quality HighCard{} = 0
quality PairOf{} = 1
...
quality StraightFlush{} = 8
Could this be replaced by an assoc list of some kind?

I was actually contemplating adding a quality field to the record.
...
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
What I'd really like to do is collapse the three types of comparison (i.e. -
hand, rank hand, rank minorrank hand) into one comparison each.

    Thanks,
     <mike