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

[Mike Meyer]

On Tue, Oct 18, 2011 at 2:47 PM, Daniel Fischer <
daniel.is.fischer at 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
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