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

[Mike Meyer]

On Tue, Oct 18, 2011 at 4:01 PM, Daniel Fischer <
daniel.is.fischer at googlemail.com> wrote:

> On Wednesday 19 October 2011, 00:27:19, Mike Meyer wrote:
> > On Tue, Oct 18, 2011 at 2:47 PM, Daniel Fischer <
> > I was actually contemplating adding a quality field to the record.
> If you don't export the constructors and take care to create only hands
> with the correct quality, that is possible. I'm not sure if it gains
> anything, though.
>

That's pretty much what playing with it turned up.


> >
> > > 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.
> I don't see what you mean, could you elaborate?
>

The hand types break down into three cases:
ordered by the cards in the hand (HighCard, Straight, Flush, StraightFlush)
ordered by a rank and then the cards (PairOf, ThreeOfAKind, FourOfAKind)
ordered by two ranks and then the cards (TwoPair, FullHouse)

I'd like to be able to match against the pattern ignoring the constructor,
like so:

instance Order Hand where
   compare h1 h2 = case compare (quality h1) (quality h2) of
             EQ -> ex_compare h1 h2
             ne -> ne
      where   ex_compare (_ c1) (_ c2) = compare c1 c2
                   ex_compare (_ r1 c1) (_ r2 c2) = case compare r1 r2 of
...
                   ex_compare (_ r1 s1 c1) (_ r2 s2 c2) = case compare r1 r2
of ...

Or something similar.

     <mike
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