[Haskell-cafe] Trying to sort out multiparameter type classes and
jeremy at goop.org
Tue Dec 1 17:34:46 EST 2009
I'm playing around with some types to represent a game board (like Go,
Chess, Scrabble, etc).
I'm using a type class to represent the basic Board interface, so I can
change the implementation freely:
class Board b pos piece where
-- Update board with piece played at pos
play :: b pos piece -> pos -> piece -> b pos piece
-- Query pos to get piece (Nothing if off board)
at :: b pos piece -> pos -> Maybe piece
-- Empty board
empty :: b pos piece
and a Position on the board is represented thus:
class Position p where
up :: p -> p
down :: p -> p
left :: p -> p
right :: p -> p
With a concrete implementation using a tuple:
instance (Enum c,Enum r) => Position (c,r) where
up = second pred
down = second succ
left = first pred
right = first succ
My initial Board is a function: position -> Maybe piece, but I'm having
a hard time writing the instance for it. My first attempt is:
instance Board (pos -> Maybe piece) pos piece where
empty = \_ -> Nothing
at = ($)
play b pos piece = move
where move pos' | pos' == pos = Just piece
| otherwise = b pos'
but ghci complains:
Expected kind `* -> * -> *', but `pos -> Maybe piece' has kind `*'
In the instance declaration for `Board (pos
-> Maybe piece) pos piece'
Playing around with parentheses on the instance line got various similar
messages, but I couldn't get anything to work.
What am I missing here?
One thing that strikes me is that "Board (pos -> Maybe piece) pos piece"
has a lot of redundancy, and I'm wondering if I'm defining the Board
type class wrong in the first place.
Given that the "b" type parameter necessarily defines the position and
pieces, I tried using dependent types:
class Board b | b -> pos, b -> piece where ...
but this complains "Not in scope: type variable `pos'/`piece'", so I
tried sprinkling some existential types around as well:
class (forall pos piece. b pos piece) => Board b | b -> pos, b -> piece where ...
(with and without the dependent types) but this just complains
"malformed class assertion".
As you can probably tell, I'm just thrashing around trying things
without really understanding what's going on here, so I'm hoping someone
can give me some useful pointers.
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