On Thu, Jun 10, 2010 at 11:14 PM, Daniel Fischer
On Thursday 10 June 2010 22:01:38, Dupont Corentin wrote:
Hello Maciej, i tried this out, but it didn't worked.
Daniel,
I added a (Show a) constraint to Equal:
data Obs a where Player :: Obs Integer Turn :: Obs Integer Official :: Obs Bool Equ :: (Show a, Eq a) => Obs a -> Obs a -> Obs Bool
--woops!!
Plus :: (Num a) => Obs a -> Obs a -> Obs a Time :: (Num a) => Obs a -> Obs a -> Obs a Minus :: (Num a) => Obs a -> Obs a -> Obs a Konst :: a -> Obs a And :: Obs Bool -> Obs Bool -> Obs Bool Or :: Obs Bool -> Obs Bool -> Obs Bool
It works for the Show instance, but not Eq. By the way, shouldn't the Show constraint be on the instance and not on the datatype declaration?
Can't be here, because of Equ :: Obs a -> Obs a -> Obs Bool
You forget the parameter a, and you can't recover it in the instance declaration. So you have to provide the Show instance for a on construction, i.e. put the constraint on the data constructor.
Anyway, is my Obs construction revelant at all? What i want to do is to provide an EDSL to the user to test things about the state of the game (for the Nomic game i'm making). Obs will be then embedded in another EDSL to construct Nomic's rules.
I'd prefer to keep the datatype as generic as possible...
There is really no way to make my Obs datatype an instance of Eq and Show??
Show can work (should with the constraint on Equ), Eq is hairy.
instance Show t => Show (Obs t) where show (Equ a b) = show a ++ " `Equal` " ++ show b show (Plus a b) = ... show (Konst x) = "Konst " ++ show x ...
For an Eq instance, you have the problem that
Equ (Konst True) (Konst False) and Equ Player Turn
both have the type Obs Bool, but have been constructed from different types, so you can't compare (Konst True) and Player. I don't see a nice way to work around that.
These is a dirty way: compare the string representation of the rules. They should be unique. Corentin