[Haskell-beginners] Concrete instance of abstract class

[Adrian May]

Hi Brent,


> I would do this using an associated
> type, like so:
>
> class Turtlish t where
>   type TState t :: *
>   pic           :: t -> Dia
>   state         :: t -> TState t
>   move          :: TState t -> t -> t
>   (>>>),(+++)   :: t -> t -> t
>   x >>> y = x +++ move (state x) y
>
>
Bingo. Just what I needed. But is this a Haskellish thing to do or am I
showing my C++ background by even wanting something like this? Anyway, for
the sake of any future googlers, I needed  {-# LANGUAGE TypeFamilies #-} and
completed the instance with:

instance Turtlish TurtWorld where
  type TState TurtWorld = TurtState
  ...

Thanks,
Adrian.



>
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