Class ATs Question

[Ashley Yakeley]

In article 
<r02010400-1046-4B28DF02D73311DA9B60001451045A8A@[10.0.1.3]>,
 Dave Menendez <dave at zednenem.com> wrote:

> This is one of the motivating examples for associated types. You would
> define Ref as,
> 
>     class (Monad m) => Ref m where
>         data Ref m a
>         
>         newRef   :: a -> m (Ref m a)
>         readRef  :: Ref m a -> m a
>         writeRef :: Ref m a -> a -> m ()
> 
> This declares a one-to-one relation between "m" and "Ref m". That is,
> you are guaranteed that Ref (ST s1) == Ref (ST s2) iff s1 == s2.

You can also do it without associated types or MPTCs:

  data Ref m a = MkRef
  {
    readRef  :: m a
    writeRef :: a -> m ()
  }

  class (Monad m) => Ref m where
    newRef   :: a -> m (Ref m a)

This is also more general.

-- 
Ashley Yakeley, Seattle WA
WWEWDD? http://www.cs.utexas.edu/users/EWD/