Re: Class ATs Question

29 Apr 2006

      In article 
,
 Dave Menendez  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/