Types vs. Classes

19 May 2006

      Occasionally in library proposals one comes across classes of this form:

   class Thingy a where
       foo :: a -> this
       bar :: a -> that
       spong :: a -> theotherthing

Such classes can be replaced by data-types, which turn out to also be 
more general:

   data Thingy = MkThingy
   {
       foo :: this,
       bar :: that,
       spong :: theotherthing
   }

My question: is there a reason not to use types? I'm particularly 
interested in two cases that I've come across, references and streams. 
Here's the class version:

   class (Monad m) => Ref m r | r -> m, m -> r where
       newRef   :: a -> m (r a)
       readRef  :: r a -> m a
       writeRef :: r a -> a -> m ()

   class (Monad m) => Stream m h where
       vPutStrLn :: h -> String -> m ()
       vGetContents :: h -> m String
       vIsEOF :: h -> m Bool
       vClose :: h -> m ()
       -- etc.

And here's the type version:

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

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

   data Stream m = MkStream
   {
       vPutStrLn :: String -> m (),
       vGetContents :: m String,
       vIsEOF :: m Bool,
       vClose :: m (),
       -- etc.
   }

Is there any reason not to prefer using types?

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