On Tue, 2006-01-31 at 13:28 +0000, Duncan Coutts wrote:
> On Tue, 2006-01-31 at 13:59 +0100, Wolfgang Jeltsch wrote:
> > Am Montag, 30. Januar 2006 19:02 schrieb Duncan Coutts:
> > > [...]
> >
> > > I have often thought that it would be useful to have an existential
> > > corresponding to a class.
> >
> > How would this work with multi-parameter classes, constructor classes,
> > etc.?
> > If you propose something that only works in conjunction with a special kind
> > of classes I would hesitate to include such thing in a Haskell standard.
>
> As John Mecham said it'd be for single parameter type class with a
> parameter of kind *.
>
> But you're probably right that people should get more experience with
> using this technique before giving special support in the language to
> make it convenient.
>
> As Bulat noted we can already use this construction:
>
> class (Monad m) => Stream m h | h->m where
> vClose :: h -> m ()
> vIsEOF :: h -> m Bool
> .....
>
> data Handle = forall h . (Stream IO h) => Handle h
>
> instance Stream IO Handle where
> vClose (Handle h) = vClose h
> vIsEOF (Handle h) = vIsEOF h
> .....
>
> But we have to give the name of the most general instance a different
> name to the class which is rather inconvenient.
>
> So perhaps we should start with allowing a class a data type to have the
> same name and in a future standard think about making it easy to define
> Bulat's Handle instance above with a short hand like:
>
> class (Monad m) => Stream m h | h->m where
> vClose :: h -> m ()
> vIsEOF :: h -> m Bool
> .....
> deriving data Stream
Actually this is unnecessary. All we need is:
class (Monad m) => Stream m h | h->m where
vClose :: h -> m ()
vIsEOF :: h -> m Bool
newtype Handle = exists s. Stream s => Handle s
deriving Stream
So all we need is existentials and newtype-deriving.
Duncan
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