[Haskell-cafe] Class

[Sebastian Sylvan]

On 11/2/06, Slavomir Kaslev <slavomir.kaslev at gmail.com> wrote:
> On 11/2/06, Sebastian Sylvan <sylvan at student.chalmers.se> wrote:
> > On 11/2/06, Slavomir Kaslev <slavomir.kaslev at gmail.com> wrote:
> > > Little by little, I think I am getting closer.
> > >
> > > class Show a => Visible a where
> > >     toString :: a -> String
> > >     toString = show
> > >     size :: a -> Int
> > >     size = length . show
> > >
> > > Is just declaration, not definition. It doesn't define anything, even
> > > though it has default implementations for toString and size one still
> > > needs to define instance of it. Right?
> > >
> > > As Sebastian Sylvan proposed, I probably need something like this:
> > >
> > > class Visible a where
> > >     toString :: a -> String
> > >     size :: a -> Int
> > >
> > > instance Show a => Visible a where
> > >     toString = show
> > >     size = length . show
> > >
> > > But it seems ghc doesn't like instance definitions like 'instance Show
> > > a => Visible a where ...'. Why?
> >
> > This is not Haskell98 since instance declarations of this form could
> > cause the type checker to go into an infinite loop. This particular
> > one is okay, though, but you have to start ghc with
> > -fallow-undecidable-instances and -fglasgow-exts I'm afraid.
> >
> >
> > --
> > Sebastian Sylvan
> > +46(0)736-818655
> > UIN: 44640862
> >
>
> Thanks, Sebastian. That was helpful. Are there any papers on the subject?
>

The users guid to GHC explains it quite clearly, in my opinion, though
I'm sure there are some papers on the subject as well (I can't think
of any right now, though).

/S


-- 
Sebastian Sylvan
+46(0)736-818655
UIN: 44640862