[Haskell] Re: Parameterized Show

[Keean Schupke]

Do you need a language extension at all? You can certainly
do it with the existing extensions!

data ShowDict a
instance Show (ShowDict a) where
    showsPrec _ (ShowDict a) = ...

    Keean

George Russell wrote:

> Graham Klyne wrote (snipped):
> > I like the principle of parameterizing Show to allow for different 
> encoding
> > environments (indeed, I had wondered as I was writing my earlier 
> message if
> > the two cases were really sufficient).  Indeed, in the application area
> > that interests me (Semantic Web) it could be beneficial to have 
> different
> > encoding options corresponding to different serializations of RDF (e.g.
> > RDF/XML and Notation3).
>
> I like the idea too, not just for Show but for any instances.  It 
> seems to
> me that in general you should be able to combine the convenience of the
> Haskell type system with the power of Standard ML's structures and 
> functors.
> Something along these lines was done by Kahl & Scheffczyk ("Named 
> Instances for
> Haskell Type Classes", Haskell Workshop 2001).
>
> In general I suspect you can do this without even having to extend the 
> existing
> Haskell typesystem very far.  The following language extensions seem 
> sufficient.
> (1) for a class declaration, a way of declaring that a certain type 
> represents
> a dictionary of functions for that class, for example
>
>    class Show a (ShowDict,appFn) where
>      ...
>
> would define the new class "Show" but also the new type (ShowDict a) 
> representing
> dictionaries for that class.  It also declares a value appFn which I 
> shall explain
> in a moment.
>
> (2) for an instance declaration, a way of "naming" the corresponding 
> dictionary.
>
>    instance Show Int (intShowDict) where
>
> would create a value "intShowDict :: ShowDict Int"
>
> (3) a way of using the dictionary.  For this we need (appFn), 
> declaraed by the type
> declaration.  appFn has the unorthodox type
>
>    appFn :: ShowDict a -> (forall a . Show a => b) -> b
>
>
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