simulating dynamic dispatch

Fergus Henderson
Fri, 21 Mar 2003 17:49:44 +1100

On 20-Mar-2003, Hal Daume III <hdaume@ISI.EDU> wrote:
> i'm hoping to be able to simulate a sort of dynamic dispatch based on
> class instances.  basically a function which takes a value and depending
> on what classes it is an instance of, does something.

I call this feature "dynamic type class casts".

But there are some difficulties in defining the semantics of this.
What should it mean in the presence of dynamic loading?
If you're allowed to dynamically load new modules that might contain
new instance declarations, and you're allowed to check (in a
non-IO-monad context) whether a type is an instance of a type class,
and you want things to remain referentially transparent, then you've
got a problem.

> i tried something like:
> class FooBar a where 
>     wasFoo :: a -> Maybe FB  -- will be MkFoo
>     wasBar :: a -> Maybe FB  -- will be MkBar
>     wasFoo _ = Nothing
>     wasBar _ = Nothing
> instance Foo a => FooBar a where
>     wasFoo a = Just (MkFoo a)
> instance Bar a => FooBar a where
>     wasBar a = Just (MkBar a)
> but this complains about duplicate instance declarations (for obvious
> reasons).

You can use instance declarations for whichever ground types you need, e.g.

	instance FooBar Int where ...
	instance FooBar String where ...

Fergus Henderson <>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <>  |     -- the last words of T. S. Garp.