On 15/05/15 21:59, Teodor Vlasov wrote:

Hello everyone,

I'm trying to implement traversal over data structures and found Data.Typeable very useful for that. But it seems to be impossible to cast to specific typeclass, so that we can cast Typeable to something that is Show'able for example. It'd be usefull to have something like:

data CanShow = forall a. CanShow a

toConcrete :: (Typeable a) => a -> Maybe CanShow

Implementing such datatypes for needed classes would allow to do some kind of class casting.

Is it possible to implement this somehow? Is it a limitation of the implementation or a principal one?

Casting to a type works because all information is there, you only need to convince the type system that it is fine to use it. "Casting to a class" doesn't work because the type class implementation (the method dictionary) is not contained in a value of that type. Instead, compiler plugs it in during compile time where it is needed. What you can do is try to cast a value to a number of known types, and if one of those casts succeeds, get hold of the dictionary corresponding to that monomorphic type. What you *cannot* do is to find out at runtime which instances the compiler knew about at compile time (let alone recover the dictionary); that information is gone. Roman