[Haskell-cafe] Dispatch a type-function on the existence (or not) of instances?

[Hans Höglund]

Dear all,

I have been curious about the ability to detect the presence of a certain instance (ClassFoo TypeBar) in the type system.
Specifically, is it possible to "dispatch" a type on the existence (or not) of such an instance. For example given two functions:

withInstance :: (ClassFoo TypeBar) => TypeIfInstanceExists
withoutInstance :: TypeIfInstanceDoesNotExists

I would be able to consolidate them into something like this:

withOrWithoutInstance :: 
    (r ~ InstanceExists ClassFoo TypeBar, 
     a ~ If r TypeIfInstanceExists TypeIfInstanceDoesNotExists) => a

I guess what I need is:

1) A type-level "if".
2) The possibility of "converting" a constraint into a type-level bool.

I am sure (1) is possible but have no idea about (2). Anyone?

Best regards,
Hans