From: Jacek Generowicz

So my question is, what would be the Haskell approach to combining heterogeneity (as provided by variant (algebraic) types) with type-extensibility (as provided by type classes), without losing the benefits of Haskell's type system?

I haven't looked into Template Haskell, but i guess that it is likely to provide a solution. But is there a solution in plain Haskell?

It's not plain Haskell, but I'm surprised nobody mentioned the ExistentialQuantification extension, which unless I'm missing something provides exactly what you want. John