Re: [Haskell-cafe] Type classes, collections, sum types, closures, and a massive headache

If I understand your message well enough, I think you are looking for GHC's `ExistentialQuantification` extension. Building heterogeneous collections is a common example of what existential types are useful for. Take a look at this wiki page [1]; there is an example of how to accomplish this there, along with a handful of other techniques. [1] http://www.haskell.org/haskellwiki/Heterogenous_collections#Existential_type...