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

[Taylor Hedberg]

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_types
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