"Existential types" sounds a bit scary :)
It's unfortunate that they've developed a scariness feeling associated with them. They can be used in strange ways, but simple uses are quite approachable. One way to think of them is like implementing an object-oriented interface. You know it's an object, but you can't do anything with it except use the methods of the interface.
--- {-# LANGUAGE ExistentialQuantification #-} data Square = Square ... data Circle = Circle ... class Perimeter a where perimeter :: a -> Double instance Perimeter Square where perimeter (Square ...) = ... instance Perimeter Circle where perimeter (Circle ...) = ... -- The 'a' is hidden here. The interface is defined by the class constraint. data Perimeterizable = forall a . (Perimeter a) => P a -- This is the accessor method for things Perimeterizable. getPerimeter (P x) = perimeter x vals :: [Perimeterizable] vals = [P Square, P Circle] perims = map getPerimeter vals --- Regards, Sean