Polymorph.hs: Haskell 98 encoding
Existential.hs: variation that uses forall
Heterogeneous.hs: variation that uses HList
Inference.hs: illustration of full type inference
 
We compose together record types from supertype(s) and subtype data.
We use one type class per OO class.
We use default methods in the type classes for OO method inheritance.
The Haskell 98 encoding models polym. collections through Either-typed tails.
The Existential.hs encoding wraps shapes in ex. envelopes instead.
The Heterogeneous.hs encoding builds a heterogeneous list instead.