  Haskell provides type-class-bounded and parametric polymorphism as opposed to
subtype polymorphism of object-oriented languages such as Java and OCaml. It is
a contentious question whether Haskell 98 without extensions, or with common
extensions, or with new extensions can fully support conventional
object-oriented programming with encapsulation, mutable state, inheritance,
overriding, statically checked implicit and explicit subtyping, and so on. We
systematically substantiate that Haskell 98, with some common extensions,
supports all the conventional OO features plus more advanced ones, including
first-class lexically scoped classes, implicitly polymorphic classes, flexible
multiple inheritance, safe downcasts and safe co-variant arguments. Haskell
indeed can support width and depth, structural and nominal subtyping. We
address the particular challenge to preserve Haskell's type inference even for
objects and object-operating functions. The OO features are introduced in
Haskell as the OOHaskell library. OOHaskell lends itself as a sandbox for typed
OO language design.
