Haskell's overlooked object system

TL;DR

This paper demonstrates that Haskell, with certain extensions, can fully support traditional object-oriented programming features including inheritance, encapsulation, and subtyping, challenging the view that Haskell's type system is incompatible with OO paradigms.

Contribution

It systematically shows that Haskell 98 with common extensions can implement comprehensive OO features, including advanced ones like multiple inheritance and safe downcasts.

Findings

Haskell supports all conventional OO features with extensions.

Haskell can handle subtyping, inheritance, and encapsulation.

The OOHaskell library enables OO programming in Haskell.

Abstract

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.