Introduction

In F♯, object-oriented is sometimes more practical than functional style.

Object-oriented building blocks in F♯:

1. Members

   • Methods, properties, operators

   • Attach functionalities directly to the type

   • Encapsulate the object's state (particularly if mutable)

   • Used with object dotting my-object.my-member

2. Interfaces and classes

   • Support abstraction through inheritance

The four pillars of object-oriented programming

• Abstraction

• Encapsulation

• Inheritance

• Polymorphism

With the exception of inheritance, the other 3 concepts also apply to functional programming, especially in F♯, but to a lesser extent.

Abstraction

In OO, especially in C♯, abstraction is implemented using:

• Types: classes or enums

• Abstract classes and interfaces

In FP, abstraction is implemented using:

• Types: algebraic data types are more powerful to model domain concepts

• Functions: although equivalent to an interface with a single method, a function has a lower level of abstraction than an interface

Encapsulation

Encapsulation means 2 things:

• Hide elements:

  • This feature is easy in F♯: private keyword, expression nesting, partial application, .fsi files...

• Prevent a state from being invalid = Protect application invariants

  • If the state is mutable, it's definitely object-oriented.

  • If the state is immutable, it should be encapsulated only when it's possible to create an instance or copy and update an instance up to an invalid state.

Inheritance

Inheritance is an object-oriented technique. Still, F♯ supports it, with just some limitations compared to C♯.

Polymorphism

In fact, there are several polymorphisms. The main ones are:

1. By sub-typing: the one classically associated with object-orientation → Base type defining abstract or virtual members → Subtypes inheriting and implementing these members

2. Ad hoc/overloading → overloading of members with the same name

3. Parametric → generic in C♯, Java, TypeScript

4. Structural/duck-typing → SRTP in F♯, structural typing in TypeScript

5. Higher-kinded → type classes in Haskell

F♯ supports the 4 first polymorphisms.