Are FP and OO orthogonal?

Question

I have heard this time and again, and I am trying to understand and validate the idea that FP and OO are orthogonal.

First of all, what does it mean for 2 concepts t

Answer 1

The idea of objects can be implemented in an immutable fashion. An example is the book "A Theory of Objects", by Abadi and Cardelli, that aims at formalizing these ideas, and where objects are first given immutable semantics because that makes reasoning about object-oriented programs simpler.

In this case, a method that would traditionally have modified the object in-place instead returns a new object, while the previous object persists.

Answer 2

Orthogonality implies that two things are unrelated. It comes from mathematics where it means perpendicular. In common usage it can mean two decisions are unrelated or that one subject is irrelevant when considering another subject. As used here, orthogonal means that one concept doesn't either imply or exclude the other.

The two concepts object oriented programming and functional programming are not incompatible with each other. Object orientedness does not imply mutability. Many people who are introduced to object oriented programs the traditional way often first use C++, Java, C# or similar languages where mutability is common and even encouraged (standard libraries provide a varierty of mutable classes for people to use). Therefore it is understandable that many people associate object oriented programming with imperative programming and mutability, as this is how they have learned it.

However object oriented programming covers topics like:

• Encapsulation
• Polymorphism
• Abstraction

None of this implies mutability, and none of it excludes functional programming. So yes they are orthogonal in that they are different concepts. They are not opposites - you can use one, or the other, or both (or even neither). Languages like Scala and F# attempt to combine both paradigms into a single language:

Scala is a multi-paradigm programming language designed to integrate features of object-oriented programming and functional programming.

Source

F# is a succinct, expressive and efficient functional and object-oriented language for .NET which helps you write simple code to solve complex problems.

Source

Answer 3

For two concepts to be orthogonal means that they can be independently realized to any degree in any given manifestation. Considering music, for instance, you can classify a musical piece as to how harmonic it is and how rhythmic it is. The two concepts "harmonic" and "rhythmic" are orthogonal in the sense that there are harmonic and rhythmic pieces, disharmonic and arrythmic pieces, but also disharmonic and rhythmic pieces as well as harmonic and arrhythmic pieces.

Applied to original question this means that there are purely functional, non-object oriented programming lanuages such as Haskell, purely object-oriented, "non-functional" languages such as Eiffel, but also languages which are neither such as C and languages which are both such as Scala.

Simply speaking, Scala being object-oriented means that you can define data structures ("classes" and "traits") which encapsulate data with the methods that manipulate this data, guaranteeing that instances of these structures ("objects") are always in a defined state (the object's contract laid out in its class).

On the other hand, Scala being a functional language means that it favors immutable over mutable state and that functions are first class objects, which can be used just like any other object as local variables, fields or parameters to other functions. In addition to this, almost every statement in Scala has a value, which encourages you to use a functional programming style.

Orthogonality of object-orientated programming and functional programming in Scala additionaly means that you as a programmer are free to choose any mixture of these two concepts you see fit for your purpose. You can write your programs in a purely imperative style, using mutable objects only and not using functions as objects at all, on the other hand you can also write purely functional programs in Scala not using any of its object-oriented features.

Scala really does not require you to use one style or the other. It lets you choose the best of both worlds to solve your problem.

Answer 4

First of all, what does it mean for 2 concepts to be orthogonal?

It means they don't affect each other. I.e. a functional language isn't less functional because it's also object oriented.

They seem like opposites. How does it affect their orthogonality?

If they were opposites (i.e. a purely functional language could not possibly be object oriented), they would by definition not be orthogonal. However I do not believe that this is the case.

and OO seems like something that is built for state and mutation(a slightly organized version of imperative programming?). And I do realize that objects can be immutable. But OO seems to imply state/change to me.

While this is true for most mainstream OO languages, there is no reason that an OO language needs to have mutable state.

If a language has objects, methods, virtual inheritance and ad-hoc polymorphism, it's an object oriented language - whether it also has mutable state or not.