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Comment by rimantas | Hacker News Reader

The other reason as I see is some quite weird perception that OOP == inheritance.

OOP is different things to different people. While I agree with you that OOP != inheritance, I believe that polymorphism ∈ OOP and inheritance is a part of polymorphism.

I don't follow you here. Are you saying that inheritance is mandatory to OOP? (I don't agree nor disagree. I'd just like to know what you mean.)

Polymorphism is often implemented as at least inheritance of interface in statically typed languages. Without polymorphism you don't get dynamic dispatch, especially open to extension at runtime.

I am not saying that inheritance is mandatory to OOP. I think "mandatory" is too strong of a word here. As I said, OOP means different things to different people and I am okay with that.

Speaking for myself, OOP is a way to orchestrate programming on a large scale. To grow software without growing the frequency, severity, and difficulty of bugs and without growing costs associated with new features is the desired result. Component-ized architecture takes great steps to achieve this result. For this reason, I view OOP as being some combination of: message passing, encapsulation, and polymorphism. I do not believe this is a complete view of OOP, nor that there is no viable alternative view of OOP.

Ad hoc polymorphism ∈ OOP

Parametric polymorphism ∈ FP

I disagree. Polymorphism and inheritance are separate concepts.

Yes, some statically typed languages like C++ and Java enforce certain inheritance constraints via classes or explicit interfaces before you can use polymorphism.

But almost all dynamically typed languages allow for ad-hoc polymorphism. For instance, in Python this is called "duck typing". Just because two objects provide a "foo()" method doesn't mean they are in any inheritance relation to each other. But polymorphism should still work.

Quite right. We can correct that by showing the classification of main programming paradigms[1]. It shows that, OO just needs records, procedures, named state and closures. (I personally prefer to say the more understandable "OO needs structs, assignment, and closures".)

Now I have a problem, however: what is the difference between OO and stateful functional programming?

[1]: http://www.info.ucl.ac.be/~pvr/paradigms.html

Polymorphism is at the core of most OO language features. Sum types aren't a viable substitute in most large apps because they are closed to extension at runtime. Doing it right functionally means rolling your own protocol system.

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