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Comment by vog | Hacker News Reader

I was hoping to see a good article about inheritance vs. object composition.

Although Ruby's "modules" are indeed preferable to inheritance they are usually as inappropriate as inheritance.

To my experience, object composition solves most of the design issues in a more elegant way. And I wonder why this is still so less understood by most programmers. It isn't even particularly new. Even the good old GoF patterns recommend object composition over inheritance, especially in their Smalltalk examples.

(In their book, they demonstrate each pattern in a static (C++) and a dynamic (Smalltalk) language, so Ruby programmers should have a deeper look at the Smalltalk part of the book, rather than imitating the C++ variants of the patterns.)

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I think the reason people prefer inheritance is because it gives them the illusion that their code is simpler. They don't see the huge interfaces implied by inheritance because they are implicit.

Composition, on the other hand, is explicit. When you use it, you notice the complexities. That gives you an incentive to simplify your design, which makes you write actually simpler code.

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 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.

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.)

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.

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.

Ad hoc polymorphism ∈ OOP

Parametric polymorphism ∈ FP

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.

The problem is that lots of different software engineering styles are implemented using the language feature of inheritance. For example in C++ implementing interface X looks the same syntactically as inheriting from some class X to modify it.

Sometimes people refer to the specific problematic inheritance style as "implementation inheritance". I found it very useful just to know that such a distinction could be made.

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