[tome]

> > Haskell type classes have nothing to do with OOP in the least, and as far as I know

> They surely do

That misses the point. The original challenge was

> > Can you name a contemporary expert or a platform that started or spent a good chunk of time as FP and ran back towards OOP

and didibus offered

> Hum, I think most did ... Haskell added TypeClasses

But that's simply not true that Haskell added type classes as an attempt to run back to OOP.

[pjmlp]

Well all the FP languages without any kind of OOP like features, are doomed to stay as academia research projects, so.

[tome]

Yes, if you define OOP-like broadly enough, that is surely true.

[didibus]

That's my point. We should discuss the specifics and stop generalizing.

For example, OO has polymorphism as a core idea. TypeClasses are an implementation of ad-hoc polymorphism. This concept is a core part of OOP as well as all practical FP languages.

Now if we talk about OOP being bad, and we start talking about ad-hoc polymorphism. That's actually a great concept. It's one of the best concepts to appear in programming languages. So here we are talking about something great which most OOP languages have and make a core part of their offering. Something which Haskell also has and makes a core part of its offering.

Now you can try to move the goalpost, say nah, OOP doesn't mean ad-hoc polymorphism... okay what does it mean then? And maybe you mention class inheritance. Okay, is that it? What about abstraction, data encapsulation, overloading, access scopes, data instantiation, polymorphism, message passing, information hiding, actor-concurency, user defined types, etc. ?

So if you meant to say that inheritance is bad, then say that inheritance is bad. But even here, TypeClasses support inheritance?

> Haskell also supports a notion of class extension. For example, we may wish to define a class Ord which inherits all of the operations in Eq. [...] Note the context in the class declaration. We say that Eq is a superclass of Ord (conversely, Ord is a subclass of Eq), and any type which is an instance of Ord must also be an instance of Eq.

From: https://www.haskell.org/tutorial/classes.html

So what now? Maybe we actually mean that field inheritance is bad? Or what is it? And that's the kind of technicalities we should be discussing, not just say OOP bad, FP good.

[tome]

Subclass polymorphism in typical OO languages (Java, C++, Python) is nothing like type class polymorphism. Class inheritance is nothing like type class sub/superclassing.

It's quite possible for one to be good and the other to be bad (and I would claim that is indeed the case).

[didibus]

I'm talking about ad-hoc polymorphism, for example, in Java it would be interfaces and interface inheritance.

I believe in OO this concept comes from Self from 1987, where they first implemented Traits. In Standard ML TypeClasses were introduced in 1988 I believe. So it actually seems that OO would have had a form of ad-hoc polymorphism before ML. Now I don't want to debate where the ideas came from, I'm trying to say the "camps" are stereotyping which shouldn't be brought up in a technical conversation. Traits and TypeClasses share the same fundamental idea and purpose, and Haskell TypeClasses is its own variant of that idea, which is slightly different to Standard ML TypeClasses as well, and to how other FP languages do ad-hoc polymorphism, and in OO languages there are many variant on that idea as well.

[tome]

If you define "OO" to mean "has any form of ad hoc polymorphism" then I agree that Haskell is an OO language. On the other hand, I suspect most of the profession would assume that the polymorphism requirements in the definition of OO extend only so far as what Java, C++ and Python provide, that is, subclass-based polymorphism implemented using a vtable reference per object (or equivalent). The Haskell community certainly does not consider type classes to be a feature that provides "OO".

When I enter these debates I assume we're talking about OO in the de facto sense described above. Under that nomenclature I agree that ad hoc polymorphism is a useful language feature but believe that the OO implementation of it is bad.

[pjmlp]

And then there is prototype polymorphism (SELF, JavaScript), pattern based polymorphism (BETA), component based polymorphism (COM, SOM, UWP), protocol based polymorphism (Objective-C, Swift, Smalltalk), ....

That is the thing, there isn't one OOP.

[andrekandre]

how does smalltalk use protocols? is it more ad-hoc like "responds to selector" or something else?

[pjmlp]

Just as broadly as FP can be defined.