[zdimension]

Eli5:

Haskell type classes are not classes (like Java or PHP classes); they are comparable to Rust traits -- which are different from PHP traits which are comparable to Java/C# interfaces (with default impls; if you just want contracts you have... PHP interfaces).

A fundamental difference is that you can instantiate/implement a type class (or Rust trait) for any* type, compared to interfaces where each class declares the interfaces it implements. You can therefore create generic (forall) instances, higher kinded type classes, etc.

[pjmlp]

That conflates type classes with extension types, in type theory.

Actually in modern Java you can simulate type classes approach with a mix of interfaces and default methods implementations.

In C# you can have the experience more straightforward with extensions types introduced in C#13.

Then we have yet another way to approach type classes in Scala, with traits and implicits.

And so on, as I haven't yet run out of examples.

[derriz]

> Actually in modern Java you can simulate type classes approach with a mix of interfaces and default methods implementations.

Can you? The beauty of traits/type classes is that you can attach them to any type - in a world where 90% of the functionality of any piece of software is supplied by dependencies - external types which you cannot change - this is a vital feature.

[pjmlp]

Simulate was the word, not map 1:1 the exact experience.

It isn't pretty, but one can try to achieve a similar approach.

https://godbolt.org/z/TjPha3obs

Failing that, there are always Clojure, Kotlin and Scala on the JVM, which expose language features to achieve the same, which you naturally can mix and match with plain old Java.

[derriz]

Why the reference to “modern Java” then? Writing adaptor classes is not “modern Java” nor does it involve using “a mix of interfaces and default methods implementations”.

I was responding to your original claim and I’m well aware of such facilities in both Kotlin and Scala - having used both extensively. I was genuinely curious if the latest Java was in the process of adding support for trait/typeclasses - so I don’t understand why you’d bother to reply with something that completely changes your original claim.