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Comment by unsafecast | Hacker News Reader

Basically, before this you couldn't have generic types in traits, like this:

    trait X {
        type Y<Z>;
    }

You couldn't express "a thing that contains another thing" in one of those type expressions before, and also use the contained type. It's needed for Mappable, because it's an abstraction over the concept of modifying a value inside a container.

IMO the implementation that they have is a bit obtuse, but I don't know if you can fit something better in this language.

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I really wonder about the usefulness of implementing a language feature to reduce the need to copy paste in an editor.

Generics are a language feature to reduce the need to copy and paste in an editor. For that matter, functions are a language feature to reduce the need to copy and paste in an editor.

As a library maintainer, you can't copy-paste a line for a type that isn't written yet.

As a library user, you can't copy-paste a line for a type you just created in a library you don't maintain.

Being able to do what you describe lets a maintainer create an interface for all the future types that satisfy given requirements.

Isn't that basically all language features? I could also write

    print("Hello")

    print("Hello2")

    print("Hello3")

    print("Hello4")

    print("Hello5")

(which I partially copy-pasted above) but I use a for-loop instead.

Really? You don't see the problem with trying to maintain code that's been copy-pasted everywhere with slight differences? Doesn't it seem easier to maintain one implementation of something than N implementations?

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