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Comment by vbsteven | Hacker News Reader

A language like Kotlin can do some of these things using delegates, interfaces and extension methods.

For example a MutableList<T> can be dropped down to a List<T> which is not mutable. And a generic conversion from Collection<T> to NonEmptyCollection<T> should be trivial to write as an extension method.

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Can Kotlin handle multiple "tags" on a type as a set, and not a sequence? I'm not familiar with the language, so I'll use a C++ analogy. If you tried to tag types in C++, you'd end up with something like:

  TaggedType<std::vector, NonEmpty, Sorted>

but such type is strictly not the same as:

  TaggedType<std::vector, Sorted, NonEmpty>

What I mean by "set" instead of a "sequence" is to have the two lines above represent the same type, i.e. the order of tags should not matter.

You can maybe get there in kotlin with a generic type with multiple constraints in a where clause. Let’s say you have Sorted and NonEmpty as interfaces (could be empty marker interfaces so they behave like tags). Then you can write a method

  fun <T> doSomething(values: T) where T: Sorted, T: NonEmpty {}

And that function will take any type that has both Sorted and NonEmpty interfaces.

You don't need Kotlin to do this, even Java can do it:

    <T extends Sorted & NonEmpty> void doSomething(T values)

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