[tsimionescu]

But Range is a concrete type, not an interface, right? And most of the discussion here is about the implementation of Range, not about the interface it exposes to users.

Note also that the clone issue and the borrow issue are not applicable to Haskell, and that the performance characteristics of Range may be hard to replicate while implementing Foldable or Traversable.

[centimeter]

Right, the trick is to not use concrete types when unnecessary. As the OP makes clear, it doesn't make sense to stuff all these use cases into a single concrete type.

> Note also that the clone issue and the borrow issue are not applicable to Haskell

No, but Rust switching to a typeclass-based iterator syntax should help with this too.

> the performance characteristics of Range may be hard to replicate while implementing Foldable or Traversable.

I don't see why - rustc generally does (and must do) a great job of specializing parametric code.