Implement SortedSet#update_in and List#update_in

[xaviershay]

Seems desirable for consistency with Hash and Vector?

[dubek]

Nowadays #update_in is part of the Associable module:

  # Including `Associable` in your container class gives it an `update_in`
  # method.
  #
  # To mix in `Associable`, your class must implement two methods:
  #
  # * `fetch(index, default = (missing_default = true))`
  # * `put(index, item = yield(get(index)))`
  #
  # See {Vector#fetch}, {Vector#put}, {Hash#fetch}, and {Hash#put} for examples.

So for List we can implement #fetch (in terms of #at) and probably make some not-really-efficient #put.

As for Set and SortedSet, I'm not sure what these operations (fetch/put) mean. What is the index? What is the item's value?

[alexdowad]

For SortedSet, the index is integral, starting from the far "left" of the tree. For Set, you could say that each item is its own index, but I don't know how useful that will actually be. Probably Set will not have #update_in.

[dubek]

Indeed there's already SortedSet#fetch(index, default = (missing_default = true)) so this side is OK.

As for #put, it'll be something like #delete and then #add. Of course it's very weird because in the SortedSet ["a", "b", "c", "d"] if your perform s.put(2, "z") you'll get ["a", "b", "d", "z"], so a subsequent s.fetch(2) will return "d"...