[mncharity]

An object type can implement interface types, as you say.

Object type, and identity, can also be decoupled from implementation type. Dynamically, statically, or with advice. V8 javascript numbers and arrays can have several different underlying implementations, which get swapped depending on how the thing is used (and with extensions, you can even introspect and dispatch on them).

Typing can also be fine grained. So things can narrowly describe what they provide, and algorithms what they require. "I require a readable sequence, with an element type that has a partial order, and with a cursor supporting depth-one backtracking".

Object identity can also be decoupled from type. Ruby's 'refinements' permits lexically scoped alternative dispatch, so an object can locally look like something else. That, and just a few other bits, might have made the Python 2 to 3 transition vastly less painful - 'from past import python2'.

We are regrettably far from having a single language which combines the many valuable things we've had experience with.