> for typing proposes, the signature looks the same: This operation yields a rectangle. It's really just a matter of where the return value lives. Well, it yie

> for typing proposes, the signature looks the same: This operation yields a rectangle. It's really just a matter of where the return value lives. Well, it yields a type union of square and rectangle. A runtime, whether the original language appears mutable or not, can play games with that. Like resolving types lazily. If for example some object is only drawn, and draw happens to dispatch it independently of squareness, on say a predicate type DistantObjectThatLooksLikeADot, then there's no need to resolve whether it was square. No one will ever know. Or ever have to pay the cost of finding out. It has say a MightBeSquareMightBeRectangleHaventHadToCareYet dictionary. :) Which can become important as types get more expressive, and expensive to prove/test.