The trick there is to use an explicitly polymorphic instance, which can "decide what it should have been all along" later on. I've written about the idea: https

The trick there is to use an explicitly polymorphic instance, which can "decide what it should have been all along" later on. I've written about the idea: https://eighty-twenty.org/2015/01/25/monads-in-dynamically-typed-languages https://eighty-twenty.org/2015/01/25/monads-in-dynamically-t.... (OO folklore has long had some form of this idea; in limited forms, you see it in Smalltalk images stretching back decades.)