Making that happen in a language design should involve some subtle shifts in the way data is conceptualized. That isn’t a digression in a discussion of types, because the way we conceptualize data has deep, not to say insidious, effects on the nature of typing.
As for the types themselves, I suggest we abandon the whole notion of types in favor of a lightweight mathematical notion of sets — and avoid using the word “type” as it naturally drags us back toward the conceptual morass of type theory that we need to escape.
http://fexpr.blogspot.com/2011/11/where-do-types-come-from.html