> I can't speak to why run-time checking wasn't built into the language as a default behavior tldr: you other get either huge overhead or need to type annotate

> I can't speak to why run-time checking wasn't built into the language as a default behavior tldr: you other get either huge overhead or need to type annotate everything like it's Java, and it'll still be pretty limited/not super useful --- there's a bunch of libraries that do this, and they're all fundamentally limited [though i guess some of these may just be failures of imagination on my part]: - hard to check generics: if it's a builtin, you have to traverse the whole structure and check every element. and for user-defined collections, the programmer has to to manually instantiate every generic like: CoolSequence[int]([1,2,3]) (which sucks) and every method call/property access needs to be checked - if your function takes a callback, there's no way to typecheck that callback at runtime (without calling it, at which point it's not terribly useful. which extends to all functions i guess) - not much you can do with iterators/generators. can't check if something really is an `Iterator[int]` without exhausting it, and e.g. type-erroring after processing 20 elements because it yielded not-an-int isn't very useful so in the end, even if you wrap Every Damn Thing in a type assertion, you still get the errors very late, not much sooner than your usual "AttributeError: object None has no property named 'foobar'". kind of a worst-of-both-worlds situation