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Yeah, that's really something that feels like it should be a part of the language, because it would be so useful!

It's explicitly one of the language's non-goals[1].

> Add or rely on run-time type information in programs, or emit different code based on the results of the type system. Instead, encourage programming patterns that do not require run-time metadata.

[1] https://github.com/Microsoft/TypeScript/wiki/TypeScript-Desi...

I understand it (there's already enough to do at type level that runtime level is a non-goal), but types are purely indicative at IO boundaries. Validation still needs to happen

That's right. So since it won't be a part of the language, libraries like this are being developed.

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