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TypeScript supports runtime type assertions, but it doesn't mandate them or write them for you.

I should have said your type system cannot be sound without runtime overhead. And I don't believe that choosing automatic runtime overhead is the right move.

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The problem is not that it doesn't write them for you, but that `as MyType` is way too ergonomic of a syntax for unchecked assertions, whereas checked assertions require effectively parsing the structure manually with typeof checks.

TypeScript desperately needs a way to derive type-guarding functions from types (and so does every gradually-typed language). There're libraries for this but you need to define the types themselves through combinators.

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