Comment by codebje
Isn't that just saying you can do dependent types in TypeScript easily so long as you're willing to give up type checking?
Isn't that just saying you can do dependent types in TypeScript easily so long as you're willing to give up type checking?
No, because 1) you can't do (full) dependent types in TypeScript, 2) it's not clear what "dependent types" would even mean without type checking, and 3) using the union type you aren't giving up type checking at all.
The limitation is what comes later in the article - the dependent type can express the relationship between the argument and the returned type. But that's "out of scope" of the question of "typing a function that can return either a number or a string depending on the boolean argument". It only becomes a problem later on because to use the result you need to test the type (using `typeof`) even if you know you which boolean you passed in; that information doesn't end up tracked by the type system - although there actually are a couple of different ways to do this that satisfy this example in TypeScript, as suggested by some other comments.
Just using a union type in TypeScript isn't dissimilar to my suggestion of wrapping the result in an ADT; it's just that TypeScript makes this lightweight.