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Comment by ElFitz | Hacker News Reader

> There is no other programming language that has union types as powerful as TypeScript

How about Swift and OCaml? ReasonML comes to mind too. Haven't tried actively comparing any of them though.

OCaml/Reason have discriminated unions, which means that you have to explicitly specify which variant of the union you're using, such as

  type union = Bool of bool | Char of char | Nothing

  let (x, y, z) = (Nothing, Bool true, Char 'x')

Is this not what enums with associated values are?

Probably... but in any case TypeScript also supports naked (edit: I just made up the name) unions, where you don't need to declare them upfront, and where values of different types can be included directly. Such as:

  let a = random() ? 'a' : 1

TypeScript infers type

  string | number

This is still a discriminated union, only that the discriminator is provided by Javascript and is thus hidden: it's `typeof`.

Yeah, that's the runtime perspective... but for the type system, they're different... with the "naked" union being much harder (as they're non-algebraic - `a | b | b` always equals `a | b` and might even equal just `a` for some values of `a` and `b`)

No, this works on any overlapping types too - even unsound combinations such as `T | Array<T>`

Interesting! Thanks!

What features are you thinking of in those languages? I don't know Swift but I can't think of anything comparable in OCaml.

Sorry; I don't have much of an answer here.

I merely know from experience that they have a pretty good type system, but not much more. So I asked :-)

Answers have been enlightening

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