How so? It looks a lot like a less-rich Java, whose type system has existed for literally decades. In fact, the generics syntax and type erasure was directly influenced by Java.
I haven’t used Java in many years, but TypeScript’s structural types give it a different flavor than I remember from Java. Unlike in Java, most of the types I define aren’t classes or interfaces. It’s just data that has an expected shape.
Check out the design of Zod, for example. I have a bunch of Zod types for the validation of incoming JSON messages. This automatically generates the corresponding TypeScript types through the magic of type inference.
It’s quite easy to create discriminated unions, which are just structs where one field has a fixed value.
> In functional programming, a type is just a function that returns a list of possible values
Its literally not; there may be a language which does this or something close (actually, I’ve seen narrow, purpose-focussed relational languages where this is the case, and all types are enumerable), but it's definitely not generally the case in functional languages.
Conceptually, you can think of a type as a set of values, but no, it’s not how static type-checking is implemented. Some sets are very large (consider the set of all JSON values) and enumerating them isn’t useful.
