never type (and unions)

[emil14]

never type is something like "null that you can't use". with unions it allows to describe invariant better

suppose we have {a, b int?} rec where only 2 states must be possible: {a: null, b: int} and {a: int, b: null}

so instead of e.g. { a: int?, b: int? } where could say t = {a: int, b: never} | {a: never, b: int}

Problem 1: need for type inference

Makes type-system more complex because we need to introduce some sort of type inference. To make it possible to do some form of

var v t
if v.b == nil {
  // v is {a: int, b: never} or even just {a: int}
} else {
  // vice versa
}

# Problem 2: unions as not just constraints

If unions are only used as a constraints, then concrete type for type parameter can be always resolved. It's not true otherwise. However, there are other problems with unions used that way that relates to problem #1 probably

[emil14]

Thought

Type inference needed anyway. If component accepts foo | bar (whether this union is a constraint or not) there should be way in infer (or explicitly type cast) inside component's source code