How can I create a function type Creator<X> that will only allow creating A or Bs while keeping the quality that you know which type you get?
This is not what I want:
type C = A|B
type Creator = () => C; // requires a type predicate for type safety
Using type predicates, one can discern whether a value qualifies as one type or another. That puts some extra work on the consumer of the interface I would like to avoid. Instead I would like a client that is to implement the interface to say
export creator : Creator<A> = () => return new A(); // should be OK!
while disallowing
export creator : Creator<number> = () => return 42; // NOT OK!
I have a TS Playground setup to make playing around with this easy.
CodePudding user response：
You just need a type constraint on X:
type Creator<X extends C> = () => X;