Type safe event emitter with Flow

type_safe_event_emitter.md

// A is a phantom type that ties an event instance...
class Event<A> {}
// ...to its handler
type Handler<A> = (a: A, ...rest: Array<void>) => void;

declare class EventEmitter {
  on<A, F: Handler<A>>(event: Event<A>, handler: F): void;
  emit<A>(event: Event<A>, a: A): void;
}

type User = { name: string, surname: string };
const save: Event<User> = new Event()
const emitter = new EventEmitter()

emitter.on(save, (u: User) => {})
// $ExpectError
emitter.on(save, (u: number) => {})
// $ExpectError
emitter.on(save, (u: User, unknown: number) => {})

emitter.emit(save, { name: 'Giulio', surname: 'Canti' })
// $ExpectError
emitter.emit(save, {})