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Created November 5, 2016 08:38
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Type safe event emitter with Flow
// A is a phantom type that ties an event instance...
class Event<A> {}
// its handler
type Handler<A> = (a: A, 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, {})
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