export interface Subscription {
  closed: boolean
  unsubscribe: () => void
}

export type ObserverStart = (subscription: Subscription) => void
export type ObserverNext<T> = (() => void) | ((value: T) => void)
export type ObserverError = (error: Error) => void
export type ObserverComplete = (value?: any) => void

export interface SubscriptionObserver<T> {
  closed: boolean
  error: ObserverError
  complete: ObserverComplete
  next: ObserverNext<T>
}

interface OptionalObserver<T> {
  start?: ObserverStart
  error?: ObserverError
  complete?: ObserverComplete
  next?: ObserverNext<T>
}

export type SloppyObserver<T> = OptionalObserver<T> | ObserverNext<T>

export type SubscriberFunction<T> = (
  observer: SubscriptionObserver<T>
) => Subscription | { unsubscribe: () => void } | (() => void) | void

export class Observable<T> {
  public constructor(callback: SubscriberFunction<T>)
  public subscribe(observer: SloppyObserver<T>): Subscription
}