Observable.pipe

class Observable<T, E> {

  // cant be typed in flow, but perhaps we can
  // somehow declare several overloads? 
  // pipe(Observable<T0, E0> => Observable<T1, E1>): Observable<T1, E1>
  // pipe(Observable<T0, E0> => Observable<T1, E1>, Observable<T1, E1> => Observable<T2, E2>): Observable<T2, E2>
  // etc...
  pipe(...operators) {
    return operators.reduce((acc, curr) => curr(acc), this);
  }
}

// example: outputs an Observable of 2,3,4
Observable.
  of(1,2,3).
  // map returns Observable<T, E> => Observable<R, E> (faking partial application in JS)
  pipe(map(x => x + 1));