https://github.com/kefirjs/kefir

// @flow

import Kefir from 'kefir'

function foo(numberStream: Kefir.Observable<number>) {
  numberStream.onValue(x => {
    // Flow knows x is a number here
  });
}

const s = Kefir.constant(5);
// Flow can automatically infer the type of values in the stream and determine
// that `s` is of type Kefir.Observable<number> here.
foo(s);