flowtype brilliant type inference

/* @flow */
// http://flowtype.org/
class DumbPromise<T>{
  value: T;
  constructor(value:T){
    this.value = value;
  };
  then<Y>(f:(x:T)=>Y): DumbPromise<Y> {
    return new DumbPromise(f(this.value));
  };
}

new DumbPromise(1)
 .then(x => x.toString())
 .then(x => x.toLowerCase())
 .then(x => x.length)
 // next line contains type error
 .then(x => x.toLowerCase())
;

And Scala:

    class DumbPromise[T](value: T) {
        def then[Y](f: T => Y) = new DumbPromise(f(value))
    }

    new DumbPromise(1)
    .then(_.toString)
    .then(_.toLowerCase)
    .then(_.length)
    //.then(_.toLowerCase) This line contains type error