Как сделать последовательные вызовы http api в angular2?

this.serviceInst.firstAPIMethod()
    .flatMap(firstMethodResult => this.serviceInst.secondAPIMethod(firstMethodResult))
    .flatMap(secondMethodResult => this.serviceInst.thirdAPIMethod(secondMethodResult))
    .subscribe(thirdMethodResult => {
          console.info(thirdMethodResult);
     });

let inSequence = (tasks) => { // tasks being an array of functions returning promises
  return tasks.reduce((p, task) => p.then(task), Promise.resolve());
}

inSequence(tasks).then(() => {
  console.info('all requests done');
});