'use strict';
// Promiseオブジェクトをそのまま返す
let readAsync = new Promise(function (resolve, reject) {
    fs.readFile('./README.md', function (err, data) {
      resolve(data);
    })
  });
// Promiseオブジェクトを返す関数
let writeAsync = function (data) {
  return new Promise(function (resolve, reject) {
    fs.writeFile('./README-copy.md', data, function (err) {
      resolve();
    });
  });
}
// Defferedオブジェクトのpromiseを返す
let renameAsync = function () {
  var d = Promise.defer();
  fs.rename('./README-copy.md', './README-copy-modified.md', function (err) {
    d.resolve();
  });
  return d.promise;
};
// Generator
let AppendAsync = function* (){
  fs.appendFile('./README-copy-modified.md', 'this string is appended.');
  yield;
};
readAsync
.then(writeAsync)
.then(renameAsync)
.then(function () {
  AppendAsync().next();
});