`Either` 类型可以转换为 `Task` 类型吗？答案

【问题标题】：Can `Either` types be transformed to `Task` types?`Either` 类型可以转换为 `Task` 类型吗？
【发布时间】：2016-05-07 22:34:24
【问题描述】：

如果我有一个 Task 有一个 Either err b 作为正确（成功）值，我如何组合/合并/转换它们以便成功值直接在 .fork() 中可用，而不是包含在Either?

const Task = require('data.task'); // folktale
const Either = require('data.either');

// eitherYayNay :: Bool → Either String String
const eitherYayNay = bool =>
  bool ? Either.Right('yay') : Either.Left('nay');

// theTask :: Bool → Task Either a b
const theTask = yn =>
  new Task((reject, resolve) => {
    resolve(eitherYayNay(yn));
    // reject();
  });

// niceTask :: Bool → Task a b
// ???

// the desired result...
niceTask(something).fork(
  err => { 
    // err could be the left value of the Task, or of the Either
  },
  val => { 
    console.log(val); // a string, not an Either
  }
);

【问题讨论】：

标签： javascript functional-programming folktale

【解决方案1】：

这就是我会做的：

var Task   = require("data.task");
var Either = require("data.either");

   // eitherYayNay :: Bool -> Either String String
const eitherYayNay = bool =>
    bool ?
        Either.Right("yay") :
        Either.Left("nay");

   // theTask :: Bool -> Task a (Either String String)
const theTask = bool => Task.of(eitherYayNay(bool));

   // niceTask :: Bool -> Task String String
const niceTask = bool => theTask(bool).chain(makeNice);

   // makeNice :: Either String String -> Task String String
const makeNice = either =>
    either.isRight ?
        Task.of(either.value) :
        Task.rejected(either.value);

const fork = bool => niceTask(bool).fork(onError, onValue);

const onError = error => console.log("Error: " + error);

const onValue = value => console.log("Value: " + value);

fork(true);  // Value: yay
fork(false); // Error: nay

查看演示：

var eitherYayNay = function (bool) {
    return bool ?
        Either.Right("yay") :
        Either.Left("nay");
};

var theTask = function (bool) {
    return Task.of(eitherYayNay(bool));
};

var niceTask = function (bool) {
    return theTask(bool).chain(makeNice);
};

var makeNice = function (either) {
    return either.isRight ?
        Task.of(either.value) :
        Task.rejected(either.value);
};

var fork = function (bool) {
    return niceTask(bool).fork(onError, onValue);
};

var onError = function (error) {
    alert("Error: " + error);
}

var onValue = function (value) {
    alert("Value: " + value);
}

fork(true);  // Value: yay
fork(false); // Error: nay

<script src="https://cdn.rawgit.com/aaditmshah/0b27bf3abfaba225b479/raw/f9c6af5e548d27c0d1932b80a9af7e0568c4a89e/task.js"></script>
<script src="https://cdn.rawgit.com/aaditmshah/5bf5e66c37663f3777ee/raw/3110fa24652ed42f005ebc40a39b5138db0063f9/either.js"></script>

希望对您有所帮助。

【讨论】：