PureEvalMaybeAsync
此处阅读到的 Maybe 可能与其他函数式语言（如 Haskell）中的 Data Maybe 并不相同。PureEval 中的 Maybe 是一个 Data Maybe 与 Class Monad 的融合。
这里与 Maybe 一节不同的地方是，MaybeAsync 提供了一个支持异步的 Monad。
MaybeAsync
MaybeAsync 类型内包含一个不确定的 Promise 值。
undefined 与 null 值都将被认为 NothingAsync。
- Type
MaybeAsync.of()
将一个 Promise 提升为
- Type
- Example
js
const foo = new Promise((res, rej) => {
res('Hello, World!');
});
const task = MaybeAsync.of(foo);const foo = new Promise((res, rej) => {
res('Hello, World!');
});
const task = MaybeAsync.of(foo);MaybeAsync.is()
判断一个值是否为
- Type
- Example
js
MaybeAsync.is(JustAsync(new Promise((res) => res(1)))); //true
MaybeAsync.is(NothingAsync); //trueMaybeAsync.is(JustAsync(new Promise((res) => res(1)))); //true
MaybeAsync.is(NothingAsync); //trueMaybeAsync.isNothing()
判断该
- Type
- Example
js
JustAsync(1).isNothing(); //false
JustAsync(null).isNothing(); //trueJustAsync(1).isNothing(); //false
JustAsync(null).isNothing(); //trueMaybeAsync.map()
将 MaybeAsync 中的值经过函数处理映射为新的值，并构造新的 MaybeAsync 来保存。
- Type
- Example
js
JustAsync(new Promise((res) => res('Hello, World!'))).map(toLowerCase); // MaybeAsync "hello, world!"JustAsync(new Promise((res) => res('Hello, World!'))).map(toLowerCase); // MaybeAsync "hello, world!"MaybeAsync.fold()
WARNING
请注意：由于异步的特点，该函数的返回值是一个 Promise 类型的值，如有需要，请使用 await 等语句获取其值。
将函数应用到
传入两个单元函数参数，分别代表
当匹配到
- Type
- Example
js
await JustAsync(new Promise((res) => res('Hello, World!'))).fold(() => 'fuck', id); //"Hello, World!"
await JustAsync(new Promise((res) => res(null))).fold(() => 'fuck', id); //"fuck"await JustAsync(new Promise((res) => res('Hello, World!'))).fold(() => 'fuck', id); //"Hello, World!"
await JustAsync(new Promise((res) => res(null))).fold(() => 'fuck', id); //"fuck"MaybeAsync.chain()
在 MaybeAsync 后链接一个返回 MaybeAsync 类型的函数，可作为链式调用。
传入一个返回 MaybeAsync 类型的函数，返回值为链接后的 MaybeAsync 对象。
- Type
- Example
js
const getUserById = async (id) => {
// some async operation to fetch user by id
};
const getUserEmail = async (user) => {
// some async operation to fetch user email
};
const sendEmail = async (email) => {
// some async operation to send email
};
const sendEmailToUserById = (id) =>
MaybeAsync.of(getUserById(id))
.chain((user) => (user ? JustAsync(user) : NothingAsync))
.chain(getUserEmail)
.chain(sendEmail);
sendEmailToUserById(123);const getUserById = async (id) => {
// some async operation to fetch user by id
};
const getUserEmail = async (user) => {
// some async operation to fetch user email
};
const sendEmail = async (email) => {
// some async operation to send email
};
const sendEmailToUserById = (id) =>
MaybeAsync.of(getUserById(id))
.chain((user) => (user ? JustAsync(user) : NothingAsync))
.chain(getUserEmail)
.chain(sendEmail);
sendEmailToUserById(123);JustAsync()
实际同 MaybeAsync.of。
- Type
- Example
js
JustAsync(1); //Just 1JustAsync(1); //Just 1