PureEvalMaybe(Monad)
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。
Maybe
Maybe 类型内包含一个不确定的值。
如果一个确定的值在 Maybe 中执行发生错误，该 Maybe 也会变为 Nothing。
undefined 与 null 值都将被认为 Nothing。
- Type
Maybe.of()
将一个值提升为
- Type
- Example
js
Maybe.of(1); //Just 1
Maybe.of(null); //NothingMaybe.of(1); //Just 1
Maybe.of(null); //NothingMaybe.is()
判断一个值是否为
- Type
- Example
js
Maybe.is(Just(1)); //true
Maybe.is(Nothing); //trueMaybe.is(Just(1)); //true
Maybe.is(Nothing); //trueMaybe.isNothing()
判断该
- Type
- Example
js
Just(1).isNothing(); //false
Just(null).isNothing(); //trueJust(1).isNothing(); //false
Just(null).isNothing(); //trueMaybe.map()
将 Maybe 中的值经过函数处理映射为新的值，并构造新的 Maybe 来保存。
- Type
- Example
js
Just(1).map(add(114513)); //Just 114514Just(1).map(add(114513)); //Just 114514Maybe.fold()
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
Just('The body next door').fold(() => 'Nothing', id); //"The body next door"Just('The body next door').fold(() => 'Nothing', id); //"The body next door"Maybe.chain()
在 Maybe 后链接一个返回 Maybe 类型的函数，可作为链式调用。
传入一个返回 Maybe 类型的函数，返回值为链接后的 Maybe 对象。
- Type
- Example
js
const getUser = (id) => {
if (id === 1) {
return Just({ id: 1, name: 'Alice' });
} else {
return Nothing;
}
};
const getAddress = (user) => {
if (user.name === 'Alice') {
return Just({ street: '123 Main St', city: 'Anytown' });
} else {
return Nothing;
}
};
const getCity = (address) => {
if (address.street === '123 Main St') {
return Just(address.city);
} else {
return Nothing;
}
};
const city = Just(1)
.chain(getUser)
.chain(getAddress)
.chain(getCity)
.fold(
() => 'Unknown',
(city) => city
);
console.log(city); // 'Anytown'const getUser = (id) => {
if (id === 1) {
return Just({ id: 1, name: 'Alice' });
} else {
return Nothing;
}
};
const getAddress = (user) => {
if (user.name === 'Alice') {
return Just({ street: '123 Main St', city: 'Anytown' });
} else {
return Nothing;
}
};
const getCity = (address) => {
if (address.street === '123 Main St') {
return Just(address.city);
} else {
return Nothing;
}
};
const city = Just(1)
.chain(getUser)
.chain(getAddress)
.chain(getCity)
.fold(
() => 'Unknown',
(city) => city
);
console.log(city); // 'Anytown'Just()
实际同 Maybe.of。
- Type
- Example
js
Just(1); //Just 1Just(1); //Just 1