Question

我仍然对函数式编程一窍不通，并试图找出Monads的一个卷曲问题。我遇到的情况是，为了论证，将来会有一个HTTP请求并返回值列表。然后，我希望能够检查该列表中是否存在特定值，我本以为在这里返回Maybe monad将是胜利。但是，如果存在该值，则我希望能够基于该返回值发出另一个HTTP请求，但只有在该值存在的情况下它才应运行。这是一个使用Future＆Maybe https://codesandbox.io/s/2xvy3m1qmy

的ramda-fantasy实现的示例

我很可能将整个事情弄错了，因此，如果您需要更多信息，或者需要更改以上任何内容，请坚持下去。只是为了澄清这就是我的伪代码工作流程：

请求所有值
过滤值并可能找到匹配的值
如果找到，请请求获取其余数据

这是最初的，经过评论的尝试，如上所述：

import R from "ramda";
import Fantasy from "ramda-fantasy";
const Future = Fantasy.Future;
const Maybe = Fantasy.Maybe;

// make a fake HTTP request and return a list of values
// response :: Future Array String
const response = Future.of(['one', 'two', 'three'])

const maybeOrNothing = val => val ? Maybe.Just(val) : Maybe.Nothing()

// Maybe return our given value
// getVal :: String -> Maybe String
const getVal = input => response.map(R.find(R.equals(input))).map(maybeOrNothing)

// make another fake request
// let's pretend that this takes an ID and makes a fake ajax request to get teh data
// getValueData :: String -> Future
const getValueData = id => Future((reject, resolve) => {
  // fake HTTP request
  setTimeout(() => {
    resolve({
      id: id,
      foo: 'bar'
    })
  }, 100)
})

// 'one' should then run getValueData
// something isn't right here, do I need to map then chain?
getVal('one')
.chain(getValueData)
.fork(console.error, console.log)

// 'five' isn't there, so shouldn't run getValueData
// something isn't right here as getValueData still runs
// map(R.chain) works to not run getValueData but then it causes issues later on
getVal('five')
.chain(getValueData)
.fork(console.error, console.log)

Answer 1

您必须寻找sequence operations，它可以将Maybe<Future<…>>变成Future<Maybe<…>>，然后可以加入您的外部未来。

使用traverse：

getVal('one')
.chain(traverse(Future.of, getValueData))
.fork(console.error, console.log)

Answer 2

设法解决了我自己的问题，部分原因是Bergi的建议。 Bergi在上面的回答非常接近，但是实现并不十分正确。问题是我试图处理一个Future，然后将其更改为Maybe，所以我用以下方法处理它：

import R from "ramda";
import Fantasy from "ramda-fantasy";
const Future = Fantasy.Future;
const Maybe = Fantasy.Maybe;

// make a fake HTTP request and return a list of values
// response :: Future Array String
const response = Future.of(['one', 'two', 'three'])

const maybeOrNothing = val => val ? Maybe.Just(val) : Maybe.Nothing()
const maybeToFuture = m => Maybe.isNothing(m) ? Future.reject() : Future.of(m.value)

// Maybe return our given value
// getVal :: String -> Maybe String
const getVal = input => response.map(R.find(R.equals(input))).map(maybeOrNothing)

// make another fake request
// let's pretend that this takes an ID and makes a fake ajax request to get teh data
// getValueData :: String -> Future
const getValueData = id => Future((reject, resolve) => {
  // fake HTTP request
  setTimeout(() => {
    resolve({
      id: id,
      foo: 'bar'
    })
  }, 100)
})

// 'one' should then run getValueData
getVal('one')
  .chain(maybeToFuture)
  .chain(getValueData)
  .fork(console.error, console.log)

// five isn't there so getValueData doesn't run
getVal('five')
  .chain(maybeToFuture)
  .chain(getValueData)
  .fork(console.error, console.log)

在Future中处理嵌套的Mayad monad

2 个答案: