在 Future 中处理嵌套的 Maybe monad

Question

我仍然掌握函数式编程的窍门，并试图找出 Monads 的一个小问题。我有一种情况，我有一个未来会发出一个 HTTP 请求并返回一个值列表，为了参数的缘故。然后我希望能够检查该列表中是否存在特定值，我会认为在这里返回一个 Maybe monad 会是胜利。但是，如果该值存在，我希望能够基于该返回值发出另一个 HTTP 请求，但它应该仅在该值存在时运行。这是一个使用 Future & Maybe https://codesandbox.io/s/2xvy3m1qmy 的 ramda-fantasy 实现的示例

我很可能把这整件事弄错了，所以如果您需要更多信息，或者需要更改上述任何内容，那就去做吧。只是为了澄清这将是我在伪代码中的工作流程：

1. 请求所有值
2. 过滤值并可能找到匹配的值
3. 如果找到，请求获取其余数据

这是一个初步的、带注释的尝试，如上链接所示：

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

您一定在寻找可以将Maybe<Future<…>> 变成Future<Maybe<…>> 的sequence operations，然后您可以加入您的外部未来。

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 上面的答案非常接近，但实施并不完全正确。问题是我试图处理 Future 然后将其更改为 Maybe，所以我处理了以下内容：

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)