Question

我尝试使用IO和Maybe monads 来制作一个简单的示例。程序从DOM读取一个节点并向其写入一些innerHTML。

我所依赖的是IO和Maybe的组合，例如： IO (Maybe NodeList)。

如何使用此设置短路或抛出错误？

我可以使用getOrElse来提取值或设置默认值，但将默认值设置为空数组并不会有任何帮助。

import R from 'ramda'; import { IO, Maybe } from 'ramda-fantasy'; const Just = Maybe.Just; const Nothing = Maybe.Nothing; // $ :: String -> Maybe NodeList const $ = (selector) => { const res = document.querySelectorAll(selector); return res.length ? Just(res) : Nothing(); } // getOrElse :: Monad m => m a -> a -> m a var getOrElse = R.curry(function(val, m) { return m.getOrElse(val); }); // read :: String -> IO (Maybe NodeList) const read = selector => IO(() => $(selector)); // write :: String -> DOMNode -> IO const write = text => (domNode) => IO(() => domNode.innerHTML = text); const prog = read('#app') // What goes here? How do I short circuit or error? .map(R.head) .chain(write('Hello world')); prog.runIO();

https://www.webpackbin.com/bins/-Kh2ghQd99-ljiPys8Bd

Answer 1

您可以尝试编写EitherIO monad变换器。 Monad变换器允许您将两个monad的效果组合成一个monad。它们可以用通用的方式编写，这样我们就可以根据需要创建monad的动态组合，但是在这里我将演示Either和IO的静态耦合。

首先，我们需要一种从IO (Either e a)到EitherIO e a的方法，以及从EitherIO e a到IO (Either e a)的方式

EitherIO :: IO (Either e a) -> EitherIO e a
runEitherIO :: EitherIO e a -> IO (Either e a)

我们需要一些辅助函数来将其他平面类型带到我们的嵌套monad

EitherIO.liftEither :: Either e a -> EitherIO e a
EitherIO.liftIO :: IO a -> EitherIO e a

为了符合梦幻之地，我们的新EitherIO monad具有chain方法和of功能，并遵守monad法律。为方便起见，我还使用map方法实现了仿函数接口。

<强> EitherIO.js

import { IO, Either } from 'ramda-fantasy'
const { Left, Right, either } = Either

// type EitherIO e a = IO (Either e a)
export const EitherIO = runEitherIO => ({
  // runEitherIO :: IO (Either e a)
  runEitherIO, 
  // map :: EitherIO e a => (a -> b) -> EitherIO e b
  map: f =>
    EitherIO(runEitherIO.map(m => m.map(f))),
  // chain :: EitherIO e a => (a -> EitherIO e b) -> EitherIO e b
  chain: f =>
    EitherIO(runEitherIO.chain(
      either (x => IO.of(Left(x)), (x => f(x).runEitherIO))))
})

// of :: a -> EitherIO e a
EitherIO.of = x => EitherIO(IO.of(Right.of(x)))

// liftEither :: Either e a -> EitherIO e a
export const liftEither = m => EitherIO(IO.of(m))

// liftIO :: IO a -> EitherIO e a
export const liftIO = m => EitherIO(m.map(Right))

// runEitherIO :: EitherIO e a -> IO (Either e a)
export const runEitherIO = m => m.runEitherIO

调整您的程序以使用EitherIO

有什么好处是你的read和write函数没问题 - 你的程序中没有任何内容需要更改，除了我们如何在prog中构建调用

import { compose } from 'ramda'
import { IO, Either } from 'ramda-fantasy'
const { Left, Right, either } = Either
import { EitherIO, liftEither, liftIO } from './EitherIO'

// ...

// prog :: IO (Either Error String)
const prog =
  EitherIO(read('#app'))
    .chain(compose(liftIO, write('Hello world')))
    .runEitherIO

either (throwError, console.log) (prog.runIO())

补充说明

// prog :: IO (Either Error String)
const prog =
  // read already returns IO (Either String DomNode)
  // so we can plug it directly into EitherIO to work with our new type
  EitherIO(read('#app'))
    // write only returns IO (), so we have to use liftIO to return the correct EitherIO type that .chain is expecting
    .chain(compose(liftIO, write('Hello world')))
    // we don't care that EitherIO was used to do the hard work
    // unwrap the EitherIO and just return (IO Either)
    .runEitherIO

// this actually runs the program and clearly shows the fork
// if prog.runIO() causes an error, it will throw
// otherwise it will output any IO to the console
either (throwError, console.log) (prog.runIO())

检查错误

继续将'#app'更改为某个不匹配的选择器（例如）'#foo'。重新运行程序，您将看到适当的错误被禁止进入控制台

Error: Could not find DOMNode

Runnable演示

你做到了这一点。这是一个可运行的演示作为奖励：https://www.webpackbin.com/bins/-Kh5NqerKrROGRiRkkoA

使用EitherT的通用转换

monad变换器将monad作为参数并创建一个新的monad。在这种情况下，EitherT会占用一些monad M并创建一个有效行为为M (Either e a)的monad。

所以现在我们有办法创建新的monads

// EitherIO :: IO (Either e a) -> EitherIO e a
const EitherIO = EitherT (IO)

我们还有将平面类型提升为嵌套类型的功能

EitherIO.liftEither :: Either e a -> EitherIO e a
EitherIO.liftIO :: IO a -> EitherIO e a

最后一个自定义运行函数，可以更容易地处理我们的嵌套IO (Either e a)类型 - 注意，一层抽象（IO）被删除，所以我们只需要考虑{{1} }

Either

<强> EitherT

是面包和黄油 - 你在这里看到的主要区别是runEitherIO :: EitherIO e a -> Either e a接受monad EitherT作为输入并创建/返回一个新的Monad类型

<强> EitherIO

现在可以用

// EitherT.js
import { Either } from 'ramda-fantasy'
const { Left, Right, either } = Either

export const EitherT = M => {
   const Monad = runEitherT => ({
     runEitherT,
     chain: f =>
       Monad(runEitherT.chain(either (x => M.of(Left(x)),
                                      x => f(x).runEitherT)))
   })
   Monad.of = x => Monad(M.of(Right(x)))
   return Monad
}

export const runEitherT = m => m.runEitherT

实现

- 一种极为简化的实现

EitherT

我们计划的更新

import { IO, Either } from 'ramda-fantasy'
import { EitherT, runEitherT } from './EitherT'

export const EitherIO = EitherT (IO)

// liftEither :: Either e a -> EitherIO e a
export const liftEither = m => EitherIO(IO.of(m))

// liftIO :: IO a -> EitherIO e a
export const liftIO = m => EitherIO(m.map(Either.Right))

// runEitherIO :: EitherIO e a -> Either e a
export const runEitherIO = m => runEitherT(m).runIO()

使用EitherT运行演示

这是使用EitherT的可运行代码：https://www.webpackbin.com/bins/-Kh8S2NZ8ufBStUSK1EU

Answer 2

如果给定的谓词返回true，您可以创建一个辅助函数，它将有条件地与另一个IO生成函数链接。如果返回false，则会生成IO ()。

// (a → Boolean) → (a → IO ()) → a → IO ()
const ioWhen = curry((pred, ioFn, val) =>
  pred(val) ? ioFn(val) : IO(() => void 0))

const $ = document.querySelector.bind(document)

const read = selector => 
  IO(() => $(selector))

const write = text => domNode =>
  IO(() => domNode.innerHTML = text)

const prog = read('#app').chain(
  ioWhen(node => node != null, write('Hello world'))
)

prog.runIO();

将Maybe和IO monads组合用于DOM读/写

2 个答案: