结合 Maybe 和 IO monad 进行 DOM 读/写

Question

我正在尝试编写一个简单的示例使用 IO 和 Maybe monads。程序从 DOM 中读取一个节点并向其写入一些 innerHTML。

我最关心的是 IO 和 Maybe 的组合，例如IO (Maybe NodeList).

如何在此设置中短路或引发错误？

我可以使用getOrElse 来提取值或设置默认值，但是将默认值设置为空数组并没有任何帮助。

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

如果给定的谓词返回 true，您可以创建一个辅助函数，该函数将有条件地与另一个 IO 生成函数链接。如果它返回 false，它将产生一个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();

Answer 2

您可以尝试编写一个EitherIO monad 转换器。 Monad 转换器允许您将两个 monad 的效果组合成一个 monad。它们可以用一种通用的方式编写，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) 的方法

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

我们需要几个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

为了符合幻想世界，我们新的EitherIO monad 具有chain 方法和of 函数并遵守monad 法则。为方便起见，我还使用map 方法实现了函子接口。

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

这样做的好处是您的 read 和 write 函数可以正常工作 - 除了我们在 prog 中构造调用的方式之外，您的程序中没有任何内容需要更改

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' 更改为一些不匹配的选择器（例如）'#foo'。重新运行程序，您会看到相应的错误提示到控制台中

Error: Could not find DOMNode

---

可运行演示

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 转换器将 monad 作为参数并创建一个新的 monad。在这种情况下，EitherT 将采用一些 monad M 并创建一个有效行为的 monad M (Either e a)。

所以现在我们有了一些方法来创建新的 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函数

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

最后一个自定义的 run 函数可以更轻松地处理我们嵌套的 IO (Either e a) 类型 - 请注意，一层抽象 (IO) 被移除，因此我们只需要考虑 Either

runEitherIO :: EitherIO e a -> Either e a

---

要么

是面包和黄油 - 您在这里看到的主要区别是 EitherT 接受 monad M 作为输入并创建/返回新的 Monad 类型

// 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

EitherIO

现在可以用EitherT 来实现——一个大大简化的实现

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()

我们的计划更新

import { EitherIO, liftEither, liftIO, runEitherIO } from './EitherIO'

// ...

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

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

---

使用 EitherT 的可运行演示

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