组合用于DOM读/写的可能和IO单体

Question

我正在尝试用IO和monads来制作一个简单的示例。程序从DOM中读取一个节点并向其写入一些innerHTML。

我要挂的是IO和可能的组合，例如IO (Maybe NodeList)。

如何短路或在此设置中抛出错误？

我可以使用getOrElse提取一个值或设置一个默认值，但是将默认值设置为一个空数组没有任何帮助。

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

您可以尝试编写一个EitherIO单台转换器。monad变压器允许您将两个单体的效果组合成一个单一的单体。它们可以以一种通用的方式编写，这样我们就可以根据需要创建monads的动态组合，但是在这里，我将演示Either和IO的静态耦合。

首先我们需要一条从IO (Either e a)到EitherIO e a的路，还有一条从EitherIO e a到IO (Either e a)的路

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

我们需要一对辅助函数来将其他的平面类型带到嵌套的monad中

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

为了与幻境相一致，我们的新的EitherIO monad采用了chain方法和of函数，并遵守了monad定律。为了方便您，我还用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

更好的是，您的read和write函数都很好--程序中除了在prog中构造调用之外，什么都不需要改变

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())

附加解释

// 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，并将'#app'改为一些不匹配的选择器(例如) '#foo'。重新运行该程序，您将看到控制台中出现了相应的错误。

Error: Could not find DOMNode

可运行演示

你走了这么远。下面是一个可运行的演示，作为您的奖励：https://www.webpackbin.com/bins/-Kh5NqerKrROGRiRkkoA

使用的EitherT泛型变换

monad转换器将单模作为参数并创建一个新的monad。在本例中，EitherT将使用一些monad M并创建一个具有M (Either e a)的有效行为的monad。

所以现在我们有了一些方法来创造新的单簧管

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

此外，我们还具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，一个使处理嵌套IO (Either e a)类型更容易的自定义运行函数--注意，一个抽象层(IO)被移除，所以我们只需要考虑Either。

runEitherIO :: EitherIO e a -> Either e a

EitherT

是面包和黄油-您在这里看到的主要区别是EitherT接受一个单一M作为输入，并创建/返回一个新的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实现--这是一个非常简化的实现。

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()

更新到我们的程序

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运行的演示程序

下面是使用EitherT：https://www.webpackbin.com/bins/-Kh8S2NZ8ufBStUSK1EU运行的代码

Answer 2

如果给定谓词返回true，您可以创建一个助手函数，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，它将生成一个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();