帮助我了解以下Scala代码:scalaz IO Monad [英] Help me understand this Scala code: scalaz IO Monad

问题描述

这是我要理解的代码(来自

Here's the code I'm trying to understand (it's from http://apocalisp.wordpress.com/2010/10/17/scalaz-tutorial-enumeration-based-io-with-iteratees/):

object io {
  sealed trait IO[A] {
    def unsafePerformIO: A
  }

  object IO {
    def apply[A](a: => A): IO[A] = new IO[A] {
      def unsafePerformIO = a
    }
  }

  implicit val IOMonad = new Monad[IO] {
    def pure[A](a: => A): IO[A] = IO(a)
    def bind[A,B](a: IO[A], f: A => IO[B]): IO[B] = IO {
      implicitly[Monad[Function0]].bind(() => a.unsafePerformIO,
                                        (x:A) => () => f(x).unsafePerformIO)()
    }
  }
}

此代码的用法如下(我假设隐含import io._)

This code is used like this (I'm assuming an import io._ is implied)

def bufferFile(f: File) = IO {   new BufferedReader(new FileReader(f)) }

def closeReader(r: Reader) = IO {   r.close }

def bracket[A,B,C](init: IO[A], fin: A => IO[B], body: A => IO[C]): IO[C] = for { a <- init
      c <- body(a)
      _ <- fin(a) }   yield c

def enumFile[A](f: File, i: IterV[String, A]): IO[IterV[String, A]] =  bracket(bufferFile(f),
          closeReader(_:BufferedReader),
          enumReader(_:BufferedReader, i))

让我们从bufferFile定义开始.我认为调用io.IO的apply方法是否正确?该apply方法采用无参数函数，该函数返回一个值(正确吗?).我想这就是我要坚持的地方.有人可以解释bufferFile的定义如何工作吗?

Let's start with the bufferFile definition. Am I correct in thinking that the apply method of io.IO is called? That apply method takes a parameterless function that returns a value (correct?). I guess this is where I'm stuck. Can someone explain how the definition of bufferFile works?

推荐答案

是的，您说得对，几乎可以； io.IO.apply用所谓的按名称"参数调用，该参数基本上是一个不占用任何内容(Unit)并返回A的函数.有趣的是，当您像new BufferedReader(new FileReader(f))这样直接传递A的实例时，它将转换为类似() => new BufferedReader(new FileReader(f))的事物.

Yes you are right, well almost; io.IO.apply gets called with a so called "by name" parameter which is basically a function which takes nothing (Unit) and returns A. The cool thing about is that when you pass an instance of A directly like new BufferedReader(new FileReader(f)), it will be converted to something like () => new BufferedReader(new FileReader(f)).

作为apply的结果，您将获得IO[BufferedReader]的实例，该实例定义了方法def unsafePerformIO，该方法仅返回捕获的BufferedReader的实例.

As a result of apply you get an instance of an IO[BufferedReader] which defines a method def unsafePerformIO which simply returns the instance of the captured BufferedReader.

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