Help me understand this Scala code: scalaz IO Monad

Question

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)()
    }
  }
}

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))

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?

Answer 1

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)).

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.

Answer 2

Complementing agilesteel's answer, the code

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

Is equivalent to

def bufferFile(f: File) = new IO[A] {
  def unsafePerformIO = { new BufferedReader(new FileReader(f)) }
}