Scalaz迭代器，为BufferedReader创建枚举器

Question

Scalaz迭代器，为BufferedReader创建枚举器

3

如何为BufferedReader创建枚举器？

我发现一篇比较旧的文章：http://apocalisp.wordpress.com/2010/10/17/scalaz-tutorial-enumeration-based-io-with-iteratees/，看起来它在Scalaz 6.0.4上无法使用。

我尝试基于这里的示例创建枚举器：Idiomatic construction to check whether a collection is ordered。

 implicit val ListEnumerator = new Enumerator[List] {
   def apply[E, A](e: List[E], i: IterV[E, A]): IterV[E, A] = e match {
      case List() => i
      case x :: xs => i.fold(done = (_, _) => i,
                       cont = k => apply(xs, k(El(x))))
   }
 }

但我不明白如何将IO Monad与Enumerator结合使用。

- Eugene Zhulenev

1个回答

网页内容由stack overflow 提供, 点击上面的

可以查看英文原文，
原文链接

- lester · Accepted Answer

Rúnar的文章有什么问题？下面这个版本对我来说可行（Scalaz 6.0.4）：

object FileIteratee {
  def enumReader[A](r: BufferedReader, it: IterV[String,  A]) : IO[IterV[String,  A]] = {
    def loop: IterV[String,  A] => IO[IterV[String,  A]] = {
      case i@Done(_, _) => i.pure[IO]
      case i@Cont(k) => for {
        s <- r.readLine.pure[IO]
        a <- if (s == null) i.pure[IO] else loop(k(El(s)))
      } yield a
    }
    loop(it)
  }

  def bufferFile(f: File) = new BufferedReader(new FileReader(f)).pure[IO]

  def closeReader(r: Reader) = r.close().pure[IO]

  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))
}