scala - Scalaz 的 Reader Monad

Question

我尝试用 scalaz 定义 Reader monad，如下所示:

import scalaz._
import Scalaz._

final class Reader[E,A](private[Reader] val runReader: E => A)

object Reader {
  def apply[E,A](f: E => A) = new Reader[E,A](f)
  def env[E]: Reader[E,E] = Reader(identity _)

  implicit def ReaderMonad[E] = new Monad[PartialApply1Of2[Reader,E]#Apply] {
    def pure[A](a: => A) = Reader(_ => a)

    def bind[A,B](m: Reader[E,A], k: A => Reader[E,B]) =
      Reader(e => k(m.runReader(e)).runReader(e))
  }
}


object Test {
  import Reader._

  class Env(val s: String)

  def post(s: String): Reader[Env, Option[String]] =
    env >>= (e => if (e.s == s) some(s).pure else none.pure)
}

但我得到一个编译器错误:

reader.scala:27: reassignment to val
     env >>= (e => if (e.s == s) some(s).pure else none.pure)
         ^

这是为什么？

谢谢，
列维

Answer 1

即使按照 Scala 的标准，这个错误也是相当不透明的。以 = 结尾的方法名称被特殊对待——它们首先被视为普通标识符，否则，它们将被扩展为自赋值。

scala> def env[A] = 0
env: [A]Int

scala> env >>= 0
<console>:7: error: reassignment to val
       env >>= 0
           ^

scala> env = env >> 0
<console>:6: error: reassignment to val
       env = env >> 0
           ^

如果您对程序的语法解释感到困惑，最好运行 scalac -Xprint:parser看看发生了什么。同样，您可以使用 -Xprint:typer或 -Xprint:jvm查看程序转换的后期阶段。

那么，如何调用 >>=在您的 Reader 上?首先，您需要显式传递类型参数 Env至 env .结果 Reader[Env, Env]然后必须转换为 MA[M[_], A] .对于简单类型构造函数，隐式转换 MAs#ma就足够了。然而两个参数类型的构造函数 Reader必须部分应用——这意味着它不能被推断出来，而是你必须提供一个特定的隐式转换。

如果 Adriaan 找到一个空闲的下午到 implement higher-order unification for type constructor inference，情况将会大大改善。 . :)

在那之前，这是你的代码。还有一些评论是内联的。

import scalaz._
import Scalaz._

final class Reader[E, A](private[Reader] val runReader: E => A)

object Reader {
  def apply[E, A](f: E => A) = new Reader[E, A](f)

  def env[E]: Reader[E, E] = Reader(identity _)

  implicit def ReaderMonad[E]: Monad[PartialApply1Of2[Reader, E]#Apply] = new Monad[PartialApply1Of2[Reader, E]#Apply] {
    def pure[A](a: => A) = Reader(_ => a)

    def bind[A, B](m: Reader[E, A], k: A => Reader[E, B]) =
      Reader(e => k(m.runReader(e)).runReader(e))
  }

  // No Higher Order Unification in Scala, so we need partially applied type constructors cannot be inferred.
  // That's the main reason for defining function in Scalaz on MA, we can create one implicit conversion
  // to extract the partially applied type constructor in the type parameter `M` of `MA[M[_], A]`.
  //
  // I'm in the habit of explicitly annotating the return types of implicit defs, it's not strictly necessary
  // but there are a few corner cases it pays to avoid.
  implicit def ReaderMA[E, A](r: Reader[E, A]): MA[PartialApply1Of2[Reader, E]#Apply, A] = ma[PartialApply1Of2[Reader, E]#Apply, A](r)
}


object Test {
  import Reader._

  class Env(val s: String)

  def post(s: String): Reader[Env, Option[String]] =
    // Need to pass the type arg `Env` explicitly here.
    env[Env] >>= {e =>
      // Intermediate value and type annotation not needed, just here for clarity.
      val o: Option[String] = (e.s === s).guard[Option](s)
      // Again, the partially applied type constructor can't be inferred, so we have to explicitly pass it.
      o.pure[PartialApply1Of2[Reader, Env]#Apply]
    }
}