读者单子与Scalaz

Question

我尝试使用scalaz这样定义读者单子：

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？

感谢， 列维

Answer 1

此错误是相当不透明的，甚至Scala的标准。以=结尾的方法名称会被专门处理 - 它们首先被视为正常标识符，如果失败，则将其扩展为自我赋值。

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 
     ^

如果你弄不清你的程序的语法解释，它的运行scalac -Xprint:parser，看看发生了什么事情是一个好主意。同样，您可以使用-Xprint:typer或-Xprint:jvm来查看程序转换的后期阶段。

那么，您如何在您的Reader上拨打>>=？首先，您需要明确地将类型参数Env传递给env。必须将生成的Reader[Env, Env]转换为MA[M[_], A]。对于简单类型的构造函数，隐式转换MAs#ma就足够了。然而，这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，而是你必须提供一个特定的隐式转换。

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

在此之前，这是您的代码。还有几条评论是内联的。

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] 
    } 
}