使用单位法

Question

我有以下定义和实现：

case class State[S, +A](run: S => (A, S)) {
  def map[B](f: A => B): State[S, B] =
    flatMap(a => unit(f(a)))

  def map2[B, C](sb: State[S, B])(f: (A, B) => C): State[S, C] =
    flatMap(a => sb.map(b => f(a, b)))

  def flatMap[B](f: A => State[S, B]): State[S, B] = State(s => {
    val (a, s1) = run(s)
    f(a).run(s1)
  })
}

object State {

  def unit[S, A](a: A): State[S, A] =
    State(s => (a, s))

  def get[S]: State[S, S] = State(s => (s, s))

}

trait RNG {
  def nextInt: (Int, RNG) // Should generate a random `Int`. We'll later define other functions in terms of `nextInt`.
}

object RNG {

  // NB - this was called SimpleRNG in the book text

  case class Simple(seed: Long) extends RNG {
    def nextInt: (Int, RNG) = {
      val newSeed = (seed * 0x5DEECE66DL + 0xBL) & 0xFFFFFFFFFFFFL // `&` is bitwise AND. We use the current seed to generate a new seed.
      val nextRNG = Simple(newSeed) // The next state, which is an `RNG` instance created from the new seed.
      val n = (newSeed >>> 16).toInt // `>>>` is right binary shift with zero fill. The value `n` is our new pseudo-random integer.
      (n, nextRNG) // The return value is a tuple containing both a pseudo-random integer and the next `RNG` state.
    }
  }
}

我的问题是，如何在 State 对象上使用 unit 函数？我试过如下：

val s2 = State.unit[RNG, Int](4563)
println(s2.run((x: RNG) => x))

但是编译器抱怨：

Error:(12, 29) type mismatch;
 found   : state.RNG => state.RNG
 required: state.RNG
    println(s2.run((x: RNG) => x)._1)

怎么了？

Answer 1

编译错误是由于您调用了s2.run。 run 是类型为 S => (A, S) 的函数，因此给定状态类型的值 returns 包含结果和新状态的一对。由于 s2 的类型为 State[RNG, Int]，s2.run 的类型为 RNG => (Int, RNG)，因此您需要提供 RNG 的值，例如：

s2.run(new RNG.Simple(1))

您正在提供功能 RNG => RNG。

使用单位法

Using unit method

monads

functional-programming

scala