scalaz List [StateT] .sequence - 找不到参数n的隐式值:scalaz.Applicative

Question

我试图找出如何使用基于对我的Scalaz状态monad示例答案的评论StateT来组合两个State状态变换器.

看起来我很亲密,但在尝试申请时遇到了问题sequence.

import scalaz._
import Scalaz._
import java.util.Random

val die = state[Random, Int](r => (r, r.nextInt(6) + 1))

val twoDice = for (d1 <- die; d2 <- die) yield (d1, d2)

def freqSum(dice: (Int, Int)) = state[Map[Int,Int], Int]{ freq =>
  val s = dice._1 + dice._2
  val tuple = s -> (freq.getOrElse(s, 0) + 1)
  (freq + tuple, s)
}

type StateMap[x] = State[Map[Int,Int], x]

val diceAndFreqSum = stateT[StateMap, Random, Int]{ random =>
  val (newRandom, dice) = twoDice apply random
  for (sum <- freqSum(dice)) yield (newRandom, sum)
}

所以我得到了一个StateT[StateMap, Random, Int]可以解开初始随机和空地图状态的东西:

val (freq, sum) = diceAndFreqSum ! new Random(1L) apply Map[Int,Int]()
// freq: Map[Int,Int] = Map(9 -> 1)
// sum: Int = 9

现在我想生成一个列表StateT并使用,sequence以便我可以调用list.sequence ! new Random(1L) apply Map[Int,Int]().但是当我尝试这个时,我得到:

type StT[x] = StateT[StateMap, Random, x]
val data: List[StT[Int]] = List.fill(10)(diceAndFreqSum)
data.sequence[StT, Int]

//error: could not find implicit value for parameter n: scalaz.Applicative[StT]
          data.sequence[StT, Int]
                       ^

任何的想法？我可以在最后一段时间使用一些帮助 - 假设它是可能的.

Answer 1

啊看着scalaz Monad源代码,我注意到有一个implicit def StateTMonad确认StateT[M, A, x]是类型参数x的monad.monad也是应用程序,通过查看特征的定义Monad并在REPL中查看来确认:

scala> implicitly[Monad[StT] <:< Applicative[StT]]
res1: <:<[scalaz.Monad[StT],scalaz.Applicative[StT]] = <function1>

scala> implicitly[Monad[StT]]
res2: scalaz.Monad[StT] = scalaz.MonadLow$$anon$1@1cce278

所以这给了我定义一个隐式Applicative[StT]来帮助编译器的想法:

type StT[x] = StateT[StateMap, Random, x]
implicit val applicativeStT: Applicative[StT] = implicitly[Monad[StT]]

这就是诀窍:

val data: List[StT[Int]] = List.fill(10)(diceAndFreqSum)
val (frequencies, sums) =
  data.sequence[StT, Int] ! new Random(1L) apply Map[Int,Int]()

// frequencies: Map[Int,Int] = Map(10 -> 1, 6 -> 3, 9 -> 1, 7 -> 1, 8 -> 2, 4 -> 2)
// sums: List[Int] = List(9, 6, 8, 8, 10, 4, 6, 6, 4, 7)