双眼皮功能型图案

Question

让玩具类Counter如：

class Counter private( val next: Int, val str2int: Map[String,Int] ) {
  def apply( str: String ): (Int,Counter)  = str2int get str match {
    case Some(i) => ( i, this )
    case None => ( next, new Counter( next+1, str2int + (str -> next) ) )
  }
}
object Counter {
  def apply() = new Counter( 0, Map() )
}

该类提供了String和自然数之间的映射，每次查询新的String时都会延迟扩展该映射。

然后我可以编写一个方法，可以将字符串序列转换为整数序列，从而在遍历期间更新映射。我得到的第一个实现是使用 foldLeft：

def toInt( strs: Seq[String], counter: Counter ): ( Seq[Int], Counter ) =
  strs.foldLeft( (Seq[Int](), counter) ) { (result, str) =>
    val (i, nextCounter) = result._2( str )
    ( result._1 :+ i, nextCounter )
  }

这按预期工作：

val ss = Seq( "foo", "bar", "baz", "foo", "baz" )
val is = toInt( ss, Counter() )._1
            //is == List(0, 1, 2, 0, 2)

但我对 toInt 实现不是很满意。问题是我折叠了两个不同的值。是否有函数式编程模式来简化实现？

Answer 1

您正在寻找的模式是 State monad：

import scalaz._
import Scalaz._

case class Counter(next: Int = 0, str2int: Map[String,Int] = Map()) {
  def apply( str: String ): (Counter, Int) = (str2int get str) fold (
    (this, _),
    (new Counter(next+1, str2int + (str -> next)), next)
  )}

type CounterState[A] = State[Counter, A]

def count(s: String): CounterState[Int] = state(_(s))

def toInt(strs: Seq[String]): CounterState[Seq[Int]] =
  strs.traverse[CounterState, Int](count)

那里的类型注释很不幸，也许可以通过某种方式消除它。无论如何，这是一个运行过程：

scala> val ss = Seq( "foo", "bar", "baz", "foo", "baz" )
ss: Seq[java.lang.String] = List(foo, bar, baz, foo, baz)

scala> val is = toInt(ss) ! Counter()
is: Seq[Int] = List(0, 1, 2, 0, 2)

Answer 2

您可以通过进行更多模式匹配来使折叠看起来更好一点：

strs.foldLeft((Seq[Int](), counter)) { case ((xs,counter), str) =>
  val (i, nextCounter) = counter(str)
  (xs :+ i, nextCounter)
}

然后，如果您有 管道运算符 |> 在某处定义，并且您对 /: 别名感到满意>向左折叠，你 就可以使其变得紧凑且可读。

((Seq[Int](), counter) /: strs) { case ((xs,counter), str) =>
  counter(str) |> { case (i,nextCounter) => (xs +: i, nextCounter) }
}

一旦您熟悉了语法，

Answer 3

我认为 state monad 就是您正在寻找的。

Answer 4

折叠两个值并没有什么问题。它可以稍微改进：

import scalaz._
import Scalaz._

def toInt( strs: Seq[String], counter: Counter ): ( Seq[Int], Counter ) =
  strs.foldLeft( (Seq[Int](), counter) ) { case ((xs, counter), str) =>
    counter(str).mapElements(xs :+ _, identity)
  }

或者，如果你愿意，

def toInt( strs: Seq[String], counter: Counter ): ( Seq[Int], Counter ) =
  strs.foldLeft( (Seq[Int](), counter) ) { case ((xs, counter), str) =>
    (xs :+ (_: Int)) <-: counter(str)
  }

或者甚至，

def toInt( strs: Seq[String], counter: Counter ): ( Seq[Int], Counter ) =
  strs.foldLeft( (Seq[Int](), counter) ) { case (result, str) =>
    result.fold((xs, counter) =>
      counter(str).mapElements(xs :+ _, identity))
  }