在 Scala 中，如何定义一个带有元组参数的函数变量

Question

在 Scala class 中，objective 是实现一个 Builder 模式，构造的 class 的参数之一是一个函数，它接受一个元组和 returns一个双。

在生成器 class 中，需要一个变量 tupleFunc 来保存具有默认实现的此类函数。 Builder 还将有一个方法 tupleFuncIs 来覆盖默认值。

class FunctionProblem private (val tupleFunc: (Boolean, Double, Int) => Double,
                               val func: Double => Double) {

}

object FunctionProblem {
  class Builder {
    // problem line follows
    private var tupleFunc: ((Boolean, Double, Int) => Double) = (x: (Boolean, Double, Int)) => if (x._1) x._2 else x._3.toDouble
    private var func: (Double => Double) = (x: Double) => 0.75 * x

    def tupleFuncIs(x: (Boolean, Double, Int) => Double): Builder = { tupleFunc = x; this }
    def funcIs(x: Double => Double): Builder = { func = x; this }

    def build(): FunctionProblem = new FunctionProblem(tupleFunc, func)
  }

  def builder(): Builder = new Builder
}

但是，Scala 编译器抱怨类型不匹配

[ant:scalac]  found   : ((Boolean, Double, Int)) => Double
[ant:scalac]  required: (Boolean, Double, Int) => Double
[ant:scalac]     private var tupleFunc: ((Boolean, Double, Int) => Double) = (x: (Boolean, Double, Int)) => if (x._1) x._2 else x._3.toDouble
[ant:scalac]                                                                                             ^

为什么会发现双括号，或者更好的是，应该如何重写tupleFunc以满足编译器？

Answer 1

你需要更多parens

class FunctionProblem private (val tupleFunc: ((Boolean, Double, Int)) => Double,
  val func: Double => Double) {

}

object FunctionProblem {
  class Builder {
    // problem line follows
    private var tupleFunc: ((Boolean, Double, Int)) => Double = (x: (Boolean, Double, Int)) => if (x._1) x._2 else x._3.toDouble
    private var func: (Double => Double) = (x: Double) => 0.75 * x

    def tupleFuncIs(x: ((Boolean, Double, Int)) => Double): Builder = { tupleFunc = x; this }
    def funcIs(x: Double => Double): Builder = { func = x; this }

    def build(): FunctionProblem = new FunctionProblem(tupleFunc, func)
  }

  def builder(): Builder = new Builder
}

请输入您的类型

((Boolean, Double, Int) => Double)

只是一个接受 3 个参数而不是元组的函数，对于元组，你需要将它包装在另一个 () 中，如下所示：

(((Boolean, Double, Int)) => Double)

顺便说一句，尽管您的代码中到处都是函数，但功能不是很好，它看起来很像 java。您也许应该考虑使用不可变值和 return 新构建器进行中间化。

我建议以更多的 scala 方式实现此任务是使用 case class

import FunctionProblem._

case class FunctionProblem(tupleFunc: TupleFunc = defaultTupleFunc, func: DoubleFunc = defaultDoubleFunc)

object FunctionProblem {
  type TupleFunc = ((Boolean, Double, Int)) => Double
  type DoubleFunc = Double => Double

  private val defaultTupleFunc: TupleFunc = x => if (x._1) x._2 else x._3.toDouble
  private val defaultDoubleFunc: DoubleFunc = 0.75 * _
}

FunctionProblem().func(3) //res0: Double = 2.25
FunctionProblem(func = _ + 5).func(3) //res1: Double = 8.0
FunctionProblem(tupleFunc = _ => 32).tupleFunc((true, 5, 1)) //res2: Double = 32.0

如果您想链式调用，可以使用自动生成的 copy 方法

FunctionProblem().copy(func = _ + 9).copy(tupleFunc = _ => 3).func(1) //res3: Double = 10.0

在 Scala 中，如何定义一个带有元组参数的函数变量

In Scala, how to define a variable that is a function with a tuple argument

scala

tuples