在Scala中，如何使用元组参数定义一个函数变量

Question

在一个Scala类中，目标是实现一个Builder模式，并且构造类的参数之一是一个函数，它接收一个元组并且返回一个Double。

在Builder类中，变量tupleFunc需要用默认实现来保存此类型的函数。 Builder还会有一个方法tupleFuncIs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 
} 

然而，Scala编译器抱怨类型不匹配的

[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]                       ^

为什么有发现，双括号或更好，但应该如何tupleFunc被改写，以满足编译器？

Answer 1

你需要更多的括号

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 
} 

这里

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

你的类型是一个简单的函数，有3个参数不是一个元组，对元组需要将其包装在一个更()这样：

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

顺便说一句，尽管你的代码中有函数不是ve ry功能性，它看起来很像java。你也许应该考虑使用不可变的值，并在中立时返回新的构建器。

我的建议，以实现更多的Scala方式这个任务是使用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 

如果你想，你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 

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