我试图使用集合的sum
方法使重载加法运算符工作。例如,对于
final case class Probability(val value: Double) {
def +(that: Probability): Probability =
Probability(this.value + that.value)
}
List(Probability(0.4), Probability(0.3)).sum
我收到以下错误消息:
Error:(6, 109) could not find implicit value for parameter num:
Numeric[A$A42.this.Probability]
def get$$instance$$res0 = /* ###worksheet### generated $$end$$ */
List(Probability(0.4), Probability(0.3)).sum;}
Error:(6, 109) not enough arguments for method sum:
(implicit num: Numeric[A$A42.this.Probability])A$A42.this.Probability.
Unspecified value parameter num.
def get$$instance$$res0 = /* ###worksheet### generated $$end$$ */
List(Probability(0.4), Probability(0.3)).sum;}
我认为这必须与implicit
s做一些事情(正如错误信息明确指出的那样),但我不知道应该在哪里定义或添加(在类Probability
,它的人体对象,或者在sum
之后?),或者如何应该看。
Sum方法适用于类型Numeric [T],因此如果要通过自己的“数字类型”调用sum,则需要定义Numeric [Probability]的实例。
放置自定义类型定义的最佳位置是对象伴随。
object Probability {
implicit val probabilityNumberic = new Numeric[Probability] {
def plus(x: Probability, y: Probability): Probability = ???
def minus(x: Probability, y: Probability): Probability = ???
def times(x: Probability, y: Probability): Probability = ???
def negate(x: Probability): Probability = ???
def fromInt(x: Int): Probability = ???
def toInt(x: Probability): Int = ???
def toLong(x: Probability): Long = ???
def toFloat(x: Probability): Float = ???
def toDouble(x: Probability): Double = ???
def compare(x: Probability, y: Probability): Int = ???
}
}
或者,如果您不想定义Numeric [T]实例,或者如果您希望将Probability作为结果,则可以使用reduce而不是sum。
List(Probability(0.4), Probability(0.3)).reduce(_ + _)
如果列表可能为空,则为reduceOption。
关于你可以定义Numeric[Probability]
的位置,你必须考虑implicits的范围。在对象伴侣上定义默认实现是很常见的,但你可以在求和之前通过导入创建它,或者你可以明确地使用它。这取决于你需要什么。