访问类型的伴随对象 [英] access to a type's companion object

问题描述

我有一个某种类型的变量，我想从伴随对象中获取信息.例如，我认为我可以做这样的事情:

I have a variable of some type and I'd like to get information from the companion object. For example, I thought I might be able to do something like this:

def foo[I: Integral](i:I): = {
  val minVal = i match {
    case _:Byte  => Byte.MinValue
    case _:Char  => Char.MinValue
    case _:Int   => Int.MinValue
    case _:Long  => Long.MinValue
    case _:Short => Short.MinValue
  }
  // compare i and minVal
}

但这相当冗长，minVal 以 :Long 的形式出现，这使得与 i: I 的比较变得复杂.

But this is rather verbose and minVal comes out as :Long which complicates comparisons with i: I.

我希望我能找到一些简洁直接的东西，但我怀疑这需要反思，而这往往两者都不是.

I was hoping I could find something concise and direct but I suspect this requires reflection, which is often neither.

推荐答案

您可以使用类型类来获取最小值:

You could use a type class to get the minumum value :

trait MinValue[T] { def minValue: T }
object MinValue {
  implicit val minByte = new MinValue[Byte] { def minValue = Byte.MinValue }
  implicit val minChar = new MinValue[Char] { def minValue = Char.MinValue }
  implicit val minLong = new MinValue[Long] { def minValue = Long.MinValue }
  implicit val minInt  = new MinValue[Int]  { def minValue = Int.MinValue }
}

我们可以使用这个类型类来获取传递给foo函数的值的类型的最小值:

We can use this type class to get the minumum value for the type of the value passed to the foo function :

def foo[I: Integral](i: I)(implicit min: MinValue[I]) = 
  implicitly[Integral[I]].compare(i, min.minValue)
// or 
def foo2[I: Integral: MinValue](i: I) = {
  val minVal = implicitly[MinValue[I]].minValue
  implicitly[Integral[I]].compare(i, minVal)
}

foo(5) // Int = 1
foo(Int.MinValue) // Int = 0

foo2(-127.toByte) // Int = 1
foo2(-128.toByte) // Int = 0

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