问题描述
学习 Scala3 扩展和 CanEqual 概念,但在扩展 Int 的某些功能时发现困难。
在下面的示例中,我可以轻松地向 Int 添加 >= 功能以将其与 RationalNumber 案例类进行比较,但无法修改 == 的行为。 (注意 1~2 与 RationalNumber(1,2) 相同)。
问题似乎与基本的 AnyVal 类型以及 Scala 如何传递给 Java 来处理 equals 和 == 相关。
case class RationalNumber(val n: Int,val d: Int):
def >=(that:RationalNumber) = this.num * that.den >= that.num * this.den
//... other comparisons hidden (note not using Ordered for clarity)
private def gcd(a: Int,b: Int): Int = if (b == 0) a else gcd(b,a % b)
val sign = if (n<0 ^ d<0) -1 else 1
private val (an,ad) = (math.abs(n),math.abs(d))
val num = sign * (an / gcd(an,ad))
val den = if(an == 0) 1 else ad / gcd(an,ad)
override def equals (that: Any): Boolean =
that match
case t: RationalNumber => t.den == den && t.canEqual(this) && t.num == num
case t: Int => equals(RationalNumber(t,1))
case _ => false
override lazy val toString = s"$num/$den"
object RationalNumber:
def apply (r: Int): RationalNumber = new RationalNumber(r,1)
import scala.language.implicitConversions
implicit def intToRat (i: Int): RationalNumber = i ~ 1
given CanEqual[RationalNumber,Int] = CanEqual.derived
given CanEqual[Int,RationalNumber] = CanEqual.derived
extension (i: Int)
def ~(that: Int) = new RationalNumber(i,that)
def >=(that: RationalNumber) = i ~ 1 >= that
def equals (that: AnyVal) : Boolean =
println("this never runs")
that match
case t: RationalNumber => t.den == 1 && t.num == i
case _ => i == that
def ==(that: RationalNumber) =
println ("this never runs")
i~1 == that
object Main:
@main def run =
import RationalNumber._
val one = 1 ~ 1
val a = 1 == one // never runs extension ==
val b = one == 1
val c = 1 >= one
val d = one >= 1
val ans = (a,b,c,d) // (false,true,true)
println(ans)
解决方法
仅当不存在同名的限定方法时才尝试扩展方法。因此,至少以下符合条件的 ==
已经在 Int
def ==(arg0: Any): Boolean
它不会呼叫您的分机。如果您将名称更改为 ===
那么它会起作用
def ===(that: RationalNumber)
如果需要,您可以强制使用类型归属 (1: RationalNumber) == one
进行隐式转换。 (不鼓励隐式转换)。
尝试扩展 ScalaNumericConversions
,然后扩展 ScalaNumber
case class RationalNumber(val n: Int,val d: Int) extends ScalaNumericConversions {
def intValue: Int = ???
def longValue: Long = ???
def floatValue: Float = ???
def doubleValue: Double = ???
def isWhole: Boolean = false
def underlying = this
...
override def equals (that: Any): Boolean = {
that match {
case t: RationalNumber => t.den == den && t.canEqual(this) && t.num == num
case t: Int => equals(RationalNumber(t,1))
case _ => false
}
}
}
所以现在Scala最终会调用BoxesRuntime#equalsNumNum
public static boolean equalsNumNum(java.lang.Number xn,java.lang.Number yn) {
...
if ((yn instanceof ScalaNumber) && !(xn instanceof ScalaNumber))
return yn.equals(xn);
}
...
哪个音符翻转了参数的顺序,因此会调用RationalNumber#equals
,所以实际上
1 == one
变成
one.equals(1)
通过查看 REPL 中的 :javap -
for 1 == BigInt(1)
30: invokestatic #54 // Method scala/runtime/BoxesRunTime.equals:(Ljava/lang/Object;Ljava/lang/Object;)Z
然后按照BoxesRunTime.equals