什么可能导致以下运算符重载以交换操作数？

Question

我正在使用 scala 语言定义 2 个运算符：:++ 和 ++:，它们彼此完全相同：a :++ b == b ++: a，它们显然不可交换：a :++ b != a ++: b.

这是我用于测试的 Scala 代码：

import org.scalatest.FunSpec

import scala.collection.immutable.ListMap

case class Example(self: ListMap[String, String] = ListMap.empty) {

  def :++(v: Example) = this.copy(
    self ++ (v.self -- self.keys.toSeq)
  )

  def ++:(v: Example) = {
    println("forward: " + :++(v))
    println("reverse: " + (v :++ this))
    v :++ this
  }
}

class OperatorOverrideSuite extends FunSpec {

  val p1 = Example(ListMap("a" -> "1"))
  val p2 = Example(ListMap("a" -> "2"))

  it(":++ operator should preserve first value") {
    assert(p1 :++ p2 == p1)
  }

  it("++: operator should preserve second value") {
    assert(p1 ++: p2 == p2)
  }
}

第一个测试看起来一切正常，但是当运行我得到以下错误：

forward: Example(Map(a -> 2))
reverse: Example(Map(a -> 1))

Example(Map(a -> 1)) did not equal Example(Map(a -> 2))
ScalaTestFailureLocation: com.schedule1.datapassports.params.OperatorOverrideSuite$$anonfun at (OperatorOverrideSuite.scala:30)
Expected :Example(Map(a -> 2))
Actual   :Example(Map(a -> 1))
 <Click to see difference>

org.scalatest.exceptions.TestFailedException: Example(Map(a -> 1)) did not equal Example(Map(a -> 2))
    at org.scalatest.Assertions$class.newAssertionFailedException(Assertions.scala:528)
    at org.scalatest.FunSpec.newAssertionFailedException(FunSpec.scala:1630)
    at org.scalatest.Assertions$AssertionsHelper.macroAssert(Assertions.scala:501)
    at ...

从打印的消息来看，scala 似乎覆盖了我的运算符并自行反转操作数，什么可能导致 scala 编译器以这种方式运行？这是一个错误吗？

我正在使用最新的 scala 2.11 和最新的 Java 8u181 进行测试。

Answer 1

来自Scala的语言specification:

The associativity of an operator is determined by the operator's last character. Operators ending in a colon `:' are right-associative. All other operators are left-associative.

这是一个演示：

scala> case class Test(name: String) {
     |     def ++:(that: Test) = println(s"Called ++: on $this with argument $that")
     |     def :++(that: Test) = println(s"Called :++ on $this with argument $that")
     | }
defined class Test

scala> val (x, y) = (Test("x"), Test("y"))
x: Test = Test(x)
y: Test = Test(y)

scala> x ++: y
Called ++: on Test(y) with argument Test(x)

scala> x :++ y
Called :++ on Test(x) with argument Test(y)

因此，当您在代码中说 p1 ++: p2 时，执行的是 p2.++:(p1)，相当于 p1 :++ p2。这意味着您的两个运算符实际上是严格等价的。

什么可能导致以下运算符重载以交换操作数？

What may have caused the following operator overloading to swap operands?

scala

operator-overloading

scala-compiler