Coding

Right associativity in Scala

We define two methods here, ++ and ++:

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class Foo {
  def ++(n: Int): Unit = println(n + 1)
  def ++:(n: Int): Unit = println(n + 1)
}
object ValFunctionTest {
  def main(args: Array[String]): Unit = {
    val foo = new Foo
    foo.++(1)
    foo.++:(1)
  }
}

Nothing special, right? Yes, for now, until we try removing the parentheses in it.

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class Foo {
  def ++(n: Int): Unit = println(n + 1)
  def ++:(n: Int): Unit = println(n + 1)
}
object ValFunctionTest {
  def main(args: Array[String]): Unit = {
    val foo = new Foo
    foo ++ 1
    1 ++: foo
    
    foo ++: 1 // error
    1 ++ foo // error
  }
}

So the difference is, foo can only be placed on the left side when using ++, and it can only be placed on right side when using ++:. The latter is called right associativity, and methods ending with : are used in the right associativity.

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