Understanding Scala Notation syntax

Question

I have the following code:

abstract class AList {
  def head:Int
  def tail:AList
  def isEmpty:Boolean
  def ::(n: Int): AList = SimpleList(n, Empty)
}

object Empty extends AList {

  def head = throw new Exception("Undefined")

  def tail = throw new Exception("Undefined")

  def isEmpty = true

}

case class SimpleList(head: Int, tail: AList = Empty) extends AList {

  def isEmpty = false

}

1 :: 2 :: Empty

I wonder how the last line actually works. There is no implicit conversion from Int to SimpleList. Hence I do not understand the method call mechanism.

Object.method(Arg)

I do not see that pattern here. I think a clarification of Scala notation (infix, suffix, postfix, etc...) would help. I'd like to understand the syntactic sugar.

Answer 1

In Scala, method names ending with a colon..

• form right-associative expressions
• are additionaly invokend on the right operand.

So 1 :: 2 :: Empty is actually Empty.::(2).::(1) . .

Answer 2

:: is a method of the right operand. In scala if a method name ends in a colon the method is invoked on the right operand. So 1 :: 2 :: Empty is actually Empty.::(2) which returns a SimpleList .

The subsequent 1 :: <the-new-simple-list> is easy to follow once you understand that :: is a method of the right operand.