Question

In Scala, I can make a caseclass, case class Foo(x:Int), and then put it in a list like so:

List(Foo(42))

Now, nothing strange here. The following is strange to me. The operator :: is a function on a list, right? With any function with one argument in Scala, I can call it with infix notation.
An example is 1 + 2 is a function (+) on the object Int. The class Foo I just defined does not have the :: operator, so how is the following possible?

Foo(40) :: List(Foo(2))

In Scala 2.8 RC1, I get the following output from the interactive prompt:

scala> case class Foo(x:Int)
defined class Foo

scala> Foo(40) :: List(Foo(2))
res2: List[Foo] = List(Foo(40), Foo(2))

I can go on and use it, but what is the explanation?

Answer 1

From the Spec:

6.12.3 InfixOperations An infix operator can be an arbitrary
identifier. Infix operators have
precedence and associativity defined
as follows.

…

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.

You can always see how these rules are applied in Scala by printing the program after it has been through the ‘typer’ phase of the compiler:

scala -Xprint:typer -e "1 :: Nil"

val r: List[Int] = {
  <synthetic> val x$1: Int = 1;
  immutable.this.Nil.::[Int](x$1)
};