Operand order in Scala List.prepend (::)

Question

Odersky has brilliantly optimized Java syntax, enabling object calls without dots and parenthesis. I.e. instead of list.prepend(item), you now simply write list :: item, which also turns language operators into simple object methods. Here, List defines :: (prepend) operator. However, you normally write it vice-verse in Scala, using item :: list. It suggests that every (listable) type defines the ::(List) operator, which is unbelievable. How is the operand reversal implemented? I cannot figure it out from the source code

class List[+A] extends AbstractSeq[A]
  def ::[B >: A] (x: B): List[B] =
    new scala.collection.immutable.::(x, this)

It seems that ::(val head, val tail) also stands for a type name. But it does not fit the head :: tail pattern anyway.

`::` is Scala's "cons", the non-empty list and counterpart to `Nil`. Note that you can also use infix notation for binary type constructors - think of a generic type as a "type function" which produces a resultant type: `type \/[+A, +B] = Either[A, B]` then `def foo: String \/ Int = ???`. — J Cracknell, Jun 24 '15 at 05:07

Carcigenicate · Accepted Answer · 2015-06-23T17:33:45.613

4

Any operator with a : on its right side has its operands flipped. There's other operators that make use of this too (can't think of any examples off the top of my head though).

edited Jun 23 '15 at 17:33

answered Jun 23 '15 at 10:56

Carcigenicate

43,494
9
68
117

1

With your info, I have found [a similar question/answer](http://stackoverflow.com/a/2827570/4550451). Thanks. – Val Jun 23 '15 at 11:00
@RecognizeEvilasWaste Np. It's nice thinking about Scala. I miss it :/ – Carcigenicate Jun 23 '15 at 11:02

Operand order in Scala List.prepend (::)

1 Answers1