Is cons a method or a class?

Question

Marting Odersky in his book writes:

Class ::, pronounced “cons” for “construct,” represents non-empty lists.

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Answer 1

Short answer: both.

There is a subclass of list called ::, but you won't refer to it explicitly very often.

When you write e.g. 1 :: 2 :: Nil, the :: is a method on List, which creates an instance of the class :: behind the scenes.

:: is best thought of not as a method or a class, though, but as a constructor in the algebraic data type (ADT) sense. Wikipedia calls ADT constructors "quasi-functional entit[ies]", which makes them sound more complicated than they are, but isn't necessarily a bad way to think about them.

List has two constructors, :: (called cons in some languages) and Nil. The Wikipedia article I've linked above gives a good introduction to the idea of lists as algebraic data types.

It's worth noting that in some languages (like Haskell) ADT constructors don't have their own types associated with them—they are just functions that create an instance of the ADT. This is typically effectively the case in Scala as well, where it's fairly rare to refer to the type of an ADT constructor like ::. It is possible, though—we can write this:

def foo(xs: ::[Int]) = ???

Or this (where Some is one of the constructors for the Option ADT).

def bar(s: Some[Int]) = ???

But this is in general not very useful, and could be considered an artifact of the way Scala implements algebraic data types.