For “trait Queue[T]”, is `Queue` a type?

Question

Defined a trait in scala:

trait Queue[T]

Is Queue a type? Or something else, eg a type constructor?

From http://artima.com/pins1ed/type-parameterization.html#19.3 of book "programming in scala", it says:

Queue, as defined in Listing 19.4, is a trait, but not a type.

But someone thinks it is a type, so I totally confused.

Is it a type or not? If not, what it is exactly?

Answer 1

The following sentence answers your question:

Queue, as defined in Listing 19.4, is a trait, but not a type. Queue is not a type because it takes a type parameter.

We call Queue a generic type . You cannot use it alone, otherwise the compiler will complain

trait Queue takes type parameters

Try this:

type Q1 = Queue[Int]
//type Qwrong = Queue