Why is it possible to mix in classes in a type field?

Question

Why can I say that a type field has the type of a class with another class mixed into it (when only traits can be mixed in a class) ?

Example:

scala> class A
defined class A

scala> class B extends A
defined class B

Mixing in B to A is not allowed:

scala> new A with B
<console>:10: error: class B needs to be a trait to be mixed in
             new A with B
                        ^

But this is possible:

scala> class E {type T = A with B}
defined class E

scala> new E
res1: E = E@1f2bc83

Answer 1

There is a difference between the mixin instantiation and the compound type definition. First of all the type A with B exists and is exactly the type B, alas it is perfectly legal in scala to write

val x: A with B = new B

as is

val y: Any with AnyRef with A with B = new B

as it describes exactly the same type. You are just introducing restrictions in the type of the value you can assign to a variable of that type. These restrictions of course always hold in that case.

Furthermore you have to keep in mind that Scala does not necessarily need a type to be inhabited - i.e. the bottom type Nothing may not be instantiated at all. But as Nothing is a subtype of every type that can be expressed in Scala it is even valid to write an expression like

def foo: AnyRef with AnyVal = sys.error("IMPOSSIBRU!")

Nothing is a subtype of AnyRef with AnyVal by definition thus that declaration typechecks.

Answer 2

This is called a compound type and has nothing to do with traits. It allows you to express that a type is a subtype of several other types.

For more information where they can occur see the Scala tag info in section "type handling".