Trait A can only be extended by class that extends Trait B

Question

Say I have two traits (A and B) and a class C which mixes them in and implements their methods:

trait A { def foo } trait B { def bar } class C extends A with B { def foo = "Foo" def bar = "Bar" }

Is there any way in Scala to specify that the class that extends trait B must extend trait A and then use the implemented method of trait A's defined method in trait B?

So that B could call this.foo() and access the value that has been returned by C's implementation?

om-nom-nom · Accepted Answer · 2013-07-22 15:20:35Z

6

Just specify what you want this to be:

trait B { this: A => def bar = this.foo }

This is so called self type and this here is an alias rather than keyword (so self: A, that: A and so on are perfectly legal).

edited Jul 22, 2013 at 15:20

om-nom-nom

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answered Jul 22, 2013 at 15:18

Debilski

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2 Comments

gzm0 Over a year ago

A note aside: It often raises some confusion that the self type is not part of the public API of B. I.e. if you have an instance of B, you won't be able to call foo on it.

mirelon Over a year ago

Is the inaccessibility of foo from outside the only difference with trait B extends A ?

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