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What is the difference between scala self-types and trait subclasses?
I understand a self-annotation as a promise to the compiler, where the programmer manifests that a trait will be mixin with the annotated one. For example:
scala> trait X defined trait X scala> trait Y { this: X => } defined trait Y scala> new Y {} <console>:10: error: illegal inheritance; self-type Y does not conform to Y's selftype Y with X new Y {} ^ scala> new Y with X {} res1: Y with X = $anon$1@1125a40 In the previous example, the third expression failed because we did not set a valid X to the new instance. Obviously, the last one works nice. So far, so good. And now, let's see another example which involves an object.
scala> object Z { this: X => } defined module Z I understand the object is being instantiated failing with the X promise (we are creating an instance now with a future promise!), as represented in the next lines, where the traits have been slightly modified:
scala> trait X { class X1 } defined trait X scala> trait Y { this: X => new X1 } defined trait Y scala> object Z { this: X => new X1 } <console>:8: error: not found: type X1 object Z { this: X => new X1 } ^ So, what does the object self-annotation imply?
