type arguments [W] do not conform to trait type parameter bounds

问题

trait SomeClass {
    ...
}

trait AnotherClass[+V <: SomeClass] {
    ...
}

trait SomeAnotherClass[+V <: SomeClass] {
     protected def someFunc[W >: V](anotherClass: AnotherClass[W]) = {
          ...
     }
}

I'm getting this error:

type arguments [W] do not conform to trait AnotherClass's type parameter bounds [+V <: SomeClass]
[error]   protected def someFunc[W >: V](anotherClass: AnotherClass[W]) = ...
[error]                                                    ^
[error] one error found

I don't get error when I do [W >: V <: SomeClass] instead of just [W >: V] , but in this case, it is shadowing the variable. Please help, how to resolve this.

UPDATE:

protected def someFunc(anotherClass: AnotherClass[V]) = {
              ...
}

I get error covariant type V occurs in contravariant position in type

回答1:

When you say W >: V, you're saying that the type parameter W of someFunc must have a lower type bound of V. That means that W can be V or any super-type of it, which would break the type bounds that V <: SomeClass.

For example Any >: SomeClass, so W in this hypothetical situation could be Any, but Any = V <: SomeClass is not also not true, so the type bounds break.

When you say W >: V <: SomeClass, then W has a lower bound of V and an upper bound of SomeClass. The upper bound of SomeClass for W is important, because the contained type of AnotherClass also has an upper bound of SomeClass. Without that, it would try to allow AnotherClass[Any] (or some other super-type that isn't SomeClass), which of course it can't.

How can you solve this? You have you pick one way to do it. You can't have W >: V and AnotherClass[+V <: SomeClass] at the same time. It just won't work, because the type bounds conflict.

来源：https://stackoverflow.com/questions/28383284/type-arguments-w-do-not-conform-to-trait-type-parameter-bounds

标签

scala

scala-2.10