Scala Language Type Variance Covariance on an invariant trait
Example
There is also a way to have a single method accept a covariant argument, instead of having the whole trait covariant. This may be necessary because you would like to use T in a contravariant position, but still have it covariant.
trait LocalVariance[T]{
/// ??? throws a NotImplementedError
def produce: T = ???
// the implicit evidence provided by the compiler confirms that S is a
// subtype of T.
def handle[S](s: S)(implicit evidence: S <:< T) = {
// and we can use the evidence to convert s into t.
val t: T = evidence(s)
???
}
}
trait A {}
trait B extends A {}
object Test {
val lv = new LocalVariance[A] {}
// now we can pass a B instead of an A.
lv.handle(new B {})
}
