Function[Any, String] Trait and Variance

Question

Scala in Depth presents the following variance example.

scala> trait Function[-Arg, +Return] {
     |  def apply(arg: Arg): Return
     | }
defined trait Function

scala> val foo = new Function[Any, String] {
     |  override def apply(arg: Any): String =
     |    "Hello. I received " + arg
     | }
foo: Function[Any,String] = $anon$1@5db0e244

scala> val bar: Function[String, Any] = foo
bar: Function[String,Any] = $anon$1@5db0e244

What's the reason that we can assign foo to bar? I have a rough idea, but figured I'd ask outright.

Answer 1

Exactly because the variance annotations make Function[Any, String] a subtype of Function[String, Any].

The covariance annotation, + means a narrower type argument yields a narrower constructed type and the contravariance annotation -, means a wider type parameter yields a narrower constructed type.