If trait A[+T] extends B, should I be able to pass classes of type A[T] where B is needed?

Question

Here is the exact situation I am in (I do not want to simplify it for fear of missing out the cause of the error):

In the framework I am working in there is a

trait RequestHeader{...}

and another trait

trait Request[+A] extends RequestHeader{...}

There is a function that expects to receive an argument of type:

def f(arg: RequestHeader => Result) = {...}

I would expect to be able to pass in

arg': Request[AnyContent] => Result

However, this causes the compiler to complain. Why is this?

Answer 1

Function1 is contravariant in its input type, which makes sense if you think about it. Consider the following simpler example:

trait Foo
case object Bar extends Foo
case object Baz extends Foo

def f(g: Foo => Foo): Foo = g(Baz)

If I give f a function of type Bar => Foo , it's not going to be able to apply that to Baz . (A function of type Foo => Bar , on the other hand, would be perfectly fine.)