[Scala] Generics tip du jour

[wtd]

So you create a generic class Foo.

code:

class Foo[T]

Now, you have classes A and B.

code:

class A class B extends A

Now, we can do something very simple.

code:

val bar: Foo[A] = new Foo[A] val baz: Foo[B] = new Foo[B]

It works fine. Now, let's extend Foo.

code:

class Foo[T] class Bar[T] extends Foo[T]

And now the following works fine too.

code:

val bar: Foo[A] = new Bar[A] val baz: Foo[B] = new Bar[B]

But... what about...

code:

val bar: Foo[A] = new Foo[B]

This fails!

But, why should it? B is a subtype of A, so Foo[B] should be a subtype of Foo[A], right?

We can make it work.

code:

class Foo[+T] class Bar[+T] extends Foo[T]

Now we can even...

code:

val baz: Foo[A] = Bar[B]