How Scala allows overriding with type parameters and not with type class

Question

Any thoughts why approach 2 is not supported, or am I missing any syntax ?

trait Bar { }
class BarImpl extends Bar{ }

1 Scala allows to override with generic type parameter

abstract class Foo {
  type T <: Bar
  def bar1(f: T): Boolean
}

class FooImpl extends Foo {
  type T = BarImpl
  override def bar1(f: BarImpl): Boolean = true 
}

2 While it doesn't allow with generic type class

abstract class Foo2[T <: Bar] {
  def bar1(f: T): Boolean
}

class FooImpl2[BarImpl] extends Foo2 {
  // Error: Method bar1 overrides nothing
  override def bar1(f: BarImpl): Boolean = true
}

score 3 · Accepted Answer · answered Apr 23 '19 at 18:36

3

In your implementation of FooImpl2 you are passing BarImpl as a new Type parameter for FooImpl2 instead of passing it to Foo2 (that is the one that is expecting a type parameter).

So what you have to do is:

class FooImpl2 extends Foo2[BarImpl] {
    override def bar1(f: BarImpl): Boolean = true
}

answered Apr 23 '19 at 18:36

Gonzalo Guglielmo

629
3
10

Any idea when do we use approach 1 vs 2 – skjagini Apr 23 '19 at 18:59
1

Check https://typelevel.org/blog/2015/07/13/type-members-parameters.html – Gonzalo Guglielmo Apr 23 '19 at 19:09
Also: https://stackoverflow.com/questions/3170821/abstract-types-versus-type-parameters – Gonzalo Guglielmo Apr 23 '19 at 19:11

How Scala allows overriding with type parameters and not with type class

1 Answers1