Using a context bound in a class type parameter
implicit, parametric-polymorphism, scala, typeclass
Solution
The `A: Foo` notation for context bounds is only a shortcut for asking for an implicit value parameter of type `Foo[A]`. Since traits do not have constructor value parameters, you can not use this with a trait:
trait Foo[A]
trait Bar[A: Foo] // "error: traits cannot have type parameters with context bounds..."
Whereas in classes it's possible:
class Bar[A: Foo] {
def foo: Foo[A] = implicitly[Foo[A]]
}
Which is just a different way of writing
class Bar[A](implicit foo: Foo[A])
You provide the evidence like you do in any other normal method call:
new Bar[Int]()(new Foo[Int] {}) // explicitly
Or:
implicit val iFoo = new Foo[Int] {}
new Bar[Int] // implicitly
Problem
I was under the impression that context bounds would work only on methods: ``` trait Target[T] class Post { def pinTo[T : Target](t:T) } ``` apparently context bounds can be used in `class` too (but not in `trait`): ``` trait Target[T] class Post[T: Target] { def pintTo[T](t:T) } ``` Now I'm confused as to how the evidence can be provided to `Post`? ``` class Business implicit object ev extends Target[Business] // is implicit necessary here ? val p = new Post[Business] // ?? how do I provide ev ? ``` related to Modeling a binary relationship between two types