问题
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 inclass
(and presumably trait
) too:
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
回答1:
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
来源:https://stackoverflow.com/questions/20440279/using-a-context-bound-in-a-class-type-parameter