上下文绑定到更高种类的类型？

Question

I want to do something like this:

def fold[C[A]](implicit ev: Foldable[A]): A

I am getting not found: type A

I know, I can do this instead:

def fold[C[_], A: Foldable]: A

But, I would rather invoke as fold[List[Int]] than fold[List, Int]

Answer 1

Here's what I came up with:

trait Foo[T, A]

implicit def makeFoo[A, M[_]] = new Foo[M[A], A] {}

class Helper[T] {
  def apply[A]()(implicit ev: Foo[T, A]) = ev 
}

def bar[T] = new Helper[T]

bar[List[Int]]()
//Foo[List[Int],Int] = $anon$1@7edf6563

The empty pair of parens might not be ideal if you really want an a no-arg method, but I can't see how to get around that at the moment.

Answer 2

I played a bit with it and came up with a helper type class:

trait Helper[M[_], CA] {
  type C[_]
  type A
  implicit def ma: M[A]
}
object Helper {
  implicit def instance[M0[_], C0[_], A0](implicit ma0: M0[A0]) = new Helper[M0, C0[A0]] {
    type C[X] = C0[X]
    type A = A0
    val ma: M0[A0] = ma0
  }
}

I know the names are pretty generic, I'd suggest finding more meaningful names.

Now instead of requiring an implicit of type Foldable[A] you require instead an implicit of Helper[Foldable, CA] where CA is the type that must match against List[Int] in your example:

def fold[CA](implicit helper: Helper[Foldable, CA]): helper.A

As an example:

def fold[CA](implicit helper: Helper[Foldable, CA]): helper.A = {
  import helper._
  println(implicitly[Foldable[A]])
  null.asInstanceOf[A]
}

scala> :paste
// Entering paste mode (ctrl-D to finish)
case class Foldable[A](name: String)
implicit val stringFoldable = Foldable[String]("String")
implicit val intFoldable = Foldable[Int]("Int")
implicit val floatFoldable = Foldable[Float]("Float")

def fold[CA](implicit helper: Helper[Foldable, CA]): helper.A = {
  import helper._
  println(implicitly[Foldable[A]])
  null.asInstanceOf[A]
}
// Exiting paste mode, now interpreting.

defined class Foldable
stringFoldable: Foldable[String] = Foldable(String)
intFoldable: Foldable[Int] = Foldable(Int)
floatFoldable: Foldable[Float] = Foldable(Float)
fold: [CA](implicit helper: Helper[Foldable,CA])helper.A

scala> fold[List[String]]
Foldable(String)
res0: String = null

scala> fold[List[Int]]
Foldable(Int)
res1: Int = 0

scala> fold[List[Float]]
Foldable(Float)
res2: Float = 0.0