Scala：从通用特征调用方法时类型不匹配

Question

Let's look at some code first:

trait Foo[T] {
  def fooize(value : T) : Unit
}
object TestFoo {
  def testFooForType[T[_] <: Foo[_]](obj : T[Int]) {
    obj.fooize(5)
  }
}

Why doesn't it typecheck and how can I overcome this difficulty? What I want to do is to test any class that implements the given trait Foo for a particular type T ( Int in the example).

Answer 1

T[_] <: Foo[_] means "T forSome type" extends "Foo forSome type".

It has to allow for cases where the types are different. (T[A] extends Foo[B])

Binding T with Int type has no effect on Foo's type, which is still unbounded.

You can either specify Foo's type:

def testFooForType[T[_] <: Foo[Int]](obj : T[_]) {
    obj.fooize(5)
}

or you can make the type relationship explicit:

def testFooForType[T[V] <: Foo[V]](obj : T[Int]) {
    obj.fooize(5)
}