为什么scala不会将这种类型的lambda与底层类型统一起来？

Question

trait A {
  type T
  def test(t: T): Unit
}

case class B[S <: A](a: S, t : S#T) {
  def test() = a.test(t) // Error: type mismatch;
    // found   : B.this.t.type (with underlying type S#T)
    // required: B.this.a.T
}

Am I wrong to expect the above to compile? Can my code be fixed?

Answer 1

Compiler has not sufficient evidence that S#T can be used as argument for test in concrete instance.

Consider this hypotecical example for weakened scala compiler

trait A2 extends A{
  type T <: AnyRef
}

class A3 extends A2{
  override type T = Integer

  def test(t: Integer): Unit = println(t * 2)
}

So B[A2] should accept instance of A3 along with anything that is <: AnyRef while A3 needs exactly Integer for its own test implementation

You can catch concrete type in the definition of B , to make sure what type will be used

case class B[S <: A, ST](a: S {type T = ST}, t: ST) {
  def test() = a.test(t) 
}

Answer 2

I could come up with encodings (removed the type parameters for simplification):

scala> :paste
// Entering paste mode (ctrl-D to finish)

def test0(a: A)(t : a.T) = a.test(t) 

abstract class B{
  val a: A
  val t: a.T
  def test = a.test(t)
}

// Exiting paste mode, now interpreting.

test0: (a: A)(t: a.T)Unit
defined class B

This on the other hand didn't work with case classes arguments (nor classes' for that matter).

One of the reasons your encoding wouldn't work:

scala> def test1(a: A)(t : A#T) = a.test(t) 
<console>:12: error: type mismatch;
 found   : t.type (with underlying type A#T)
 required: a.T
       def test1(a: A)(t : A#T) = a.test(t)

The important part is required: aT (versus A#T ). The test method in A doesn't take any T, it takes T this.T , or in other words, the T belonging to one particular instance of A.

Answer 3

Instead of a type projection you can use the dependent type aT :

trait A {
  type T
  def test(t: T): Unit
}

case class B[S <: A](a: S)(t : a.T) {
  def test() = a.test(t)
}