Why doesn't Try[List[MyClass]] conform to Try[List[T <: MyClass]]?

Question

I am trying to define a method with the following signature:

def parse[T <: MyClass](statement: String): Try[List[T]] = {

My class is an abstract class:

sealed abstract class MyClass { }

case class MyClassChild(v: Int) extends MyClass

my parse method returns a Success(List[MyClassChild])

But the compiler complains with the following error:

Error:(124, 19) type mismatch;
 found   : scala.util.Try[List[parser.MyClass]]
 required: scala.util.Try[List[T]]

Why doesn't scala.util.Try[List[parser.MyClass]] conform to scala.util.Try[List[T]] , since T <: MyClass ?

Thank you

Answer 1

T should be >: MyClass and Try[List[T]] >: Try[List[MyClass]] (List and also Try is covariant , so it will work) to confirm return type - because your function can't return bigger type than declared (see Liskov substitution principle ):

scala> trait A { type MT; def aaa[T <: MT]: List[T] = null.asInstanceOf[List[MT]] }
<console>:7: error: type mismatch;
 found   : List[A.this.MT]
 required: List[T]


scala> trait A { type MT; def aaa[T >: MT]: List[T] = null.asInstanceOf[List[MT]] }
defined trait A

If you want T <: MyClass - you should change return type to Try[List[MyClass]] :

scala> trait A { type MT; def aaa[T <: MT]: List[MT] = null.asInstanceOf[List[MT]] }
defined trait A

In other words, you can't "shrink" MyClass to T even theoretically because it's bigger by T <: MyClass definition. MyClass.asInstanceOf[T] will give you type cast error with probability > 0, it's like Any.asInstanceOf[String] .