Relationship between parameterized class with different type, Scala - scala

Question

In Scala,
What is the relationship between parameterized class with different type?
For example,
class A[T]
What is the relationship between A[Int] (or A[String]) and A?
and what is the relationship between A[Int] and A[String] ?
I want to know, because I would like to do something like
case class A[T](v: T) { def print = println(v) }
def iter(s: Seq[A[???]]) = s.map(print) // What is the proper annotation of '???'
iter(Seq(A(1), A("Hello")) // 1 Hello

Answer 1

What is the relationship between A[Int] and A?
A is a type constructor of kind * → *, and A[Int] is one of the possible results of apply a type (Int) to the type constructor A.
What is the relationship between A[Int] and A[String]?
The least upper bound between these two types is A[_ >: String with Int], which can only be instantiated to A[Any] since it's the only super class of String and Int.
What is the proper annotation of '???'
In your example that would be Any, or a type parameter you could add to your def iter that would itself be instantiated to Any.

Answer 2

Types are related using Variance.
To answer your question, Java's wildcards equivalent in Scala is Existential types. So you can specify something like this:
def iter(s: Seq[A[_]]) = s.map(_.print)
which is equivalent to:
def iter(s: Seq[A[T] forSome {type T}]) = s.map(_.print)
You can also specify bounds like below:
def iter(s: Seq[A[_ <: CharSequence]]) = s.map(_.print)
println(iter(Seq(A[StringBuilder](new StringBuilder("Test")), A[String]("Hello"))))
[Explicit type specified in constructors to avoid implicit conversion to required type CharSequence].
Note that the following would not compile though:
def iter(s: Seq[A[CharSequence]]) = s.map(_.print)
println(iter(Seq(A[StringBuilder](new StringBuilder("Test")), A[String]("Hello"))))
This is where variance specification in class would help:
case class A[+T](v: T) {
def print = v
}
def iter(s: Seq[A[CharSequence]]) = s.map(_.print)
println(iter(Seq(A[StringBuilder](new StringBuilder("Test")), A[String]("Hello"))))