Add code tabs for _tour/abstract-types

Author: flomebul

_tour/abstract-type-members.md

@@ -17,31 +17,57 @@ Abstract types, such as traits and abstract classes, can in turn have abstract t
 This means that the concrete implementations define the actual types.
 Here's an example:
 
+{% tabs abstract-types_1 class=tabs-scala-version %}
+{% tab 'Scala 2' for=abstract-types_1 %}
 ```scala mdoc
 trait Buffer {
   type T
   val element: T
 }
 ```
+{% endtab %}
+{% tab 'Scala 3' for=abstract-types_1 %}
+```scala
+trait Buffer:
+  type T
+  val element: T
+```
+{% endtab %}
+{% endtabs %}
+
 Here we have defined an abstract `type T`. It is used to describe the type of `element`. We can extend this trait in an abstract class, adding an upper-type-bound to `T` to make it more specific.
 
+{% tabs abstract-types_2 class=tabs-scala-version %}
+{% tab 'Scala 2' for=abstract-types_2 %}
 ```scala mdoc
 abstract class SeqBuffer extends Buffer {
   type U
   type T <: Seq[U]
   def length = element.length
 }
 ```
+{% endtab %}
+{% tab 'Scala 3' for=abstract-types_2 %}
+```scala
+abstract class SeqBuffer extends Buffer:
+  type U
+  type T <: Seq[U]
+  def length = element.length
+```
+{% endtab %}
+{% endtabs %}
+
 Notice how we can use yet another abstract type `U` in the specification of an upper-type-bound for `T`. This `class SeqBuffer` allows us to store only sequences in the buffer by stating that type `T` has to be a subtype of `Seq[U]` for a new abstract type `U`.
 
 Traits or [classes](classes.html) with abstract type members are often used in combination with anonymous class instantiations. To illustrate this, we now look at a program which deals with a sequence buffer that refers to a list of integers:
 
+{% tabs abstract-types_3 class=tabs-scala-version %}
+{% tab 'Scala 2' for=abstract-types_3 %}
 ```scala mdoc
 abstract class IntSeqBuffer extends SeqBuffer {
   type U = Int
 }
 
-
 def newIntSeqBuf(elem1: Int, elem2: Int): IntSeqBuffer =
   new IntSeqBuffer {
     type T = List[U]
@@ -51,10 +77,30 @@ val buf = newIntSeqBuf(7, 8)
 println("length = " + buf.length)
 println("content = " + buf.element)
 ```
+{% endtab %}
+{% tab 'Scala 3' for=abstract-types_3 %}
+```scala
+abstract class IntSeqBuffer extends SeqBuffer:
+  type U = Int
+
+def newIntSeqBuf(elem1: Int, elem2: Int): IntSeqBuffer =
+  new IntSeqBuffer:
+    type T = List[U]
+    val element = List(elem1, elem2)
+
+val buf = newIntSeqBuf(7, 8)
+println("length = " + buf.length)
+println("content = " + buf.element)
+```
+{% endtab %}
+{% endtabs %}
+
 Here the factory `newIntSeqBuf` uses an anonymous class implementation of `IntSeqBuffer` (i.e. `new IntSeqBuffer`) to set the abstract type `T` to the concrete type `List[Int]`.
 
 It is also possible to turn abstract type members into type parameters of classes and vice versa. Here is a version of the code above which only uses type parameters:
 
+{% tabs abstract-types_4 class=tabs-scala-version %}
+{% tab 'Scala 2' for=abstract-types_4 %}
 ```scala mdoc:nest
 abstract class Buffer[+T] {
   val element: T
@@ -72,5 +118,24 @@ val buf = newIntSeqBuf(7, 8)
 println("length = " + buf.length)
 println("content = " + buf.element)
 ```
+{% endtab %}
+{% tab 'Scala 3' for=abstract-types_4 %}
+```scala
+abstract class Buffer[+T]:
+  val element: T
+
+abstract class SeqBuffer[U, +T <: Seq[U]] extends Buffer[T]:
+  def length = element.length
+
+def newIntSeqBuf(e1: Int, e2: Int): SeqBuffer[Int, Seq[Int]] =
+  new SeqBuffer[Int, List[Int]]:
+    val element = List(e1, e2)
+
+val buf = newIntSeqBuf(7, 8)
+println("length = " + buf.length)
+println("content = " + buf.element)
+```
+{% endtab %}
+{% endtabs %}
 
 Note that we have to use [variance annotations](variances.html) here (`+T <: Seq[U]`) in order to hide the concrete sequence implementation type of the object returned from method `newIntSeqBuf`.  Furthermore, there are cases where it is not possible to replace abstract type members with type parameters.