Add code tabs for _overviews/scala3-book/types-union.md

Author: jglodek

_overviews/scala3-book/types-union.md

@@ -7,12 +7,17 @@ num: 51
 previous-page: types-intersection
 next-page: types-adts-gadts
 ---
+<span class="tag tag-inline">Scala 3 only</span>
 
 Used on types, the `|` operator creates a so-called _union type_.
 The type `A | B` represents values that are **either** of the type `A` **or** of the type `B`.
 
 In the following example, the `help` method accepts a parameter named `id` of the union type `Username | Password`, that can be either a `Username` or a `Password`:
 
+{% tabs union-user %}
+
+{% tab 'Scala 3 Only' %}
+
 ```scala
 case class Username(name: String)
 case class Password(hash: Hash)
@@ -23,59 +28,109 @@ def help(id: Username | Password) =
     case Password(hash) => lookupPassword(hash)
   // more code here ...
 ```
+{% endtab %}
+
+{% endtabs %}
+
 We implement the method `help` by distinguishing between the two alternatives using pattern matching.
 
 This code is a flexible and type-safe solution.
 If you attempt to pass in a type other than a `Username` or `Password`, the compiler flags it as an error:
 
+{% tabs union-error-1 %}
+
+{% tab 'Scala 3 Only' %}
+
 ```scala
 help("hi")   // error: Found: ("hi" : String)
              //        Required: Username | Password
 ```
 
+{% endtab %}
+
+{% endtabs %}
+
 You’ll also get an error if you attempt to add a `case` to the `match` expression that doesn’t match the `Username` or `Password` types:
 
+{% tabs union-error-2 %}
+
+{% tab 'Scala 3 Only' %}
+
 ```scala
 case 1.0 => ???   // ERROR: this line won’t compile
 ```
 
+{% endtab %}
+
+{% endtabs %}
+
 ### Alternative to Union Types
 As shown, union types can be used to represent alternatives of several different types, without requiring those types to be part of a custom-crafted class hierarchy, or requiring explicit wrapping.
 
 #### Pre-planning the Class Hierarchy
 Other languages would require pre-planning of the class hierarchy, like the following example illustrates:
 
+{% tabs union-preplanning %}
+
+{% tab 'Scala 3 Only' %}
+
 ```scala
 trait UsernameOrPassword
 case class Username(name: String) extends UsernameOrPassword
 case class Password(hash: Hash) extends UsernameOrPassword
 def help(id: UsernameOrPassword) = ...
 ```
+
+{% endtab %}
+
+{% endtabs %}
+
 Pre-planning does not scale very well since, for example, requirements of API users might not be foreseeable.
 Additionally, cluttering the type hierarchy with marker traits like `UsernameOrPassword` also makes the code more difficult to read.
 
 #### Tagged Unions
 Another alternative is to define a separate enumeration type like:
 
+{% tabs union-tagged-unions %}
+
+{% tab 'Scala 3 Only' %}
+
 ```scala
 enum UsernameOrPassword:
   case IsUsername(u: Username)
   case IsPassword(p: Password)
 ```
+
+{% endtab %}
+
+{% endtabs %}
+
 The enumeration `UsernameOrPassword` represents a _tagged_ union of `Username` and `Password`.
 However, this way of modeling the union requires _explicit wrapping and unwrapping_ and, for instance, `Username` is **not** a subtype of `UsernameOrPassword`.
 
 ### Inference of Union Types
 The compiler assigns a union type to an expression _only if_ such a type is explicitly given.
 For instance, given these values:
 
+{% tabs union-tagged-unions-example %}
+
+{% tab 'Scala 3 Only' %}
+
 ```scala
 val name = Username("Eve")     // name: Username = Username(Eve)
 val password = Password(123)   // password: Password = Password(123)
 ```
 
+{% endtab %}
+
+{% endtabs %}
+
 This REPL example shows how a union type can be used when binding a variable to the result of an `if`/`else` expression:
 
+{% tabs union-tagged-unions-example-if %}
+
+{% tab 'Scala 3 Only' %}
+
 ````
 scala> val a = if true then name else password
 val a: Object = Username(Eve)
@@ -84,6 +139,10 @@ scala> val b: Password | Username = if true then name else password
 val b: Password | Username = Username(Eve)
 ````
 
+{% endtab %}
+
+{% endtabs %}
+
 The type of `a` is `Object`, which is a supertype of `Username` and `Password`, but not the *least* supertype, `Password | Username`.
 If you want the least supertype you have to give it explicitly, as is done for `b`.