Merge pull request #2625 from bernit77/add_code_tab_fun_write_map_fun…

…ction

Add code tabs in scala3/book/fun-write-map-function

_overviews/scala3-book/fun-write-map-function.md

@@ -20,68 +20,116 @@ Focusing only on a `List[Int]`, you state:
 Given that statement, you start to write the method signature.
 First, you know that you want to accept a function as a parameter, and that function should transform an `Int` into some generic type `A`, so you write:
 
+{% tabs map-accept-func-definition %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 def map(f: (Int) => A)
 ```
+{% endtab %}
+{% endtabs %}
 
 The syntax for using a generic type requires declaring that type symbol before the parameter list, so you add that:
 
+{% tabs map-type-symbol-definition %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 def map[A](f: (Int) => A)
 ```
+{% endtab %}
+{% endtabs %}
 
 Next, you know that `map` should also accept a `List[Int]`:
 
+{% tabs map-list-int-param-definition %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 def map[A](f: (Int) => A, xs: List[Int])
 ```
+{% endtab %}
+{% endtabs %}
 
 Finally, you also know that `map` returns a transformed `List` that contains elements of the generic type `A`:
 
+{% tabs map-with-return-type-definition %}
+{% tab 'Scala 2 and 3' %}
 ```scala
 def map[A](f: (Int) => A, xs: List[Int]): List[A] = ???
 ```
+{% endtab %}
+{% endtabs %}
 
 That takes care of the method signature.
 Now all you have to do is write the method body.
 A `map` method applies the function it’s given to every element in the list it’s given to produce a new, transformed list.
 One way to do this is with a `for` expression:
-
+{% tabs for-definition class=tabs-scala-version %}
+{% tab 'Scala 2' %}
+```scala
+for (x <- xs) yield f(x)
+```
+{% endtab %}
+{% tab 'Scala 3' %}
 ```scala
 for x <- xs yield f(x)
 ```
+{% endtab %}
+{% endtabs %}
 
 `for` expressions often make code surprisingly simple, and for our purposes, that ends up being the entire method body.
 
 Putting it together with the method signature, you now have a standalone `map` method that works with a `List[Int]`:
 
+{% tabs map-function class=tabs-scala-version %}
+{% tab 'Scala 2' %}
+```scala
+def map[A](f: (Int) => A, xs: List[Int]): List[A] =
+  for (x <- xs) yield f(x)
+```
+{% endtab %}
+{% tab 'Scala 3' %}
 ```scala
 def map[A](f: (Int) => A, xs: List[Int]): List[A] =
   for x <- xs yield f(x)
 ```
+{% endtab %}
+{% endtabs %}
 
 
 ### Make it generic
 
 As a bonus, notice that the `for` expression doesn’t do anything that depends on the type inside the `List` being `Int`.
 Therefore, you can replace `Int` in the type signature with the generic type parameter `B`:
 
+{% tabs map-function-full-generic class=tabs-scala-version %}
+{% tab 'Scala 2' %}
+```scala
+def map[A, B](f: (B) => A, xs: List[B]): List[A] =
+  for (x <- xs) yield f(x)
+```
+{% endtab %}
+{% tab 'Scala 3' %}
 ```scala
 def map[A, B](f: (B) => A, xs: List[B]): List[A] =
   for x <- xs yield f(x)
 ```
+{% endtab %}
+{% endtabs %}
 
 Now you have a `map` method that works with any `List`.
 
 These examples demonstrate that `map` works as desired:
 
+{% tabs map-use-example %}
+{% tab 'Scala 2 and 3' %}
 ```scala
-def double(i : Int) = i * 2
+def double(i : Int): Int = i * 2
 map(double, List(1, 2, 3))            // List(2, 4, 6)
 
-def strlen(s: String) = s.length
+def strlen(s: String): Int = s.length
 map(strlen, List("a", "bb", "ccc"))   // List(1, 2, 3)
 ```
+{% endtab %}
+{% endtabs %}
 
 Now that you’ve seen how to write methods that accept functions as input parameters, let’s look at methods that return functions.