add code tabs.

_overviews/scala3-book/methods-main-methods.md

@@ -1,5 +1,5 @@
 ---
-title: main Methods
+title: Main Methods in Scala 3
 type: section
 description: This page describes how 'main' methods and the '@main' annotation work in Scala 3.
 languages: [zh-cn]
@@ -8,18 +8,25 @@ previous-page: methods-most
 next-page: methods-summary
 ---
 
+<h5>Writing one line programs <span class="tag tag-inline">Scala 3 Only</span></h5>
 
 Scala 3 offers a new way to define programs that can be invoked from the command line: Adding a `@main` annotation to a method turns it into entry point of an executable program:
 
+{% tabs method_1 %}
+{% tab 'Scala 3 Only' for=method_1 %}
+
 ```scala
-@main def hello() = println("Hello, world")
+@main def hello() = println("Hello, World")
 ```
 
-Just save that line of code in a file named something like *Hello.scala*---the filename doesn’t have to match the method name---and run it with `scala`:
+{% endtab %}
+{% endtabs %}
+
+To run this program, save the line of code in a file named as e.g. *Hello.scala*---the filename doesn’t have to match the method name---and run it with `scala`:
 
 ```bash
 $ scala Hello.scala
-Hello, world
+Hello, World
 ```
 
 A `@main` annotated method can be written either at the top-level (as shown), or inside a statically accessible object.
@@ -36,6 +43,9 @@ Learn more about the `@main` annotation by reading the following sections, or by
 With this approach your `@main` method can handle command line arguments, and those arguments can have different types.
 For example, given this `@main` method that takes an `Int`, a `String`, and a varargs `String*` parameter:
 
+{% tabs method_2 %}
+{% tab 'Scala 3 Only' for=method_2 %}
+
 ```scala
 @main def happyBirthday(age: Int, name: String, others: String*) =
   val suffix = (age % 100) match
@@ -51,6 +61,9 @@ For example, given this `@main` method that takes an `Int`, a `String`, and a va
   println(sb.toString)
 ```
 
+{% endtab %}
+{% endtabs %}
+
 When you compile that code, it creates a main program named `happyBirthday` that’s called like this:
 
 ```
@@ -59,7 +72,7 @@ Happy 23rd Birthday, Lisa and Peter!
 ```
 
 As shown, the `@main` method can have an arbitrary number of parameters.
-For each parameter type there must be an instance of the *scala.util.FromString* type class that converts an argument `String` to the required parameter type.
+For each parameter type there must be a [given instance](./ca-given-using-clauses.html) of the `scala.util.CommandLineParser.FromString` type class that converts an argument `String` to the required parameter type.
 Also as shown, a main method’s parameter list can end in a repeated parameter like `String*` that takes all remaining arguments given on the command line.
 
 The program implemented from an `@main` method checks that there are enough arguments on the command line to fill in all parameters, and that the argument strings can be converted to the required types.
@@ -73,18 +86,19 @@ $ scala happyBirthday sixty Fred
 Illegal command line: java.lang.NumberFormatException: For input string: "sixty"
 ```
 
-
-
 ## The details
 
 The Scala compiler generates a program from an `@main` method `f` as follows:
 
 - It creates a class named `f` in the package where the `@main` method was found.
 - The class has a static method `main` with the usual signature of a Java `main` method: it takes an `Array[String]` as argument and returns `Unit`.
-- The generated `main` method calls method `f` with arguments converted using methods in the `scala.util.CommandLineParser` object.
+- The generated `main` method calls method `f` with arguments converted using methods in the `scala.util.CommandLineParser.FromString` object.
 
 For instance, the `happyBirthday` method above generates additional code equivalent to the following class:
 
+{% tabs method_3 %}
+{% tab 'Scala 3 Only' for=method_3 %}
+
 ```scala
 final class happyBirthday {
   import scala.util.{CommandLineParser as CLP}
@@ -93,7 +107,7 @@ final class happyBirthday {
       happyBirthday(
           CLP.parseArgument[Int](args, 0),
           CLP.parseArgument[String](args, 1),
-          CLP.parseRemainingArguments[String](args, 2))
+          CLP.parseRemainingArguments[String](args, 2)*)
     catch {
       case error: CLP.ParseError => CLP.showError(error)
     }
@@ -104,35 +118,42 @@ final class happyBirthday {
 > This feature is not available for user programs in Scala.
 > Regular “static” members are generated in Scala using objects instead.
 
+{% endtab %}
+{% endtabs %}
 
-
-## Scala 3 compared to Scala 2
+## Backwards Compatibility with Scala 2
 
 `@main` methods are the recommended way to generate programs that can be invoked from the command line in Scala 3.
 They replace the previous approach in Scala 2, which was to create an `object` that extends the `App` class:
 
-```scala
-// scala 2
-object happyBirthday extends App {
-  // needs by-hand parsing of the command line arguments ...
-}
-```
-
 The previous functionality of `App`, which relied on the “magic” `DelayedInit` trait, is no longer available.
 `App` still exists in limited form for now, but it doesn’t support command line arguments and will be deprecated in the future.
 
-If programs need to cross-build between Scala 2 and Scala 3, it’s recommended to use an explicit `main` method with an `Array[String]` argument instead:
+If programs need to cross-build between Scala 2 and Scala 3, it’s recommended to use an `object` with an explicit `main` method and a single `Array[String]` argument instead:
+
+{% tabs method_4 %}
+{% tab 'Scala 2 and 3' %}
 
 ```scala
-object happyBirthday:
-  def main(args: Array[String]) = println("Hello, world")
+object happyBirthday {
+  private def happyBirthday(age: Int, name: String, others: String*) = {
+    ... // same as before
+  }
+  def main(args: Array[String]): Unit =
+    happyBirthday(args(0).toInt, args(1), args.drop(2).toIndexedSeq:_*)
+}
 ```
 
+> note that here we use `:_*` to pass a vararg argument, which remains in Scala 3 for backwards compatibility.
+
+{% endtab %}
+{% endtabs %}
+
 If you place that code in a file named *happyBirthday.scala*, you can then compile it with `scalac` and run it with `scala`, as shown previously:
 
 ```bash
 $ scalac happyBirthday.scala
 
-$ scala happyBirthday
-Hello, world
+$ scala happyBirthday 23 Lisa Peter
+Happy 23rd Birthday, Lisa and Peter!
 ```