reason for should not use app

README.md

@@ -44,6 +44,7 @@ contribute.
   - [2.19. SHOULD use head/tail and init/last decomposition only if they can be done in constant time and memory](sections/2-language-rules.md#219-should-use-head-tail-and-init-last-decomposition-only-if-they-can-be-done-in-constant-time-and-memory)
   - [2.20. MUST NOT use `Seq.head`](sections/2-language-rules.md#210-must-not-use-seqhead)
   - [2.21. Case classes SHOULD be final](sections/2-language-rules.md#221-case-classes-should-be-final)
+  - [2.22. SHOULD NOT use `scala.App`](sections/2-language-rules.md#222-should-not-use-scalaapp)
 
 - [3. Application Architecture](sections/3-architecture.md)
   - [3.1. SHOULD NOT use the Cake pattern](sections/3-architecture.md#31-should-not-use-the-cake-pattern)

sections/2-language-rules.md

@@ -785,4 +785,24 @@ Example:
 
 ```scala
 final case class User(name: String, id: Long)
-```
+```
+
+### 2.22 SHOULD NOT use `scala.App`
+
+`scala.App` is ofter used to denote the entrypoint of the application:
+
+```scala
+object HelloWorldApp extends App {
+  println("hello, world!")
+}
+```
+
+`DelayedInit` one of the mechanisms used to implement `scala.App` [has been deprecated](https://github.com/scala/scala/pull/3563).
+Any variables defined in the object body will be available as fields, unless the `private` access modifier is applied.
+Prefer the simpler alternative of defining a main method:
+
+```scala
+object HelloWorldApp {
+  def main(args: Array[String]): Unit = println("hello, world!")
+}
+```