Why does EK9 reject an application that has no register statements?

← Dependency Injection · Ref: Q1313

An application is a dependency-injection registry: its whole purpose is to bind concrete components to the abstract types that programs inject. An empty application body has no purpose and is almost always an incomplete definition, so EK9 raises E07160 (implementation must be provided). Unlike a method, an application cannot be marked 'as abstract' - it must contain at least one register statement (or a block statement). The fix is to add the register lines that wire your components.

  defines application
    GreetApp
      register ConsoleGreeter() as Greeter

See Q1113 for wiring several components. See Q231 for linking a program to an application.

Example

defines module qa.di.application.body

  defines component

    Greeter as abstract
      greet() as abstract
        -> name as String
        <- message as String?

      default operator ?

    ConsoleGreeter is Greeter
      override greet()
        -> name as String
        <- message as String: `Hello, ${name}!`

      default operator ?

  defines application

    //CORRECT: the application body provides at least one register statement,
    //so it is a meaningful DI registry (not an empty body -> E07160).
    GreetApp
      register ConsoleGreeter() as Greeter

  defines program

    ApplicationBodyDemo() with application of GreetApp
      stdout <- Stdout()
      greeter as Greeter!
      stdout.println(greeter.greet("World"))

Common mistakes

E07160 — An application with no register or block statements has an empty body. An application cannot be abstract and a registry with no bindings is meaningless, so EK9 raises E07160. Add at least one register statement that binds a concrete component to its abstract type. See ek9 -h E07160 for details.

Incorrect:

  defines application
    GreetApp

Correct:

  defines application

    //CORRECT: the application body provides at least one register statement,

Coming from another language?

Spring: an empty @Configuration class compiles silently and fails only at runtime when a bean is missing. Guice: an empty AbstractModule.configure() compiles and defers the error to injector creation. .NET: an empty service-collection setup compiles and surfaces missing-service errors at first resolve. EK9: an empty application is a compile-time error (E07160) - the registry must declare its bindings up front.

Keywords: application, E07160, registry, wiring, DI, empty, implementation, body, register