Why can EK9 programs only return Integer?

← Syntax and Structure Rules · Ref: Q888

EK9 programs can only return Integer (or nothing). The return value becomes the process exit code, which the operating system uses to determine success or failure.

EXIT CODE CONVENTION

  0 — success
  1 — general error
  2 — usage error
  Non-zero — failure

CORRECT PATTERNS

  MyProgram()
    <- exitCode as Integer: 0     // Returns exit code

  MyProgram()
    ... body ...                   // No return (implicit 0)

INCORRECT PATTERNS

  MyProgram()
    <- message as String: "done"  // ERROR: E07590
  MyProgram()
    <- success as Boolean: true   // ERROR: E07590

WHY ENFORCED

The OS can only receive integer exit codes. Returning Float, Boolean, or custom types has no meaning at the OS level. EK9 makes this explicit rather than silently discarding non-integer values.

See Q871 for hello world. See Q872 for program variables.

Example

defines module qa.syntaxrules.programreturn

  defines program

    ProgramReturnIntegerDemo()
      <- exitCode as Integer: 0
      stdout <- Stdout()

      stdout.println("Program running successfully")
      stdout.println(`Exit code will be: ${exitCode}`)

Common mistakes

E07590 — Programs can only return Integer because the return value is the process exit code. Float, Boolean, String, and custom types cannot be used as exit codes. See ek9 -h E07590 for details.

Incorrect:

      <- exitCode as Float: 0.0

Correct:

      <- exitCode as Integer: 0

Coming from another language?

Java: main returns void, exit code via System.exit(int). C: main returns int (exit code). Python: sys.exit(int) or implicit 0. Go: os.Exit(int). Rust: main returns () or Result. EK9: program return type restricted to Integer at compile time.

Keywords: point, main, process, entry, exit, Integer, code, E07590, return, program