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

  defines program
    MessageProgram()
      <- message as String: "done"  // ERROR: E07590
    SuccessProgram()
      <- 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.

EVERY CODE IS YOURS

A program may return any Integer, including the numbers the ek9 command uses for its own failures (3, 8, 11 ...), so a script being replaced keeps the codes its callers expect. When EK9 itself fails - a compile error, or arguments that do not fit the program - its first stderr line says so: ek9: compilation failed (exit 8). Your program's exit never carries that line, so a caller tells the two apart from stderr, not from the number.

OTHER WAYS A PROGRAM'S STATUS IS SET

An unset return exits 1. An Exception nothing catches exits with its own exit code (or 1). A Panic, such as a failed require, always exits 1. See Q1431.

See Q3 for printing output. 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: E07590, code, Integer, exit, entry, process, main, point, program, return