Why does EK9 reject the ? modifier on function parameters?

← Code Quality · Ref: Q827

EK9 requires function parameters to always be initialised. Declaring a parameter with ? (potentially unset) is not needed because the caller must always provide a value.

THE RULE

Parameters cannot use the ? unset modifier:

  process()
    -> input as String?    // ERROR: E07300

Parameters are always set by the caller. The ? modifier would be misleading.

THE FIX

Remove the ? modifier:

  process()
    -> input as String     // CORRECT: always initialised

WHY THIS MATTERS

EK9 pushes for everything to be initialised. Parameters are provided by callers, so they are always set when the function body executes. Allowing ? on parameters would create false optionality.

See Q310 for code quality. See Q126 for function patterns.

Example

defines module qa.quality.null.not.needed

  defines function

    processInput() as pure
      -> input as String
      <- rtn as String: `Processed: ${input}`

  defines program

    NullNotNeededDemo()
      stdout <- Stdout()
      result <- processInput("hello")
      stdout.println(result)

Common mistakes

E07300 — Function parameters are always provided by the caller and cannot be unset. Remove the ? modifier. See ek9 -h E07300 for details.

Incorrect:

    -> input as String?

Correct:

    -> input as String

Coming from another language?

Java: parameters are always non-null unless explicitly @Nullable. Kotlin: parameters can be nullable with ?. Rust: parameters are always initialised. Go: parameters always have zero values. EK9: parameters are always initialised — ? modifier not allowed.

Keywords: null, unset, E07300, optional, initialised, declared, parameter