Why does EK9 reject override on constructs that cannot override?

← Code Quality · Ref: Q826

EK9 rejects the override keyword on constructs that do not support inheritance-based overriding. Programs and service methods cannot override anything.

WHERE OVERRIDE IS INVALID

- Programs: programs are entry points, not extendable
- Service methods: services do not inherit from other services

WHERE OVERRIDE IS VALID

- Class methods that replace a parent class method
- Trait method implementations in classes
- Function extensions that replace a parent function

WHY THIS MATTERS

The override keyword is a contract: 'I intend to replace a parent implementation.' Using it on a construct that has no parent hierarchy is a logic error caught at compile time.

See Q96 for inheritance. See Q155 for testing.

Example

defines module qa.quality.override.check

  defines program

    OverrideCheckDemo()
      stdout <- Stdout()
      stdout.println("Programs cannot use override")

Common mistakes

E05100 — Programs cannot use override — they are entry points with no parent to override. Remove the override keyword. See ek9 -h E05100 for details.

Incorrect:

    override OverrideCheckDemo()

Correct:

    OverrideCheckDemo()

Coming from another language?

Java: @Override on a static method or constructor is a compile error. Kotlin: override keyword validated against parent. C++: override specifier checked. Python: no enforcement. EK9: override validated at SYMBOL_DEFINITION phase.

Keywords: program, E05100, inheritance, inappropriate, service, override