What are the restrictions on this and super in EK9?

← Code Quality · Ref: Q743

EK9 enforces specific rules about how 'this' and 'super' can be used.

SUPER FOR ANY NOT REQUIRED (E05040)

All EK9 classes implicitly extend 'Any'. Calling super() in a constructor when there is no explicit parent class is unnecessary and rejected. Only call super() when the class explicitly extends another class.

INAPPROPRIATE USE OF THIS OR SUPER (E05090)

You cannot directly assign to 'this' or 'super' using ':=', ':=?', or ':'. These would attempt to replace the object pointer itself, which is not meaningful. Use ':=:' (copy) or ':~:' (merge) instead to copy state from another object.

ALLOWED OPERATIONS

- this :=: other (copy state from other)
- this :~: other (merge state from other)
- $this (string conversion)
- this? (isSet check)

See Q580 for this delegation. See Q581 for super delegation.

Example

defines module qa.codequality.thissuperrestrictions

  defines class

    Base as open
      label <- String()

      Base()
        -> label as String
        this.label: label

      describe()
        <- rtn as String: label

      default operator

    Child extends Base
      role <- String()

      Child()
        ->
          label as String
          role as String
        //super() IS valid here because Child explicitly extends Base
        super(label)
        this.role: role

      override describe()
        <- rtn as String: `${super.describe()} (${role})`

      default operator

  defines program

    ThisSuperDemo()
      stdout <- Stdout()

      child <- Child("Alice", "Engineer")
      stdout.println(child.describe())

      base <- Base("Bob")
      stdout.println(base.describe())

Common mistakes

E05040 — Base has no explicit parent, so calling super() is unnecessary. All classes implicitly extend Any. Remove the super() call. See ek9 -h E05040 for details.

Incorrect:

Base()
        -> label as String
        super()
        this.label: label

Correct:

Base()
        -> label as String
        this.label: label

E50060 — EK9 has no toString() method. Use the describe() method or string interpolation. See ek9 -h E50060 for details.

Incorrect:

stdout.println(child.toString())

Correct:

stdout.println(child.describe())

E50060 — The method is describe(), not getLabel(). EK9 does not use Java-style getter naming. See ek9 -h E50060 for details.

Incorrect:

stdout.println(base.getLabel())

Correct:

stdout.println(base.describe())

E05090 — Cannot assign directly to 'this'. Use 'this.fieldName' to access a specific field, or use :=:, :~:, += etc. for aggregate operations. See ek9 -h E05090 for details.

Incorrect:

        this: label

Correct:

        this.label: label

Coming from another language?

Java: super() implicitly inserted if omitted. Python: super().__init__() must be called explicitly. Rust: no super concept. Go: no inheritance. Kotlin: super() implicitly called. EK9: super() only valid with explicit parent, this/super cannot be reassigned.

Keywords: restriction, merge, quality, constructor, super, E05040, copy, E05090, assignment, this