Why can't I use 'this' in a function in EK9?

← Classes and OOP · Ref: Q787

The 'this' keyword refers to the current object instance. It only exists inside class methods, constructors, and operators — places where an object instance is available.

WHERE THIS IS VALID

- Class methods and operators (refers to the class instance)
- Record operators (refers to the record instance)
- Component methods (refers to the component instance)
- Constructors (refers to the object being created)

WHERE THIS IS NOT VALID

- Functions (stateless, no instance)
- Programs (not an object)
- Module-level code

THIS EXAMPLE

The Counter class uses this.count in its methods — valid because methods have an instance. The standalone increment() function has no instance, so it uses parameters instead.

See Q50 for functions vs methods. See Q93 for class basics.

Example

defines module qa.classesandoop.thisinkclasses

  defines function

    increment() as pure
      -> current as Integer
      <- rtn as Integer: current + 1

  defines class

    Counter
      count <- Integer()

      Counter()
        -> initial as Integer
        count :=: initial

      next()
        count: this.count + 1

      current()
        <- rtn as Integer: this.count

      default operator ?

  defines program

    ShowCounter()
      stdout <- Stdout()
      counter <- Counter(0)
      counter.next()
      counter.next()
      stdout.println($counter.current())
      result <- increment(10)
      stdout.println($result)

Common mistakes

E05070 — Using this() in a function attempts constructor delegation, but functions have no constructors. The this() call is only valid inside a class constructor. See ek9 -h E05070 for details.

Incorrect:

      <- rtn as Integer: current + 1
      copy <- this()

Correct:

      <- rtn as Integer: current + 1

Coming from another language?

Java: this available in all instance methods, not in static methods. Python: self is an explicit parameter (same concept). Rust: self/&self parameter in impl methods. Kotlin: this available in class members. Go: receiver parameter serves as this. EK9: this only in class/record/component methods and constructors, never in functions.

Keywords: instance, scope, method, E05070, this, stateless, class, function