Why can't I construct an abstract type directly in EK9?

← Code Quality · Ref: Q821

EK9 prevents direct construction of abstract types at compile time. Abstract types define contracts — they must be extended with concrete implementations.

THE RULE

You cannot call the constructor of an abstract class, function, or generic parameterized with an abstract type:

  Shape as abstract    // abstract — no direct construction
  s <- Shape()         // ERROR: E10030

HOW TO FIX

- Create a concrete subclass and construct that instead
- Use a dynamic class/function for inline implementation
- If using generics, parameterize with a concrete type

WHY THIS MATTERS

Abstract types may have abstract methods with no implementation. Constructing them would create an object that cannot respond to all its methods. EK9 catches this at compile time rather than allowing it to fail at runtime.

See Q97 for class vs record. See Q96 for inheritance.

Example

defines module qa.quality.abstract.construction

  defines class

    Shape as abstract
      default operator ?

    Circle extends Shape
      default Circle()
      default operator ?

  defines program

    AbstractConstructionDemo()
      stdout <- Stdout()

      // === CORRECT: construct concrete subclass ===
      shape <- Circle()
      stdout.println(`Shape created: ${shape?}`)

Common mistakes

E50080 — Shape is abstract and cannot be constructed directly. Use a concrete subclass like Circle instead. See ek9 -h E50080 for details.

Incorrect:

      shape <- Shape()

Correct:

      shape <- Circle()

Coming from another language?

Java: cannot instantiate abstract class — runtime InstantiationError if attempted via reflection. Python: ABCMeta raises TypeError at runtime. Rust: traits cannot be constructed (no inheritance). Go: interfaces have no constructors. EK9: compile-time error for abstract construction.

Keywords: abstract, concrete, E10030, implementation, type, constructor, construction