E05030: not open to be extended

What went wrong

You're trying to extend a class that isn't marked as 'open'. In EK9, classes are CLOSED by default - they cannot be extended unless explicitly designed for inheritance.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 1043 (I get E05030 'not open to be extended'. What does this...)
• ek9 -q 102 (How do I make a class extensible with 'as open'?)
• ek9 -q 1044 (I tried to extend List and got E05030. How do I wrap...)
• ek9 -q 101 (Why are types closed by default in EK9?)
• ek9 -q 23 (What basic types does EK9 have?)

How to fix it

1. If you control BaseClass: add 'as open' if inheritance is intended
2. If you don't control BaseClass: use composition (wrap the class)
3. Consider if inheritance is really needed - composition is often better

Rationale

Classes closed by default because:

• Prevents fragile base class problem (changes break subclasses)
• Forces explicit design for inheritance (intentional extension points)
• Encourages composition over inheritance (more flexible)
• Makes class contracts more predictable (sealed behavior)

Common causes

• Assuming classes are open like Java (EK9 defaults differ)
• Forgetting to add 'as open' when designing for inheritance
• Trying to extend built-in types (List, Dict, etc. - all closed)

See also

• E05030: extending closed class (this error)
• E05020: circular inheritance (different - cycle in hierarchy)

https://ek9.io/inheritance.html#closed-by-default https://ek9.io/forAI.json#$.closedByDefault

Examples

Every example is compiled by the EK9 compiler: the first fails exactly as captioned, the others fix it.

Fails with E05030: not open to be extended

#!ek9
defines module error.e05030.bad

  defines class

    Shape
      describe()
        <- description as String: "a shape"

    Circle extends Shape
      override describe()
        <- description as String: "a circle"

Fixed: Open the type you mean to extend

#!ek9
defines module error.e05030.good

  defines class

    Shape as open
      describe()
        <- description as String: "a shape"

    Circle extends Shape
      override describe()
        <- description as String: "a circle"

Questions and answers

These Q&A entries show E05030 and its fix:

• Q23: What basic types does EK9 have?
• Q51: What are abstract functions in EK9?
• Q101: Why are types closed by default in EK9?
• Q102: How do I make a class extensible with 'as open'?
• Q103: What are abstract classes and methods in EK9?
• Q105: How does method dispatch work in EK9?
• Q109: How do I use composition instead of inheritance in EK9?
• Q210: How does trait delegation with 'by' work in EK9?
• Q264: How do I implement the adapter pattern in EK9?
• Q278: Why does AI try to extend List, Dict, or closed types in EK9?
• Q574: What happens if I try to extend or override in a closed type?
• Q575: How do overrides work in deep class hierarchies?
• Q578: What is the relationship between open, abstract, and override?
• Q602: What is a circular type hierarchy and why does EK9 ban it?
• Q610: How do I refactor circular dependencies using composition?
• Q677: How does abstract implementation flow through a three-level hierarchy?
• Q784: Why can't I extend a class in EK9?
• Q1027: Review this design. Should I use composition instead of a large class?
• Q1043: I get E05030 'not open to be extended'. What does this mean and how do I fix it?
• Q1044: I tried to extend List and got E05030. How do I wrap a collection instead?
• Q1045: Review this design. Should it use composition instead of inheritance?
• Q1080: Copy a child class including parent fields.
• Q1108: Extend a BaseService component and add a version field to the child.
• Q1122: Write a dispatcher with a fallback handler for unmatched types.
• Q1123: Dispatch on a deep class hierarchy where exact match wins over parent handler.
• Q1130: Chain trait delegations so Top delegates to Middle which delegates to Final.
• Q1244: Create me a dispatcher for a deep class hierarchy (Shape -> Polygon -> Quadrilateral -> Square) where the Polygon handler catches Quadrilateral but a more specific Square handler exists.
• Q1250: Show me how to make a Repository class extensible so that InMemoryRepository can inherit from it without triggering E05030.
• Q1252: Write me an open logWriter function and a dynamic function instance that overrides its behaviour, demonstrating that 'as open' on a function allows dynamic instances to extend it.
• Q1254: Walk me through fixing error E05030 'not open to be extended' for three different cases: extending a closed class, extending a closed function, and extending a built-in type.
• Q1297: Can records extend other records in EK9?
• Q1365: How do I use a Set for unique values in EK9?