Why does EK9 reject duplicate properties in records with JSON support?

← Code Quality · Ref: Q829

EK9 requires every property name to be unique across a record's inheritance hierarchy, whatever operators it defaults. A child record that redeclares a parent property is E02010, with or without 'default operator'.

THE RULE

  defines record
    ParentRecord as open
      name as String: String()
      default operator
    ChildRecord extends ParentRecord
      name as String: String()       // ERROR E02010: property duplicated
      default operator

WHY IT ALSO PROTECTS JSON

The $$ operator generates JSON like {"name": "value"}. Because the name can never appear twice in the hierarchy, a generated $$ never has duplicate keys, so there is no separate JSON error. Writing $$ by hand does not make the redeclaration legal.

HOW TO FIX

- Use a different name in the child: childName as String
- Or set the inherited property instead of redeclaring it

See Q819 for duplicate property basics. See Q97 for records.

Example

defines module qa.quality.duplicate.json

  defines record

    BaseConfig as open
      label as String: String()
      default operator ?
      default operator $

    ChildConfig extends BaseConfig
      childLabel as String: String()
      default operator ?
      default operator $

  defines program

    DuplicateJsonDemo()
      stdout <- Stdout()
      child <- ChildConfig()
      stdout.println($child)

Common mistakes

E02010 — The parent BaseConfig already has 'label'. Redeclaring it in a child creates a duplicate property. Use a different name. See ek9 -h E02010 for details.

Incorrect:

    ChildConfig extends BaseConfig
      label as String: String()

Correct:

    ChildConfig extends BaseConfig
      childLabel as String: String()

Coming from another language?

Java: Jackson serializes duplicate fields from parent+child creating ambiguous JSON. Python: no enforcement. Rust: no inheritance. Go: embedded struct fields can shadow. EK9: compile-time error prevents ambiguous JSON serialization.

Keywords: json, duplicate, property, E02010, toJson, serialization, record