How does EK9 detect duplicate trait references and service path conflicts?

← Code Quality · Ref: Q740

EK9 detects three categories of structural duplication at compile time.

DUPLICATE TRAIT REFERENCE (E02050)

Listing the same trait multiple times in a 'with trait of' clause is redundant and indicates a mistake. Each trait should appear exactly once.

SERVICE PATH DUPLICATION (E02070)

Two service operations cannot share identical HTTP method and path pattern combinations. Even if the path variable names differ, the routing structure is the same and creates ambiguity.

DELEGATE METHOD NAME CLASH (E02080)

A function delegate field cannot have the same name as a method in the same class. The compiler cannot distinguish between calling the delegate and calling the method.

See Q93 for class traits. See Q657 for service URI mapping.

Example

defines module qa.codequality.duplicatetraits

  defines trait

    Printable
      show() as abstract
        <- rtn as String?

    Describable
      describe() as abstract
        <- rtn as String?

  defines class

    Item with trait of Printable, Describable
      name <- String()

      Item()
        -> name as String
        this.name: name

      override show()
        <- rtn as String: name

      override describe()
        <- rtn as String: `Item: ${name}`

      default operator

  defines program

    DuplicateTraitsDemo()
      stdout <- Stdout()

      item <- Item("Widget")
      stdout.println(item.show())
      stdout.println(item.describe())

Common mistakes

E02050 — The same trait 'Printable' is listed twice. Each trait should appear only once in the 'with trait of' clause. See ek9 -h E02050 for details.

Incorrect:

Item with trait of Printable, Printable

Correct:

Item with trait of Printable, Describable

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

Incorrect:

stdout.println(item.toString())

Correct:

stdout.println(item.show())

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

Incorrect:

stdout.println(item.getDescription())

Correct:

stdout.println(item.describe())

Coming from another language?

Java: duplicate interface implementation silently ignored. Python: no trait concept, multiple inheritance. Rust: duplicate trait bounds produce errors. Go: duplicate interface embedding is valid. Spring Boot: duplicate path mappings fail at runtime. EK9: all three detected at compile time.

Keywords: E02050, method, trait, E02070, delegate, clash, service, E02080, duplicate, path, quality