Resolve diamond inheritance when a class adopts two traits that share a parent.

← Sealed Types and Traits · Ref: Q1131

Resolve diamond conflicts with explicit override:

  DiamondClass with trait of LeftTrait, RightTrait
    override getShared() as pure
      <- rtn as String: "diamond-shared"

When two traits extend the same parent, conflicting methods must be overridden. Use TraitName.methodName() to call a specific trait's version.

See Q1129 for basic delegation. See Q1132 for explicit trait calls.

Example

defines module qa.sealedtraits.traitdiamond

  defines trait

    TopTrait
      getShared() as pure
        <- mainValue as String: "top-shared"

    LeftTrait is TopTrait
      override getShared() as pure
        <- mainValue as String: "left-shared"

    RightTrait is TopTrait
      override getShared() as pure
        <- mainValue as String: "right-shared"

  defines class

    DiamondClass with trait of LeftTrait, RightTrait

      //Must override to resolve the conflict
      override getShared() as pure
        <- mainValue as String: "diamond-resolved"

      //Explicit access to each trait's version
      getLeftVersion() as pure
        <- mainValue as String: LeftTrait.getShared()

      getRightVersion() as pure
        <- mainValue as String: RightTrait.getShared()

      default operator ?

  defines program

    TraitDiamondDemo()
      stdout <- Stdout()

      diamond <- DiamondClass()
      stdout.println(`Resolved: ${diamond.getShared()}`)
      stdout.println(`Left: ${diamond.getLeftVersion()}`)
      stdout.println(`Right: ${diamond.getRightVersion()}`)

Common mistakes

E05120 — Conflicting trait methods require the 'override' keyword to resolve the ambiguity.

Incorrect:

      getShared() as pure
        <- rtn as String: "resolved"

Correct:

      override getShared() as pure
        <- mainValue as String: "left-shared"

Other ways to ask this

I have LeftTrait and RightTrait both extending TopTrait — resolve the conflict
In Java interfaces with diamond inheritance need default method resolution. Show the EK9 way
Given a diamond pattern with conflicting methods, override to resolve them
Handle the case where two traits provide conflicting implementations of the same method

Coming from another language?

Java: interface default method conflict requires explicit override. C#: explicit interface implementation. EK9: override conflicting methods + TraitName.method() for explicit access.

Keywords: trait, multiple, resolve, diamond, override, conflict