Create a dynamic class that implements a Describable trait inline.

Create a dynamic class implementing a trait inline:

  myObj <- (label) with trait of Describable as class
    override describe() as pure
      <- rtn as String: label
    default operator ?

(label) captures the local variable. The dynamic class implements the trait's abstract methods -- no named class needed.

See Q1125 for dynamic functions. See Q1129 for trait delegation.

Example

defines module qa.advancedtypes.dynamicclasstrait

  defines trait

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

  defines program

    DynamicClassTraitDemo()
      stdout <- Stdout()
      label <- "Dynamic"

      //Create inline implementation of Describable
      myDescribable <- (label) with trait of Describable as class
        override describe() as pure
          <- rtn as String: label
        default operator ?

      stdout.println(myDescribable.describe())

Common mistakes

E01075 — EK9 has no 'new' keyword - create an inline implementation with '(captures) with trait of TraitName as class'. See ek9 -h E01075 for details.

Incorrect:

      myDescribable <- new Describable()

Correct:

      myDescribable <- (label) with trait of Describable as class
        override describe() as pure
          <- rtn as String: label
        default operator ?

Other ways to ask this

I need an anonymous class implementing a trait, created inline with captured state
In Java I'd use an anonymous inner class. Write the EK9 dynamic class equivalent
Given a Describable trait, create an inline implementation that captures a label variable
Build a one-off trait implementation without defining a named class

Coming from another language?

Java: new Describable() { @Override String describe() { return label; } }. Kotlin: object : Describable { }. EK9: (captures) with trait of X as class.

Keywords: inline, dynamic, trait, class, anonymous, capture