Past exam of the year 1 of the computer science course of the University of Oxford / 2022 / Imperative programming (resit) / 6 / b / iii / Solution

Construct the mixed object with a concrete document implementation beneath the plugin:

val doc: DocImp with CharCountPlugin = new DocImp with CharCountPlugin

Its type combines DocImp and CharCountPlugin, so document operations and count are available. For example, inserting two 'a' characters and then deleting one leaves doc.count('a') == 1. The plugin is on the right of the concrete superclass in Scala's composition, so its edit methods extend the underlying implementations.