Scala

A programming language supporting object-oriented programming and functional programming. Its traits support reusable mixin behavior, while type variance controls how parameterized types participate in subtyping. The Scala 2 and Scala 3 language versions differ in some syntax and typing details; examples should state the version relevant to a particular claim. Official documentation: docs.scala-lang.org

Scala Swing provides Scala wrappers for Java Swing graphical user interfaces. Its notification mechanism implements an observer pattern: a publisher emits software events, and a reactor subscribes with listenTo(publisher). Adding a handler to reactions supplies a partial function that matches the software events the reactor handles. Subscription and handling are distinct: one determines which publisher is heard, and the other determines what to do with a matching notification.^[ref]

For example, a button publishes a button-click notification, and a subscribed display component handles it by updating a label. Another publisher's notifications need a subscription of their own. Calling deafTo(publisher) ends the subscription to that publisher without requiring removal of every handler.^[ref]