Subtype polymorphism is an important feature available in most modern type systems which makes code reuse and specialization possible. Recent works on separation of crosscutting concerns have created event interfaces (types) to decouple subjects from handlers. Extending the notion of subtyping to these event interfaces is a logical step. In this paper, we define event type polymorphism in the context of the Ptolemy language. Ptolemy allows declaring quantified, typed events which provide an interface between subjects and handlers. We add the notion of polymorphic event types to the Ptolemy language, defining a subtype relation among event types which in turn allows for both depth and width subtyping with regard to event context. Since Ptolemy only has explicit event announcement, our semantics is simpler and easier to reason about when compared to previously defined approaches. We also give the first formally defined static semantics for polymorphic events as well as demonstrate its usefulness via examples
