Programming Languages For Interactive Computing

Traditional programming languages are algorithmic: they are best suited to writing programs that acquire all their inputs before executing and only produce a result on termination. By contrast most applications are interactive: they maintain ongoing interactions with their environments. Modern systems address this incompatibility by manually extending the execution model of the host language to support interaction, usually by embedding an event-driven state management scheme which executes fragments of imperative code in response to interactions, the job of each executed fragment being to restore the internal consistency of the computation. The downside of this approach to interaction is that it relies heavily on mutable stores and side-effects and mixes application logic in with behaviour which is more properly the responsibility of an execution model. I describe a programming model called declarative interaction which supports interaction directly. The distinguishing feature of the model is its modal construal of state and interaction.