Varv: Reprogrammable Interactive Sofware as a Declarative Data Structure

Most modern applications are immutable and turn-key despite the acknowledged benefts of empowering users to modify their software. Writing extensible software remains challenging, even for expert programmers. Reprogramming or extending existing software is often laborious or wholly blocked, requiring sophisticated knowledge of application architecture or setting up a development environment. We present Varv, a programming model representing reprogrammable interactive software as a declarative data structure. Varv defnes interactive applications as a set of concepts that Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proft or commercial advantage and that copies bear this notice and the full citation on the frst page. Copyrights for components of this work owned by others than the author(s) must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specifc permission and/or a fee. Request permissions from permissions@acm.org. CHI ’22, April 29-May 5, 2022, New Orleans, LA, USA consist of a schema and actions. Applications in Varv support incremental modifcation, allowing users to reprogram through addition and selectively suppress, modify, or add behavior. Users can defne high-level concepts, creating an abstraction layer and efectively a domain-specifc language for their application domain, emphasizing reuse and modifcation. We demonstrate the reprogramming and collaboration capabilities of Varv in two case studies and illustrate how the event engine allows for extensive tooling support.