Figure 1: Varv Examples: (a) A todo list web application that is inherently extensible. Here, a basic todo list is extended with the ability to complete and delete todos by adding two new concept defnitions and new modifed template defnitions. (b) A board game toolkit that defnes abstractions for board game logic. The games "Checkers" and "Othello" were implemented with the toolkit and then merged into a new "Checkers-O-Thello" game with the addition of a short concept defnition. As Varv applications are represented as data structures, higher-level tooling can be developed including a block-based editor (right), an inspector to go from an element in the view to the corresponding template or data (context menu to the left), and a data inspector for live editing application state (middle).