Structure editors designed for keyboard input often struggle to resolve the tension between maintaining hierarchical term structure and offering efficient linear editing affordances. Contemporary designs either compromise structure by deferring to text near the leaves or else maintain structure by permitting only edits that transform the selected term. However, visually adjacent sequences (e.g. of operators, operands, and individual delimiters) do not always cleave cleanly to term boundaries, so even experienced users report difficulties with selection and code restructuring tasks. We propose a novel approach to structure editing, tile-based editing, that maintains term structure while offering linear selection and modification affordances. The idea is to allow disassembly of terms into linearly sequenced tiles and shards around user selections, while guiding the user through restructuring actions and automatically inserting holes and performing automatic hole insertion and removal in a manner that ensures reassembly into a term.

This paper introduces tylr, a tiny tile-based editor designed primarily to highlight this uniquely flexible set of affordances. We evaluated tylr with a lab study where participants performed simple code transcription and modification tasks using tylr as well as a text editor and a structure editor built on JetBrains MPS, a state-of-the-art keyboard-driven structure editor generator. Our results indicate that participants frequently made use of tylr’s selection expressivity, and that this flexibility helped them complete some modification tasks significantly more quickly than with the MPS editor. We further observed that a few participants completed some tasks more quickly using tylr than with text, but were in general slowed by a number of limitations in our current design and implementation. We discuss these limitations and suggest future research and design directions aiming toward more flexible structure editing interfaces.