Pantograph: A Fluid and Typed Structure Editor

TL;DR

Pantograph is a novel structure editor that maintains valid program syntax and types during editing by generalizing selection and typing contexts, enabling fluid and correct program modifications.

Contribution

It introduces a new approach to structure editing that preserves program correctness without sacrificing editing fluidity, through novel selection and typing of tree contexts.

Findings

Students learned to program effectively with Pantograph.

Pantograph outperforms traditional text editors in editing fluidity.

The system maintains well-typed programs during editing.

Abstract

Structure editors operate directly on a program's syntactic tree structure. At first glance, this allows for the exciting possibility that such an editor could enforce correctness properties: programs could be well-formed and sometimes even well-typed by construction. Unfortunately, traditional approaches to structure editing that attempt to rigidly enforce these properties face a seemingly fundamental problem, known in the literature as viscosity. Making changes to existing programs often requires temporarily breaking program structure -- but disallowing such changes makes it difficult to edit programs! In this paper, we present a scheme for structure editing which always maintains a valid program structure without sacrificing the fluidity necessary to freely edit programs. Two key pieces help solve this puzzle: first, we develop a novel generalization of selection for tree-based structures that properly generalizes text-based selection and editing, allowing users to freely rearrange pieces of code by cutting and pasting one-hole contexts; second, we type these one-hole contexts with a category of type diffs and explore the metatheory of the system that arises for maintaining well-typedness systematically. We implement our approach as an editor called Pantograph, and we conduct a study in which we successfully taught students to program with Pantograph and compare their performance against a traditional text editor.