Bidirectional Evaluation with Direct Manipulation

TL;DR

This paper introduces a bidirectional evaluation algorithm for functional programming that allows output-driven program updates, demonstrated through an interactive HTML generation system called Sketch-n-Sketch.

Contribution

The paper presents a novel evaluation update algorithm that enables output-based program editing and supports custom domain-specific lenses in a functional language.

Findings

Effective program updates based on output changes in Sketch-n-Sketch

Interactive HTML editing reduces the edit-run-view cycle

Demonstrated utility with 10 examples totaling 1400 lines of code

Abstract

We present an evaluation update (or simply, update) algorithm for a full-featured functional programming language, which synthesizes program changes based on output changes. Intuitively, the update algorithm retraces the steps of the original evaluation, rewriting the program as needed to reconcile differences between the original and updated output values. Our approach, furthermore, allows expert users to define custom lenses that augment the update algorithm with more advanced or domain-specific program updates. To demonstrate the utility of evaluation update, we implement the algorithm in Sketch-n-Sketch, a novel direct manipulation programming system for generating HTML documents. In Sketch-n-Sketch, the user writes an ML-style functional program to generate HTML output. When the user directly manipulates the output using a graphical user interface, the update algorithm reconciles the changes. We evaluate bidirectional evaluation in Sketch-n-Sketch by authoring ten examples comprising approximately 1400 lines of code in total. These examples demonstrate how a variety of HTML documents and applications can be developed and edited interactively in Sketch-n-Sketch, mitigating the tedious edit-run-view cycle in traditional programming environments.