Willow: A Type-and-Effect System That Statically Detects React Render Cascades and Inter-Render Loops

arXiv 2607.27074·medium signalXBlueskyLinkedInCopy link

arXiv 2607.27074 (2026-07-29) presents Willow, a core calculus for React-style reactive programming with a time-aware operational semantics built around renders, paired with a type-and-effect system that tracks timing behavior as effects. A 'next' modality expresses delays in renders, network requests, or milliseconds, and a family of modalities tracks handler lifecycle — registration, firing, cancellation, removal. The key move is that the resulting effects form a temporal dependency graph, so standard graph algorithms statically detect render cascades and inter-render loops that cause non-termination; the prototype checker with automatic effect inference is evaluated on debouncing, form inputs, and API-driven updates.