Submission to the Substrates 2026 Workshop, co-located with <Programming 2026>.
Over the last few years of developing Infusion, a substrate supporting open authorship, it became increasingly clear that the key to many of its successful behaviours would be a grounding in better reactive primitives. The same values of open authorship — allowing systems to be specified in separate pieces contributed by multiple authors — need to be extended right down to the bedrock of reactivity that the system is built on. Some of these desired behaviours were flagged up in a section of last year’s Substrate Vision Statement.
Over these years I’ve been working with a “commodity” reactive system, preact-signals, and part of my journey
with that system has been described in the December 2025’s Understanding Reactivity
posting. I learned a lot from these commodity systems about what valuable regularities and guarantees a reactive system
needs to support, and also about the limitations of these commodity systems, some of which are fundamental, and some incidental. This learning fed into
the development of fluid.cell which is specifically tailored to the support of open, malleable systems.
Whilst it will be used to replace preact-signals as the underpinning of Infusion, fluid.cell is a self-contained small library with
clearly explainable guarantees and capabilities which could support the development of other malleable systems too.
Here are the new use cases and capabilities which fluid.cell supports:
