Reactive signals

Formula compiler and runtime Internals covers how formulas evaluate in dependency order. This is one layer further down — the actual primitive every reactive value in Formo is built from.

The Signal primitive

Every reactive value — a property, mouse.x, time, a tag count — is backed by a real Signal. A signal holds three things: its current value, a version number that increments on every change, and a set of subscribers (the things that need to know when it changes).

Reading a signal during formula evaluation automatically registers that formula as a subscriber — there's no manual subscribe call anywhere in a formula you write. Writing a new value notifies every subscriber to mark itself dirty, and schedules a flush.

One real optimization worth knowing: a signal skips its own change notification entirely if the new value is identical to the old one. Setting a property to the value it already holds doesn't ripple out to anything that depends on it — no dirty marking, no re-evaluation, nothing happens.

Prototype-inherited formula context

Each formula's execution context is built with Object.create() off a shared prototype, rather than assembling a fresh object by spreading properties together every time. It's a small, deliberate detail, but it's part of why the hot evaluation path — potentially dozens of formulas recalculating every frame — stays cheap: building a context is a prototype-chain hookup, not an allocation-heavy object copy.

Why vector geometry uses typed arrays

This isn't specific to formulas, but it's part of the same performance story: a vector network's own point data — x, y, and corner radius per vertex — lives in a stride-4 Float32Array rather than an array of plain JS objects. Reading a coordinate is an index into a flat numeric buffer, not a property lookup on an object, which matters when that data gets touched on every frame of an animation.

For the dependency graph itself — evaluation order, cycle detection, batching multiple changes into one pass — see Formula compiler and runtime internals; the Signal primitive here is what that graph is actually built out of.