Signals & Effects

IsoNim's reactive core is built on two primitives: **signals**, which hold a piece of state, and **effects**, which re-run automatically whenever a signal they read from changes. Together they give isonim-docs (and every IsoNim app) fine-grained updates without a virtual DOM diff.

Creating a signal

A signal is created with createSignal(initialValue) and read/written through its .val accessor. Renderers and consuming code import the reactive core through isonim/rxcore -- the one stable adapter seam, never isonim/core/* directly:

import isonim/rxcore

let count = createSignal(0)
echo count.val # 0
count.val = count.val + 1
echo count.val # 1

Reacting with effects

createRenderEffect (or the lower-level effect proc) re-runs its body every time a signal it reads changes value:

import isonim/rxcore

let count = createSignal(0)
createRenderEffect(proc() =
  echo "count is now ", count.val)

Tip

Effects track their dependencies automatically -- there is no manual subscription list to maintain. Reading count.val inside the effect body is what registers the dependency.

Warning

Don't mutate a signal from inside its own effect without a guard condition; it will re-trigger the effect and can loop forever.

Primitive cheat sheet

Primitive	Purpose
`createSignal`	Holds one piece of reactive state.
`effect`	Re-runs a side effect when its dependencies change.
`createRenderEffect`	The render-focused effect the `ui` DSL wires up automatically.

See the DSL guide for how ui(r): ... blocks turn signal reads inside text/attribute expressions into these effects automatically, and SSR basics for how the same components render without any reactivity at all on the server. Back to the site index.