ascent · v0.2.0

Reactive boundaries

Reactive boundaries are first-class AST nodes. The mount engine wires each once; updates patch only that boundary. Children are built right-to-left so a slot can find its insertion point from already-populated later siblings.

when

when(cond)(body) mounts/unmounts as a Squawk[Boolean] flips. The body is a thunk; rebuilt fresh on each true-transition so per-render local state stays correct.

{
  for open <- sq(true)
  yield E.div(
    E.button(Events.onClick(_ => open.update(!_)), "toggle"),
    when(open)(E.p("visible")),
  )
}

visible

forEach

Keyed-list reconciliation reuses DOM nodes across reorders (focus/caret preserved). The key function is identity; the render fn builds from a snapshot value.

{
  for items <- sq(Seq("a", "b", "c"))
  yield E.ul(
    forEach(items)(identity)(s => E.li(s))
  )
}
  • a
  • b
  • c

forEachSignal

Like forEach, but each row holds a persistent Squawk[A] fed new values; rows never rebuild; only boundaries bound to changed fields repaint. Prefer this when row UIs are heavy or hold local state. Eq[A] gates the per-row dedup (derives Eq on the row type).

case class Row(id: String, n: Int) derives Eq

{
  for items <- sq(Seq(Row("1", 1), Row("2", 2)))
  yield E.ul(
    forEachSignal(items)(_.id) { (id, _, signal) =>
      E.li(id, ": ", signal.map(_.n.toString))
    }
  )
}
  • 1: 1
  • 2: 2

scoped

scoped { … } opens a zio.Scope tied to a subtree's lifetime. Finalizers (conduit subscriptions, rAF cancels, widget teardown) run when the subtree unmounts.

{
  for n <- sq(0)
  yield scoped {
    ZIO.succeed(E.span(n.map(_.toString)))
  }
}
0