Computed Values

computed(|| { ... }) encapsulates a derivation: the body runs on the first read and stays cached until any signal read inside it changes. Unlike effect, it is lazy — it only recalculates when someone reads it.

Tracking is transitive: an effect that reads a computed becomes dependent on all signals that computed depends on. This makes it safe to compose computeds in a chain without losing reactivity.

doubled recalculates on every change to count; the effect sees both in sync.

03-computed-derived.zolo

Playground

// Feature: computed — a memoized signal derived from other signals

// Syntax: `computed(|| { ... })`. The body is re-run when any signal

// it reads changes. Reads are tracked transitively: an effect that

// reads a computed depends on whatever the computed depends on.

// When to use: derived values that should recompute lazily and be

// shared between effects (full name, total price, formatted string).


let count = signal(0)
let doubled = computed(|| { count.get() * 2 })

let e = effect(|| {
  print("count = {count.get()}, doubled = {doubled.get()}")
})

// expected: count = 0, doubled = 0


count.set(5)

// expected: count = 5, doubled = 10


count.set(7)
// expected: count = 7, doubled = 14


// Computed values cache: reading `doubled` twice in one effect run

// re-uses the result — the body only runs when a dep changes.

Challenge

Add a third computed called label that returns "even" or "odd" and include it in the effect's print.