Memos

Memos are cached computed values that only recalculate when dependencies change.

Creating a Memo

using @luna { signal, memo }

let count = signal(2)
let doubled = memo(fn() { count.get() * 2 })

println(doubled.get())  // 4

count.set(3)
println(doubled.get())  // 6

Memos are Cached

Unlike regular functions, memos only recalculate when needed:

let count = signal(1)

let expensive = memo(fn() {
  println("Computing...")
  count.get() * 100
})

println(expensive.get())  // "Computing..." then "100"
println(expensive.get())  // Just "100" - cached!
println(expensive.get())  // Just "100" - still cached!

count.set(2)
println(expensive.get())  // "Computing..." then "200"

Chaining Memos

Memos can depend on other memos:

let count = signal(2)
let doubled = memo(fn() { count.get() * 2 })
let quadrupled = memo(fn() { doubled.get() * 2 })

println(quadrupled.get())  // 8

count.set(3)
println(quadrupled.get())  // 12

Memos in Effects

Effects can depend on memos:

let a = signal(2)
let b = signal(3)
let sum = memo(fn() { a.get() + b.get() })

effect(fn() {
  println("Sum: " + sum.get().to_string())
})
// Prints: Sum: 5

a.set(10)
// Prints: Sum: 13

When to Use Memos

Use Memos For

• Expensive calculations

• Derived values used in multiple places

• Filtering or transforming lists

let items = signal([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

let even_items = memo(fn() {
  items.get().filter(fn(n) { n % 2 == 0 })
})

let even_count = memo(fn() {
  even_items.get().length()
})

Don't Use Memos For

• Simple property access

• Values only used once

// Don't do this - overhead isn't worth it
let name_memo = memo(fn() { user.get().name })

// Just access directly
effect(fn() {
  println(user.get().name)
})

Memos vs Effects

// Memo: returns a value
let doubled = memo(fn() { count.get() * 2 })

// Effect: performs side effects
effect(fn() {
  println(count.get())  // Side effect: logging
})

API Summary

TypeScript Comparison

Try It

Create a shopping cart with:

1. An items signal (array of prices)

2. A subtotal memo (sum of prices)

3. A tax memo (10% of subtotal)

4. A total memo (subtotal + tax)

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