Effect.ts: Absence as First-Class

State Management • Lesson 48 of 51

29. Ref: Basic Operations

make, get, and set - the three fundamental Ref operations

The Three Basics
const program = Effect.gen(function* () {
  // 1. make: Create a Ref
  const counter = yield* Ref.make(0);

  // 2. get: Read current value
  const value1 = yield* Ref.get(counter); // 0

  // 3. set: Write new value
  yield* Ref.set(counter, 42);

  const value2 = yield* Ref.get(counter); // 42

  return value2;
});
Understanding Each Operation

Ref.make(initialValue)

Creates a new Ref with an initial value.

const counter = yield* Ref.make(0);      // number Ref
const name = yield* Ref.make("Alice");   // string Ref
const items = yield* Ref.make([1,2,3]);  // array Ref

Returns: Effect<Ref<T>, never, never>

  • You get back an Effect that produces a Ref
  • The Ref is a handle to mutable state

Ref.get(ref)

Reads the current value from a Ref.

const value = yield* Ref.get(counter);
// value is the actual number, not a Ref!

Returns: Effect<T, never, never>

  • You get back the value, not the Ref
  • Safe to use like any other value

Ref.set(ref, newValue)

Replaces the entire value.

yield* Ref.set(counter, 100);
// counter is now 100, regardless of what it was before

Returns: Effect<void, never, never>

  • Just performs the update
  • No return value needed
Example: Score Tracker
const game = Effect.gen(function* () {
  const score = yield* Ref.make(0);

  console.log(yield* Ref.get(score)); // 0

  yield* Ref.set(score, 10);  // Player scored!
  console.log(yield* Ref.get(score)); // 10

  yield* Ref.set(score, 25);  // Scored again!
  console.log(yield* Ref.get(score)); // 25
});
Key Insight

Think of Ref like a box:

  • Ref.make(): Create an empty box with initial contents
  • Ref.get(): Look inside the box
  • Ref.set(): Replace the contents of the box

All three operations return Effects, so they compose with everything else in Effect!

The foundation: make, get, and set

Part 48 of 51 in the Effect.ts Absence Modeling series

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