Effect.ts: Absence as First-Class

Advanced Patterns • Lesson 43 of 51

43. PubSub - Broadcasting to Absent Subscribers

Values that exist for ALL subscribers, whenever they arrive

Code Example

const program = Effect.gen(function* () {
  // Create PubSub - no subscribers yet (all absent!)
  const pubsub = yield* PubSub.bounded<string>(10);

  // Subscriber 1: waiting for absent values
  const sub1 = yield* PubSub.subscribe(pubsub);

  // Subscriber 2: also waiting
  const sub2 = yield* PubSub.subscribe(pubsub);

  // Publish: make value available to ALL
  yield* PubSub.publish(pubsub, "Hello!");

  // Both receive it!
  const msg1 = yield* Queue.take(sub1);  // "Hello!"
  const msg2 = yield* Queue.take(sub2);  // "Hello!"

  // One message → multiple absence resolutions!
});

Interactive Example

const { PubSub, Queue } = await import('effect');
const program = Effect.gen(function* () {
      const pubsub = yield* PubSub.bounded(10);




      return yield* Effect.scoped(
              Effect.gen(function* () {
                      const sub1 = yield* PubSub.subscribe(pubsub);
                      const sub2 = yield* PubSub.subscribe(pubsub);
                      yield* PubSub.publish(pubsub, 'Hello!');
                      const msg1 = yield* Queue.take(sub1);
                      const msg2 = yield* Queue.take(sub2);
                      return [msg1, msg2];
              })
      );
});
const [m1, m2] = await Effect.runPromise(program);
return `PubSub!

Published: "Hello!"
Sub 1 received: "${m1}"
Sub 2 received: "${m2}"

✓ Broadcast to all!`;

Explanation

PubSub: broadcasting to multiple absent consumers.

Traditional approach: each subscriber needs separate delivery. PubSub types broadcast absence:

Subscribers are absent (don't exist yet)
Messages are absent (not published yet)
No assumptions about who subscribes when
publish: resolve absence for ALL subscribers
Late subscribers: still get messages!

Absence-first messaging: one source, many absent consumers!

Part 43 of 51 in the Effect.ts Absence Modeling series

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