Effect.gen — sequence effects like async/await

Effect v3.12.0

pipe and flatMap were introduced in Composing effects ↗

You already sequence async work in TypeScript with async and await. You call a step, await its value, then use it on the next line. It reads straight down the page, with no nesting at all.

In Effect, pipe is the tool you reach for first, and it is excellent at a straight line of transforms. Pull the name off the user, then format a greeting. When every step just transforms the last value, pipe reads beautifully.

The friction starts when a step returns its own effect. Greeting through an effect means each step becomes a Effect.flatMap callback, nesting inside the one before it. Chain a few of these and you are back to a staircase of callbacks.

Effect.gen gives you that async and await feel back for effects. Inside the generator, yield* unwraps an effect's value the way await unwraps a promise, so the steps read straight down the page again. It is the same program with no callbacks, and the type never changed at all.

// Plain TypeScript: async/await sequences steps, reading straight down.
const fetchUser = () => Promise.resolve({ name: "Ada" });
const greet = (name: string) => Promise.resolve(`Hello, ${name}`);

async function main() {
  const user = await fetchUser();
  const msg = await greet(user.name);
  return `${msg}!`;
}

main().then((line) => console.log(line)); // Hello, Ada!

❚❚ beat