Lesson 01 of 6 in Effect, one function at a time

Sequence steps with Effect.gen

Write sequential Effect code with yield*.

Start with Effect.gen. It lets you write steps in order, the way you would write async code, and each yield* waits for one effect and hands back its value.

import { Effect } from "effect";

const double = (n: number) => Effect.succeed(n * 2);

const program = Effect.gen(function* () {
	const a = yield* double(2);
	const b = yield* double(a);
	return a + b;
});

Effect.runPromise(program).then((n) => console.log(n)); // 12

Three things to notice:

• Each yield* runs one effect and gives you its success value.
• The value you return becomes the success value of the whole program.
• Nothing runs until a runner such as Effect.runPromise is called.

Reach for Effect.gen whenever a step depends on the one before it. A chain of .then calls reads backwards once the steps get long, and this form keeps the order visible.

Errors and dependencies come in later lessons. This one only covers sequencing.