Part 8: Services Without the Ceremony

Dependency injection sounds heavier than it needs to.

Most of the time, the idea is small:

This code needs a thing. Give it the thing at the edge.

Effect calls the thing a service.

Name the service

class UserRepo extends Context.Tag("UserRepo")<
  UserRepo,
  {
    readonly findById: (id: number) => Effect.Effect<User, UserNotFound>;
  }
>() {}

This is the part where the syntax looks bigger than the idea.

The idea is just:

There is a service called UserRepo, and this is what it can do.

Use the service

const fetchUser = (id: number) =>
  Effect.gen(function* () {
    const repo = yield* UserRepo;
    return yield* repo.findById(id);
  });

Now the requirement slot includes UserRepo.

The function does not import a concrete database module.

It asks for the service it needs.

Provide it with a Layer

const TestUserRepo = Layer.succeed(UserRepo, {
  findById: (id) => Effect.succeed({ id, name: "Ada" }),
});

A layer says:

Here is how to satisfy that service.

Keep the point small

Services are not a reason to build a giant framework in your app.

They are useful when a dependency is real: database, config, clock, logger, API client.

For a pure helper, skip it.