Part 3: flatMap: Do This, Then That

A useful program rarely does one thing.

It parses an id. Then it fetches a user. Then it checks a permission. Then it writes something.

That is where flatMap shows up.

flatMap means:

If this Effect succeeds, use its value to choose the next Effect.

A small chain

const parseUserId = (input: string) =>
  Number.isInteger(Number(input))
    ? Effect.succeed(Number(input))
    : Effect.fail(new InvalidUserId(input));

const fetchUser = (id: number) =>
  id === 1
    ? Effect.succeed({ id, name: "Ada" })
    : Effect.fail(new UserNotFound(id));

const program = parseUserId("1").pipe(Effect.flatMap((id) => fetchUser(id)));

The second step needs the value from the first step.

That is why this is flatMap.

Failure skips the rest

If parseUserId("abc") fails, fetchUser does not run.

That is the part I like.

The happy path reads straight down. The failure path stays in the type.

Effect.Effect<User, InvalidUserId | UserNotFound, never>;

The program can fail in either step.

The part that finally clicked

This:

parseUserId("1").pipe(Effect.flatMap(fetchUser));

is the Effect version of:

const id = parseUserId("1");
const user = fetchUser(id);

Except the failures are not hiding.

They are part of the result.