Application patterns

Place Result boundaries, translate errors, compose workflows, and keep framework concerns at the edge.

Parse at the edge

Turn untrusted input into domain values once:

const parseCreateUser = (input: unknown): ResultType<CreateUser, InvalidCreateUser> => {
  const parsed = CreateUserSchema.safeParse(input);
  return parsed.success
    ? Result.ok(parsed.data)
    : Result.err(new InvalidCreateUser({ issues: parsed.error.issues, message: "Invalid user" }));
};

Inner code should receive meaningful values instead of repeatedly checking unknown shapes.

One error vocabulary per boundary

A repository may expose UserNotFound | UserStoreUnavailable, while its database adapter knows driver-specific exceptions. Translate once:

const findUser = (id: UserId) =>
  queryUserRow(id).mapError((cause) =>
    cause._tag === "NoRows"
      ? new UserNotFound({ id, message: "User not found" })
      : new UserStoreUnavailable({ cause, message: "User store unavailable" }),
  );

Do not leak framework, database, or HTTP error types through domain APIs.

Compose in an application workflow

const registerUser = (input: unknown) =>
  Result.gen(function* () {
    const command = yield* parseCreateUser(input);
    yield* ensureEmailAvailable(command.email);
    const user = yield* insertUser(command);
    yield* publishUserRegistered(user);
    return Result.ok(user);
  });

The signature records every expected failure. Handle or normalize the union where the caller’s policy lives.

Map to transport once

const toHttpResponse = (result: ResultType<User, RegisterUserError>) =>
  result.match({
    ok: (user) => Response.json(user, { status: 201 }),
    err: (error) =>
      matchError(error, {
        InvalidCreateUser: (error) => Response.json(error.toJSON(), { status: 400 }),
        EmailTaken: (error) => Response.json(error.toJSON(), { status: 409 }),
        UserStoreUnavailable: () => Response.json({ message: "Try again" }, { status: 503 }),
        PublishFailed: () => Response.json({ message: "Try again" }, { status: 503 }),
      }),
  });

Avoid common traps