Narrowing and matching

Inspect Result status, use type guards, and fold both branches into one output.

Discriminant narrowing

if (result.status === "ok") {
  result.value;
} else {
  result.error;
}

status is the serializable discriminant: "ok" or "error".

Static guards

if (Result.isOk(result)) {
  use(result.value);
}

if (Result.isError(result)) {
  report(result.error);
}

Instance guards

if (result.isOk()) {
  use(result.value);
}

if (result.isErr()) {
  report(result.error);
}

isErr() is the instance spelling; Result.isError() is the static spelling.

match

Use match when both branches produce the same output type and you are ready to leave Result:

const statusCode = result.match({
  ok: () => 200,
  err: (error) => (error._tag === "NotFound" ? 404 : 500),
});

Static data-first and data-last forms are also available:

Result.match(result, {
  ok: (value) => value.name,
  err: () => "Anonymous",
});

const displayName = Result.match({
  ok: (value: User) => value.name,
  err: (_error: FindUserError) => "Anonymous",
});

displayName(result);

Match Result first, then tagged errors

const response = result.match({
  ok: (user) => ({ status: 200, body: user }),
  err: (error) =>
    error.match({
      UserNotFound: () => ({ status: 404, body: null }),
      DatabaseUnavailable: () => ({ status: 503, body: null }),
    }),
});

This keeps the two decisions explicit: Result branch first, error variant second.