A simple example of refactoring a function in baby steps from returning to boolean all the way to Either monad.

createUser.ts

// Option 1: true or false to indicate success
function createUser(id: string): boolean {
    const user = User.getById(id);

    if (!user) {
        User.create(id);
        return true;
    }

    return false;
}

// Option 2: null on failure or User object on success
function createUser(id: string): User | null {
    const user = User.getById(id);

    if (!user) {
        return User.create(id);
    }

    return null;
}

// Option 3: a Maybe monad with Nothing for failure and Just for success
function createUser(id: string): Maybe<User> {
    const user = User.getById(id);

    if (!user) {
        return Just(User.create(id));
    }

    return Nothing;
}

// Option 4: an Either monad with Left for failure and Right for success
function createUser(id: string): Either<ModelError, User> {
    const user = User.getById(id);

    if (!user) {
        return Right(User.create(id));
    }

    return Left(new ModelError(`User with ID ${id} already exists`));
}