Monadic version of the trip-service-kata by Sandro Mancuso, using Maybe and Either. Public interface still compatible

TripService.js

const { Maybe, Either } = require('monet');

class TripService {

    // Monadic version of the public function, returns an Either
    getTripsByUserEither(user, loggedUser) {
        return Maybe.fromNull(loggedUser)
            .toEither('User not logged in.')
            .flatMap(connectedUser => this._doGetTripsByUser(user, connectedUser)); 
        // flatMap instead of map as the loadFromDb Returns an Either as well, ie an Either in an Either, needs to be flattened
    }

    // Imperative version, calling the monadic one, keeping the contract of throwing
    getTripsByUserImperative(user, loggedUser) {
        let eitherTrips = this.getTripsByUserEither(user, loggedUser);
        return eitherTrips.cata(
            failure => {throw new Error(failure)},
            trips => trips);
    }

    // doGetTripsByUser :: user -> user -> Either String (List Trip)
    _doGetTripsByUser(user, loggedUser) {
        let isFriend = user.isFriendsWith(loggedUser);
        return isFriend ? this._loadTripsFromDb(user) : noTrips();
    }

    // loadTripsFromDb :: user -> Either String (List Trip)
    _loadTripsFromDb(user) {
        return TripDAO.findTripsByUser(user);
    }


}

function noTrips() {
    return Either.Right([]);
}