A flutter/dart library : either_option

About

A small typed and safe library for error handling with functionnal programming concept in Dart and utter project (MIT licence).

either_option is a simple library typed for easy and safe error handling with functional programming style in Dart. It aims to allow flutter/dart developpers to use the 2 most popular patterns and abstractions : Either and Option, mainly used in FP language like Scala, Haskell, OCaml,…

Installation

Package link on pub either_option In your pubspec.yaml dependencies add

    either_option: ^1.0.6

Overview

Either

Either Represents a value of one of two possible types. By convention we put missing or error value in an instance of Left and expected success value in an instance of Right.

For example, we fetch an url from a repository repository to get an User details and use Either<ServerError,User> :

  Future<Either<ServerError, User>> getUser(int id) async {
    final baseUrl = "https://fakerestapi.azurewebsites.net/api/Users/$id";

    final res = await http.get(baseUrl);

    if (res.statusCode == 200) {
      dynamic body = json.decode(res.body);
      User user = User.fromJson(body);
      return Right(user);
    }
    if (res.statusCode == 404)
      return Left(ServerError("This user doesn't exist"));

    return Left(ServerError("Unknown server error"));
  }

So now to consume result we can use for example fold method and say what to do with value :

main() async {
  final Repository repository = Repository();

  final Either<ServerError, User> res = await repository.getUser(3);
  final defaultUser = User(id: 0, username: "ko", password: "ko");
  final userName = res.fold((_) => defaultUser.username, (user) => user.username);
  print(userName); // "User 3"
  
//if res was a Left, print(userName) would give "ko"
}

Option

Option Represents a value of one of two possible types. By convention we consider missing value as an instance of None and expected success value in an instance of Some.

  Future<Option<User>> getUserOpt(int id) async {
    final res = await http.get(baseUrl);
    return Option.cond(
        res.statusCode == 200, User.fromJson(json.decode(res.body)));
  }
  // -----------------------------------------
  main() async {
    final Repository repository = Repository();
  
    final Option<User> res = await repository.getUserOpt(3);
    final defaultUser = User(id: 0, username: "ko", password: "ko");
    final userName = res.getOrElse(defaultUser).username;
    print(userName); // "User 3"

  //if res was a None, print(userName) would give "ko"
}

Ressources & explanation

Medium article

Features

Functions available :

	Option	Either
fold	yes	yes
map	yes	yes
flatMap	yes	yes
getOrElse	yes
orElse	yes
toLeft	yes
toRight	yes
toEither	yes
Option.empty	yes
Option.of	yes
filter	yes
exists	yes
contains	yes
swap	yes	yes
cond	yes	yes