# kotlin-result [![Release](https://api.bintray.com/packages/michaelbull/maven/kotlin-result/images/download.svg)](https://bintray.com/michaelbull/maven/kotlin-result/_latestVersion) [![Build Status](https://travis-ci.org/michaelbull/kotlin-result.svg?branch=master)](https://travis-ci.org/michaelbull/kotlin-result) [![License](https://img.shields.io/github/license/michaelbull/kotlin-result.svg)](https://github.com/michaelbull/kotlin-result/blob/master/LICENSE) [`Result`][result] is a monad for modelling success ([`Ok`][result-ok]) or failure ([`Err`][result-err]) operations. ## Installation ```groovy repositories { maven { url = 'https://dl.bintray.com/michaelbull/maven' } } dependencies { compile 'com.michael-bull.kotlin-result:kotlin-result:1.1.0' } ``` ## Introduction The [`Result`][result] monad has two subtypes, [`Ok`][result-ok] representing success and containing a `value`, and [`Err`][result-err], representing failure and containing an `error`. Scott Wlaschin's article on [Railway Oriented Programming][swalschin-rop] is a great introduction to the benefits of modelling operations using the `Result` type. Mappings are available on the [wiki][wiki] to assist those with experience using the `Result` type in other languages: - [Elm](https://github.com/michaelbull/kotlin-result/wiki/Elm) - [Haskell](https://github.com/michaelbull/kotlin-result/wiki/Haskell) - [Rust](https://github.com/michaelbull/kotlin-result/wiki/Rust) - [Scala](https://github.com/michaelbull/kotlin-result/wiki/Scala) ### Creating Results To begin incorporating the `Result` type into an existing codebase, you can wrap functions that may fail (i.e. throw an `Exception`) with [`Result.of`][result-of]. This will execute the block of code and `catch` any `Exception`, returning a `Result`. ```kotlin val result: Result = Result.of { customerDb.findById(id = 50) // could throw SQLException or similar } ``` The idiomatic approach to modelling operations that may fail in Railway Oriented Programming is to avoid throwing an exception and instead make the return type of your function a `Result`. ```kotlin fun checkPrivileges(user: User, command: Command): Result { return if (user.rank >= command.mininimumRank) { Ok(command) } else { Err(CommandError.InsufficientRank(command.name)) } } ``` Nullable types, such as the `find` method in the example below, can be converted to a `Result` using the `toResultOr` extension function. ```kotlin val result: Result = customers .find { it.id == id } // returns Customer? .toResultOr { "No customer found" } ``` ### Transforming Results Both success and failure results can be transformed within a stage of the railway track. The example below demonstrates how to transform an internal program error (`UnlockError`) into an exposed client error (`IncorrectPassword`). ```kotlin val result: Result = unlockVault("my-password") // returns Result .mapError { IncorrectPassword } // transform UnlockError into IncorrectPassword ``` ### Chaining Results can be chained to produce a "happy path" of execution. For example, the happy path for a user entering commands into an administrative console would consist of: the command being tokenized, the command being registered, the user having sufficient privileges, and the command executing the associated action. The example below uses the `checkPrivileges` function we defined earlier. ```kotlin tokenize(command.toLowerCase()) .andThen(::findCommand) .andThen { cmd -> checkPrivileges(loggedInUser, cmd) } .andThen { execute(user = loggedInUser, command = cmd, timestamp = LocalDateTime.now()) } .mapBoth( { output -> printToConsole("returned: $output") }, { error -> printToConsole("failed to execute, reason: ${error.reason}") } ) ``` ## Inspiration Inspiration for this library has been drawn from other languages in which the Result monad is present, including: - [Elm](http://package.elm-lang.org/packages/elm-lang/core/latest/Result) - [Haskell](https://hackage.haskell.org/package/base-4.10.0.0/docs/Data-Either.html) - [Rust](https://doc.rust-lang.org/std/result/) - [Scala](http://www.scala-lang.org/api/2.12.4/scala/util/Either.html) It also iterates on other Result libraries written in Kotlin, namely: - [danneu/kotlin-result](https://github.com/danneu/kotlin-result) - [kittinunf/Result](https://github.com/kittinunf/Result) - [npryce/result4k](https://github.com/npryce/result4k) Improvements on the existing solutions include: - Feature parity with Result types from other languages including Elm, Haskell, & Rust - Lax constraints on `value`/`error` nullability - Lax constraints on the `error` type's inheritance (does not inherit from `Exception`) - Top level `Ok` and `Err` classes avoids qualifying usages with `Result.Ok`/`Result.Err` respectively - Higher-order functions marked with the `inline` keyword for reduced runtime overhead - Extension functions on `Iterable` & `List` for folding, combining, partitioning - Consistent naming with existing Result libraries from other languages (e.g. `map`, `mapError`, `mapBoth`, `mapEither`, `and`, `andThen`, `or`, `orElse`, `unwrap`) - Extensive test suite with over 50 [unit tests][unit-tests] covering every library method ## Example The [example][example] module contains an implementation of Scott's [example application][swalschin-example] that demonstrates the usage of `Result` in a real world scenario. It hosts a [ktor][ktor] server on port 9000 with a `/customers` endpoint. The endpoint responds to both `GET` and `POST` requests with a provided `id`, e.g. `/customers/100`. Upserting a customer id of 42 is hardcoded to throw an [`SQLException`][customer-42] to demonstrate how the `Result` type can [map internal program errors][update-customer-error] to more appropriate user-facing errors. ### Payloads #### Fetch customer information ``` $ curl -i -X GET 'http://localhost:9000/customers/5' ``` ``` HTTP/1.1 200 OK Content-Type: application/json; charset=UTF-8 Content-Length: 93 { "id": 5, "firstName": "Michael", "lastName": "Bull", "email": "example@email.com" } ``` #### Add new customer ``` $ curl -i -X POST \ -H "Content-Type:application/json" \ -d \ '{ "firstName": "Your", "lastName": "Name", "email": "your@email.com" }' \ 'http://localhost:9000/customers/200' ``` ``` HTTP/1.1 201 Created Content-Type: text/plain; charset=UTF-8 Content-Length: 16 Customer created ``` ## Contributing Bug reports and pull requests are welcome on [GitHub][github]. ## License This project is available under the terms of the ISC license. See the [`LICENSE`](LICENSE) file for the copyright information and licensing terms. [result]: https://github.com/michaelbull/kotlin-result/blob/master/src/main/kotlin/com/github/michaelbull/result/Result.kt#L10 [result-ok]: https://github.com/michaelbull/kotlin-result/blob/master/src/main/kotlin/com/github/michaelbull/result/Result.kt#L30 [result-err]: https://github.com/michaelbull/kotlin-result/blob/master/src/main/kotlin/com/github/michaelbull/result/Result.kt#L35 [result-of]: https://github.com/michaelbull/kotlin-result/blob/master/src/main/kotlin/com/github/michaelbull/result/Result.kt#L17 [swalschin-rop]: https://fsharpforfunandprofit.com/rop/ [wiki]: https://github.com/michaelbull/kotlin-result/wiki [unit-tests]: https://github.com/michaelbull/kotlin-result/tree/master/src/test/kotlin/com/github/michaelbull/result [example]: https://github.com/michaelbull/kotlin-result/tree/master/example/src/main/kotlin/com/github/michaelbull/result/example [swalschin-example]: https://github.com/swlaschin/Railway-Oriented-Programming-Example [ktor]: http://ktor.io/ [customer-42]: https://github.com/michaelbull/kotlin-result/blob/master/example/src/main/kotlin/com/github/michaelbull/result/example/service/InMemoryCustomerRepository.kt#L38 [update-customer-error]: https://github.com/michaelbull/kotlin-result/blob/master/example/src/main/kotlin/com/github/michaelbull/result/example/service/CustomerService.kt#L50 [github]: https://github.com/michaelbull/kotlin-result