kotlin-result/README.md

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# kotlin-result
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[![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)
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[`Result<V, E>`][result] is a monad for modelling success ([`Ok`][result-ok]) or
failure ([`Err`][result-err]) operations.
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## Installation
```groovy
repositories {
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maven { url = 'https://dl.bintray.com/michaelbull/maven' }
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}
dependencies {
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compile 'com.michael-bull.kotlin-result:kotlin-result:1.1.0'
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}
```
## Introduction
The [`Result`][result] monad has two subtypes, [`Ok<V>`][result-ok]
representing success and containing a `value`, and [`Err<E>`][result-err],
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representing failure and containing an `error`.
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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<T, Exception>`.
```kotlin
val result: Result<Customer, Exception> = 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<Command, CommandError> {
return if (user.rank >= command.mininimumRank) {
Ok(command)
} else {
Err(CommandError.InsufficientRank(command.name))
}
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}
```
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<Customer, String> = customers
.find { it.id == id } // returns Customer?
.toResultOr { "No customer found" }
```
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### 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<Treasure, UnlockResponse> =
unlockVault("my-password") // returns Result<Treasure, UnlockError>
.mapError { IncorrectPassword } // transform UnlockError into IncorrectPassword
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```
### 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.
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```kotlin
tokenize(command.toLowerCase())
.andThen(::findCommand)
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.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}") }
)
```
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## 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/)
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- [Scala](http://www.scala-lang.org/api/2.12.4/scala/util/Either.html)
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It also iterates on other Result libraries written in Kotlin, namely:
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- [danneu/kotlin-result](https://github.com/danneu/kotlin-result)
- [kittinunf/Result](https://github.com/kittinunf/Result)
- [npryce/result4k](https://github.com/npryce/result4k)
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Improvements on the existing solutions include:
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- Feature parity with Result types from other languages including Elm, Haskell,
& Rust
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- Lax constraints on `value`/`error` nullability
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- Lax constraints on the `error` type's inheritance (does not inherit from
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`Exception`)
- Top level `Ok` and `Err` classes avoids qualifying usages with
`Result.Ok`/`Result.Err` respectively
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- 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`)
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- Extensive test suite with over 50 [unit tests][unit-tests] covering every library method
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## 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
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#### Fetch customer information
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```
$ 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"
}
```
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#### Add new customer
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```
$ 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
```
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## 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.
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[result]: https://github.com/michaelbull/kotlin-result/blob/master/src/main/kotlin/com/github/michaelbull/result/Result.kt#L10
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[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/
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[wiki]: https://github.com/michaelbull/kotlin-result/wiki
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[unit-tests]: https://github.com/michaelbull/kotlin-result/tree/master/src/test/kotlin/com/github/michaelbull/result
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[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
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[update-customer-error]: https://github.com/michaelbull/kotlin-result/blob/master/example/src/main/kotlin/com/github/michaelbull/result/example/service/CustomerService.kt#L50
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[github]: https://github.com/michaelbull/kotlin-result