# kotlin-result

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[`Result<V, E>`][result] is a monad for modelling success ([`Ok`][result-ok]) or
failure ([`Err`][result-err]) operations.

## Installation

```groovy
repositories {
    mavenCentral()
}

dependencies {
    implementation("com.michael-bull.kotlin-result:kotlin-result:1.1.9")
}
```

## Introduction

The [`Result`][result] monad has two subtypes, [`Ok<V>`][result-ok]
representing success and containing a `value`, and [`Err<E>`][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

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))
    }
}
```

To incorporate the `Result` type into an existing codebase that throws
exceptions, you can wrap functions that may `throw` with
[`runCatching`][result-runCatching]. This will execute the block of code and
`catch` any `Throwable`, returning a `Result<T, Throwable>`.

```kotlin
val result: Result<Customer, Throwable> = runCatching {
    customerDb.findById(id = 50) // could throw SQLException or similar
}
```

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" }
```

### 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
```

### 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}") }
    )
```

### Binding (Monad Comprehension)

The `binding` keyword allows multiple calls that each return a `Result` to be
chained imperatively. When inside a `binding` block, the `.bind()` function is
accessible on any `Result`. Each call to `bind` will attempt to unwrap the
`Result` and store its value, returning early if any `Result` is an `Err`.

In the example below, should `functionX()` return an `Err`, then execution will
skip both `functionY()` and `functionZ()`, instead storing the `Err` from
`functionX` in the variable named `sum`.

```kotlin
fun functionX(): Result<Int, DomainError> { ... }
fun functionY(): Result<Int, DomainError> { ... }
fun functionZ(): Result<Int, DomainError> { ... }

val sum: Result<Int, DomainError> = binding {
    val x = functionX().bind()
    val y = functionY().bind()
    val z = functionZ().bind()
    x + y + z
}

println("The sum is $sum") // prints "The sum is Ok(100)"
```

The `binding` keyword primarily draws inspiration from
[Bow's `binding` function][bow-bindings], however below is a list of other
resources on the topic of monad comprehensions.

- [Monad comprehensions - Arrow (Kotlin)][arrow-monad-comprehension]
- [Monad comprehensions - Bow (Swift)][bow-monad-comprehension]
- [For comprehensions - Scala][scala-for-comprehension]

#### Coroutine Support

Use of coroutines within a `binding` block requires an additional dependency:

```kotlin
dependencies {
    implementation("com.michael-bull.kotlin-result:kotlin-result:1.1.9")
    implementation("com.michael-bull.kotlin-result:kotlin-result-coroutines:1.1.9")
}
```

This allows for asynchronous binds to operate so that if a bind were to fail,
the binding block will return with the first failing async result:

```kotlin

suspend fun failsIn5ms(): Result<Int, DomainErrorA> { ... }
suspend fun failsIn1ms(): Result<Int, DomainErrorB> { ... }

runBlocking{
    val result = binding<Int, BindingError> {
        val x = async { failsIn5ms().bind() }
        val y = async { failsIn1ms().bind() }
        x.await() + y.await()
    }

    // result will be Err(DomainErrorB)
}
```

## 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/1'
```

```
HTTP/1.1 200 OK
Content-Type: application/json; charset=UTF-8
Content-Length: 84

{
  "firstName": "Michael",
  "lastName": "Bull",
  "email": "michael@example.com"
}
```

#### Add new customer

```
$ curl -i -X POST \
   -H "Content-Type:application/json" \
   -d \
'{
  "firstName": "Your",
  "lastName": "Name",
  "email": "email@example.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/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L10
[result-ok]: https://github.com/michaelbull/kotlin-result/blob/master/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L35
[result-err]: https://github.com/michaelbull/kotlin-result/blob/master/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L58
[result-runCatching]: https://github.com/michaelbull/kotlin-result/blob/master/src/commonMain/kotlin/com/github/michaelbull/result/Factory.kt#L11
[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/commonTest/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
[bow-bindings]: https://bow-swift.io/docs/patterns/monad-comprehensions/#bindings
[bow-monad-comprehension]: https://bow-swift.io/docs/patterns/monad-comprehensions
[scala-for-comprehension]: https://docs.scala-lang.org/tour/for-comprehensions.html
[arrow-monad-comprehension]: https://arrow-kt.io/docs/0.10/patterns/monad_comprehensions/
[either-syntax]: https://arrow-kt.io/docs/0.10/apidocs/arrow-core-data/arrow.core/-either/#syntax