# kotlin-result

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A multiplatform Result monad for modelling success or failure operations.

## Installation

```groovy
repositories {
    mavenCentral()
}

dependencies {
    implementation("com.michael-bull.kotlin-result:kotlin-result:2.0.0")
}
```

## Introduction

In functional programming, the result [`Result`][result] type is a monadic type
holding a returned [value][result-value] or an [error][result-error].

To indicate an operation that succeeded, return an [`Ok(value)`][result-Ok]
with the successful `value`. If it failed, return an [`Err(error)`][result-Err]
with the `error` that caused the failure.

This helps to define a clear happy/unhappy path of execution that is commonly
referred to as [Railway Oriented Programming][rop], whereby the happy and
unhappy paths are represented as separate railways.

### Overhead

The `Result` type is modelled as an
[inline value class][kotlin-inline-classes]. This achieves zero object
allocations on the happy path.

A full breakdown, with example output Java code, is available in the
[Overhead][wiki-Overhead] design doc.

### Multiplatform Support

`kotlin-result` targets all three tiers outlined by the
[Kotlin/Native target support][kotlin-native-target-support]

### Read More

Below is a collection of videos & articles authored on the subject of this
library. Feel free to open a pull request on [GitHub][github] if you would like
to include yours.

- [[EN] The Result Monad - Adam Bennett](https://adambennett.dev/2020/05/the-result-monad/)
- [[EN] A Functional Approach to Exception Handling - Tristan Hamilton](https://youtu.be/bEC_t8dH23c?t=132)
- [[EN] kotlin: A functional gold mine - Mark Bucciarelli](http://markbucciarelli.com/posts/2020-01-04_kotlin_functional_gold_mine.html)
- [[EN] Railway Oriented Programming - Scott Wlaschin](https://fsharpforfunandprofit.com/rop/)
- [[JP] KotlinでResult型使うならkotlin-resultを使おう](https://note.com/yasukotelin/n/n6d9e352c344c)
- [[JP] kotlinのコードにReturn Resultを組み込む](https://nnao45.hatenadiary.com/entry/2019/11/30/224820)

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)

## Getting Started

Below is a simple example of how you may use the `Result` type to model a
function that may fail.

```kotlin
fun checkPrivileges(user: User, command: Command): Result<Command, CommandError> {
    return if (user.rank >= command.mininimumRank) {
        Ok(command)
    } else {
        Err(CommandError.InsufficientRank(command.name))
    }
}
```

When interacting with code outside your control that may throw exceptions, wrap
the call with [`runCatching`][result-runCatching] to capture its execution as 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 the 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`][result-binding] function 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 error.

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

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

val sum: Result<Int, SumError> = 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` function 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)](https://arrow-kt.io/docs/0.10/patterns/monad_comprehensions/)
- [Monad comprehensions - Bow (Swift)](https://bow-swift.io/docs/patterns/monad-comprehensions)
- [For comprehensions - Scala](https://docs.scala-lang.org/tour/for-comprehensions.html)

#### Coroutine Binding Support

Use of suspending functions within a `coroutineBinding` block requires an
additional dependency:

```kotlin
dependencies {
    implementation("com.michael-bull.kotlin-result:kotlin-result:2.0.0")
    implementation("com.michael-bull.kotlin-result:kotlin-result-coroutines:2.0.0")
}
```

The [`coroutineBinding`][result-coroutineBinding] function runs inside a
[`coroutineScope`][kotlin-coroutineScope], facilitating _concurrent
decomposition of work_.

When any call to `bind()` inside the block fails, the scope fails, cancelling
all other children.

The example below demonstrates a computationally expensive function that takes
five milliseconds to compute being eagerly cancelled as soon as a smaller
function fails in just one millisecond:

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

runBlocking {
    val result: Result<Int, BindingError> = coroutineBinding { // this creates a new CoroutineScope
        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)

Improvements on existing solutions such the stdlib include:

- Reduced runtime overhead with zero object allocations on the happy path
- 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` functions 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 almost 100 [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.

## 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/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L10
[result-value]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L55
[result-error]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L59
[result-Ok]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L9
[result-Err]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L17
[kotlin-inline-classes]: https://kotlinlang.org/docs/inline-classes.html
[wiki-Overhead]: https://github.com/michaelbull/kotlin-result/wiki/Overhead
[rop]: https://fsharpforfunandprofit.com/rop/
[kotlin-native-target-support]: https://kotlinlang.org/docs/native-target-support.html
[github]: https://github.com/michaelbull/kotlin-result
[wiki]: https://github.com/michaelbull/kotlin-result/wiki
[result-runCatching]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Factory.kt#L11
[result-binding]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Binding.kt#L28
[bow-bindings]: https://bow-swift.io/docs/patterns/monad-comprehensions/#bindings
[result-coroutineBinding]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result-coroutines/src/commonMain/kotlin/com/github/michaelbull/result/coroutines/CoroutineBinding.kt#L42
[kotlin-coroutineScope]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/coroutine-scope.html
[unit-tests]: https://github.com/michaelbull/kotlin-result/tree/master/kotlin-result/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/repository/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

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