309 lines
14 KiB
Markdown
309 lines
14 KiB
Markdown
# kotlin-result
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[](https://search.maven.org/search?q=g:com.michael-bull.kotlin-result)
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[](https://github.com/michaelbull/kotlin-result/actions/workflows/ci.yaml)
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[](https://github.com/michaelbull/kotlin-result/blob/master/LICENSE)
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![badge][badge-android]
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![badge][badge-jvm]
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![badge][badge-js]
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![badge][badge-nodejs]
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![badge][badge-linux]
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![badge][badge-windows]
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![badge][badge-wasm]
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![badge][badge-ios]
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![badge][badge-mac]
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![badge][badge-tvos]
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![badge][badge-watchos]
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![badge][badge-js-ir]
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![badge][badge-android-native]
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![badge][badge-apple-silicon]
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A multiplatform Result monad for modelling success or failure operations.
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## Installation
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```groovy
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repositories {
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mavenCentral()
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}
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dependencies {
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implementation("com.michael-bull.kotlin-result:kotlin-result:2.0.0")
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}
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```
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## Introduction
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In functional programming, the result [`Result`][result] type is a monadic type
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holding a returned [value][result-value] or an [error][result-error].
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To indicate an operation that succeeded, return an [`Ok(value)`][result-Ok]
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with the successful `value`. If it failed, return an [`Err(error)`][result-Err]
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with the `error` that caused the failure.
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This helps to define a clear happy/unhappy path of execution that is commonly
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referred to as [Railway Oriented Programming][rop], whereby the happy and
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unhappy paths are represented as separate railways.
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### Overhead
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The `Result` type is modelled as an
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[inline value class][kotlin-inline-classes]. This achieves zero object
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allocations on the happy path.
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A full breakdown, with example output Java code, is available in the
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[Overhead][wiki-Overhead] design doc.
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### Multiplatform Support
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`kotlin-result` targets all three tiers outlined by the
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[Kotlin/Native target support][kotlin-native-target-support]
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### Read More
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Below is a collection of videos & articles authored on the subject of this
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library. Feel free to open a pull request on [GitHub][github] if you would like
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to include yours.
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- [[EN] The Result Monad - Adam Bennett](https://adambennett.dev/2020/05/the-result-monad/)
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- [[EN] A Functional Approach to Exception Handling - Tristan Hamilton](https://youtu.be/bEC_t8dH23c?t=132)
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- [[EN] kotlin: A functional gold mine - Mark Bucciarelli](http://markbucciarelli.com/posts/2020-01-04_kotlin_functional_gold_mine.html)
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- [[EN] Railway Oriented Programming - Scott Wlaschin](https://fsharpforfunandprofit.com/rop/)
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- [[JP] KotlinでResult型使うならkotlin-resultを使おう](https://note.com/yasukotelin/n/n6d9e352c344c)
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- [[JP] kotlinのコードにReturn Resultを組み込む](https://nnao45.hatenadiary.com/entry/2019/11/30/224820)
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Mappings are available on the [wiki][wiki] to assist those with experience
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using the `Result` type in other languages:
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- [Elm](https://github.com/michaelbull/kotlin-result/wiki/Elm)
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- [Haskell](https://github.com/michaelbull/kotlin-result/wiki/Haskell)
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- [Rust](https://github.com/michaelbull/kotlin-result/wiki/Rust)
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- [Scala](https://github.com/michaelbull/kotlin-result/wiki/Scala)
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## Getting Started
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Below is a simple example of how you may use the `Result` type to model a
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function that may fail.
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```kotlin
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fun checkPrivileges(user: User, command: Command): Result<Command, CommandError> {
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return if (user.rank >= command.mininimumRank) {
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Ok(command)
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} else {
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Err(CommandError.InsufficientRank(command.name))
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}
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}
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```
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When interacting with code outside your control that may throw exceptions, wrap
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the call with [`runCatching`][result-runCatching] to capture its execution as a
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`Result<T, Throwable>`:
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```kotlin
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val result: Result<Customer, Throwable> = runCatching {
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customerDb.findById(id = 50) // could throw SQLException or similar
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}
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```
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Nullable types, such as the `find` method in the example below, can be
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converted to a `Result` using the `toResultOr` extension function.
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```kotlin
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val result: Result<Customer, String> = customers
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.find { it.id == id } // returns Customer?
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.toResultOr { "No customer found" }
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```
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### Transforming Results
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Both success and failure results can be transformed within a stage of the
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railway track. The example below demonstrates how to transform an internal
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program error `UnlockError` into the exposed client error `IncorrectPassword`.
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```kotlin
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val result: Result<Treasure, UnlockResponse> =
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unlockVault("my-password") // returns Result<Treasure, UnlockError>
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.mapError { IncorrectPassword } // transform UnlockError into IncorrectPassword
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```
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### Chaining
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Results can be chained to produce a "happy path" of execution. For example, the
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happy path for a user entering commands into an administrative console would
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consist of: the command being tokenized, the command being registered, the user
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having sufficient privileges, and the command executing the associated action.
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The example below uses the `checkPrivileges` function we defined earlier.
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```kotlin
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tokenize(command.toLowerCase())
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.andThen(::findCommand)
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.andThen { cmd -> checkPrivileges(loggedInUser, cmd) }
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.andThen { execute(user = loggedInUser, command = cmd, timestamp = LocalDateTime.now()) }
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.mapBoth(
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{ output -> printToConsole("returned: $output") },
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{ error -> printToConsole("failed to execute, reason: ${error.reason}") }
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)
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```
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### Binding (Monad Comprehension)
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The [`binding`][result-binding] function allows multiple calls that each return
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a `Result` to be chained imperatively. When inside a `binding` block, the
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`bind()` function is accessible on any `Result`. Each call to `bind` will
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attempt to unwrap the `Result` and store its value, returning early if any
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`Result` is an error.
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In the example below, should `functionX()` return an error, then execution will
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skip both `functionY()` and `functionZ()`, instead storing the error from
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`functionX` in the variable named `sum`.
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```kotlin
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fun functionX(): Result<Int, SumError> { ... }
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fun functionY(): Result<Int, SumError> { ... }
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fun functionZ(): Result<Int, SumError> { ... }
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val sum: Result<Int, SumError> = binding {
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val x = functionX().bind()
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val y = functionY().bind()
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val z = functionZ().bind()
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x + y + z
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}
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println("The sum is $sum") // prints "The sum is Ok(100)"
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```
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The `binding` function primarily draws inspiration from
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[Bow's `binding` function][bow-bindings], however below is a list of other
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resources on the topic of monad comprehensions.
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- [Monad comprehensions - Arrow (Kotlin)](https://arrow-kt.io/docs/0.10/patterns/monad_comprehensions/)
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- [Monad comprehensions - Bow (Swift)](https://bow-swift.io/docs/patterns/monad-comprehensions)
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- [For comprehensions - Scala](https://docs.scala-lang.org/tour/for-comprehensions.html)
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#### Coroutine Binding Support
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Use of suspending functions within a `coroutineBinding` block requires an
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additional dependency:
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```kotlin
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dependencies {
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implementation("com.michael-bull.kotlin-result:kotlin-result:2.0.0")
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implementation("com.michael-bull.kotlin-result:kotlin-result-coroutines:2.0.0")
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}
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```
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The [`coroutineBinding`][result-coroutineBinding] function runs inside a
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[`coroutineScope`][kotlin-coroutineScope], facilitating _concurrent
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decomposition of work_.
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When any call to `bind()` inside the block fails, the scope fails, cancelling
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all other children.
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The example below demonstrates a computationally expensive function that takes
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five milliseconds to compute being eagerly cancelled as soon as a smaller
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function fails in just one millisecond:
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```kotlin
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suspend fun failsIn5ms(): Result<Int, DomainErrorA> { ... }
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suspend fun failsIn1ms(): Result<Int, DomainErrorB> { ... }
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runBlocking {
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val result: Result<Int, BindingError> = coroutineBinding { // this creates a new CoroutineScope
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val x = async { failsIn5ms().bind() }
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val y = async { failsIn1ms().bind() }
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x.await() + y.await()
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}
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// result will be Err(DomainErrorB)
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}
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```
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## Inspiration
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Inspiration for this library has been drawn from other languages in which the
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Result monad is present, including:
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- [Elm](http://package.elm-lang.org/packages/elm-lang/core/latest/Result)
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- [Haskell](https://hackage.haskell.org/package/base-4.10.0.0/docs/Data-Either.html)
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- [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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Improvements on existing solutions such the stdlib include:
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- Reduced runtime overhead with zero object allocations on the happy path
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- Feature parity with Result types from other languages including Elm, Haskell,
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& 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`)
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- Top level `Ok` and `Err` functions avoids qualifying usages with
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`Result.Ok`/`Result.Err` respectively
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- Higher-order functions marked with the `inline` keyword for reduced runtime
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overhead
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- Extension functions on `Iterable` & `List` for folding, combining, partitioning
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- Consistent naming with existing Result libraries from other languages (e.g.
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`map`, `mapError`, `mapBoth`, `mapEither`, `and`, `andThen`, `or`, `orElse`,
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`unwrap`)
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- Extensive test suite with almost 100 [unit tests][unit-tests] covering every library method
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## Example
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The [example][example] module contains an implementation of Scott's
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[example application][swalschin-example] that demonstrates the usage of `Result`
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in a real world scenario.
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It hosts a [ktor][ktor] server on port 9000 with a `/customers` endpoint. The
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endpoint responds to both `GET` and `POST` requests with a provided `id`, e.g.
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`/customers/100`. Upserting a customer id of 42 is hardcoded to throw an
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[`SQLException`][customer-42] to demonstrate how the `Result` type can
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[map internal program errors][update-customer-error] to more appropriate
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user-facing errors.
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## Contributing
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Bug reports and pull requests are welcome on [GitHub][github].
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## License
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This project is available under the terms of the ISC license. See the
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[`LICENSE`](LICENSE) file for the copyright information and licensing terms.
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[result]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L10
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[result-value]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L55
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[result-error]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L59
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[result-Ok]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L9
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[result-Err]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Result.kt#L17
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[kotlin-inline-classes]: https://kotlinlang.org/docs/inline-classes.html
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[wiki-Overhead]: https://github.com/michaelbull/kotlin-result/wiki/Overhead
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[rop]: https://fsharpforfunandprofit.com/rop/
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[kotlin-native-target-support]: https://kotlinlang.org/docs/native-target-support.html
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[github]: https://github.com/michaelbull/kotlin-result
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[wiki]: https://github.com/michaelbull/kotlin-result/wiki
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[result-runCatching]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Factory.kt#L11
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[result-binding]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result/src/commonMain/kotlin/com/github/michaelbull/result/Binding.kt#L28
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[bow-bindings]: https://bow-swift.io/docs/patterns/monad-comprehensions/#bindings
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[result-coroutineBinding]: https://github.com/michaelbull/kotlin-result/blob/master/kotlin-result-coroutines/src/commonMain/kotlin/com/github/michaelbull/result/coroutines/CoroutineBinding.kt#L42
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[kotlin-coroutineScope]: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/coroutine-scope.html
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[unit-tests]: https://github.com/michaelbull/kotlin-result/tree/master/kotlin-result/src/commonTest/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
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[swalschin-example]: https://github.com/swlaschin/Railway-Oriented-Programming-Example
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[ktor]: http://ktor.io/
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[customer-42]: https://github.com/michaelbull/kotlin-result/blob/master/example/src/main/kotlin/com/github/michaelbull/result/example/repository/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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[badge-android]: http://img.shields.io/badge/-android-6EDB8D.svg?style=flat
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[badge-android-native]: http://img.shields.io/badge/support-[AndroidNative]-6EDB8D.svg?style=flat
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[badge-jvm]: http://img.shields.io/badge/-jvm-DB413D.svg?style=flat
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[badge-js]: http://img.shields.io/badge/-js-F8DB5D.svg?style=flat
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[badge-js-ir]: https://img.shields.io/badge/support-[IR]-AAC4E0.svg?style=flat
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[badge-nodejs]: https://img.shields.io/badge/-nodejs-68a063.svg?style=flat
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[badge-linux]: http://img.shields.io/badge/-linux-2D3F6C.svg?style=flat
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[badge-windows]: http://img.shields.io/badge/-windows-4D76CD.svg?style=flat
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[badge-wasm]: https://img.shields.io/badge/-wasm-624FE8.svg?style=flat
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[badge-apple-silicon]: http://img.shields.io/badge/support-[AppleSilicon]-43BBFF.svg?style=flat
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[badge-ios]: http://img.shields.io/badge/-ios-CDCDCD.svg?style=flat
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[badge-mac]: http://img.shields.io/badge/-macos-111111.svg?style=flat
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[badge-watchos]: http://img.shields.io/badge/-watchos-C0C0C0.svg?style=flat
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[badge-tvos]: http://img.shields.io/badge/-tvos-808080.svg?style=flat
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