kotlin-result

Result<V, E> is a monad for modelling success (Ok) or failure (Err) operations.

Installation

repositories {
    mavenCentral()
}

dependencies {
    implementation("com.michael-bull.kotlin-result:kotlin-result:1.1.10")
}

Introduction

The Result monad has two subtypes, Ok<V> representing success and containing a value, and Err<E>, representing failure and containing an error.

Scott Wlaschin's article on Railway Oriented Programming is a great introduction to the benefits of modelling operations using the Result type.

Mappings are available on the wiki to assist those with experience using the Result type in other languages:

• Elm
• Haskell
• Rust
• 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.

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. This will execute the block of code and catch any Throwable, returning a Result<T, Throwable>.

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.

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).

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.

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.

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, however below is a list of other resources on the topic of monad comprehensions.

• Monad comprehensions - Arrow (Kotlin)
• Monad comprehensions - Bow (Swift)
• For comprehensions - Scala

Coroutine Support

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

dependencies {
    implementation("com.michael-bull.kotlin-result:kotlin-result:1.1.10")
    implementation("com.michael-bull.kotlin-result:kotlin-result-coroutines:1.1.10")
}

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:

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
• Haskell
• Rust
• Scala

It also iterates on other Result libraries written in Kotlin, namely:

• danneu/kotlin-result
• kittinunf/Result
• 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 almost 100 unit tests covering every library method

Example

The example module contains an implementation of Scott's example application that demonstrates the usage of Result in a real world scenario.

It hosts a 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 to demonstrate how the Result type can map internal program errors 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.

License

This project is available under the terms of the ISC license. See the LICENSE file for the copyright information and licensing terms.