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


repositories {
    maven { url = 'https://dl.bintray.com/michaelbull/maven' }

dependencies {
    compile 'com.michael-bull.kotlin-result:kotlin-result:1.1.0'


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:

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

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.

fun checkPrivileges(user: User, command: Command): Result<Command, CommandError> {
    return if (user.rank >= command.mininimumRank) {
    } else {

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


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.

    .andThen { cmd -> checkPrivileges(loggedInUser, cmd) }
    .andThen { execute(user = loggedInUser, command = cmd, timestamp = LocalDateTime.now()) }
        { output -> printToConsole("returned: $output") },
        { error  -> printToConsole("failed to execute, reason: ${error.reason}") }


Inspiration for this library has been drawn from other languages in which the Result monad is present, including:

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

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 covering every library method


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.


Fetch customer information

$ 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"

Add new customer

$ curl -i -X POST \
   -H "Content-Type:application/json" \
   -d \
  "firstName": "Your",
  "lastName": "Name",
  "email": "your@email.com"
}' \
HTTP/1.1 201 Created
Content-Type: text/plain; charset=UTF-8
Content-Length: 16

Customer created


Bug reports and pull requests are welcome on GitHub.


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