arrow-refined-types / arrow.refinement

Package arrow.refinement

A refined type is any regular type constrained by predicates expected to hold in all possible values of the type’s constructor.

The Arrow refined-types plugin verifies at compile and runtime refined types constructor calls, ensuring values in declared types truly represent the programmers intent.


The refined-types plugin is a Kotlin compiler plugin that can be enabled via Gradle in your build by adding it to the list of your Gradle plugins.


plugins {
    id "io.arrow-kt.refined-types" version "<release-version>"


buildscript {
    repositories {
        maven { url "" }
    dependencies {
        classpath "io.arrow-kt:arrow-refined-types-gradle-plugin:<snapshot-version>"
apply plugin: "io.arrow-kt.refined-types"

Issues modeling with primitives types.

Leveraging programming languages built-in types like String and Int is convenient and allows us to express a wide range of domain problems. Unfortunately, types like String or Int accept a broader range of values than desirable in our programs and functions. By choosing overly broad types for our models like Int or String, we move the responsibility of validating the value to each call site.

Consider the following type that attempts to model a machine port number using the type Int.

val x: Int = 5555

While there does not seem to be anything wrong at first sight with this definition, simply using Int does not prevent invalid out-of-range inputs of what’s considered a valid port number.

val x: Int = 70000

Because the valid range for port numbers is 0..65353, the value 70000 is incorrect as it would be other negative values that Int accepts. In this case, the best we can do is create a new type and require in its initialization that input arguments get validated before construction.

value class Port(val value: Int) {
  init {
    require(value in 0..65353) { "$value should be in range of 0..65353 " }

Adding these requirements to init static blocks guarantees Port instances are not constructed but invoking the constructor with an invalid input will result in an IllegalArgumentException thrown at runtime. Throwing exceptions imposes the burden in the call site to preemptively try/catch all constructor calls or crashing the program at runtime.

try { 
} catch (e: IllegalArgumentException) { 
// 70000 should be in range of 0..65353 

Runtime exceptions are what most programming languages offer as the solution to validating constructor arguments. Since the init static initialization blocks can’t produce values and only accept statements, we can only resort to throwing IllegalArgumentException if we encounter a case where the value is considered invalid.

Refining custom types.

The refined-types plugin monitors all calls to Refined type constructors, ensuring arguments provided are verifiable in the range of the declared predicates constraining the type.

Considering the example above, instead of using Int to describe a port number, we will create our own Port type and enable the refined-types capabilities by making the Port companion extend the Refined class.

import arrow.refinement.Refined
import arrow.refinement.ensure

value class Port /* private constructor */ (val value: Int) {
  companion object : Refined<Int, Port>(::Port, {
    ensure((it in 0..65535) to "$it should be in the closed range of 0..65535 to be a valid port number")

In the declaration above the Refined class takes as first argument a function (Int) -> Port with value ::Port. It’s important to make constructors private so they are not visible and instead handled by the Refined<Int, Port> implementation.

When attempting to instantiate a Port with invalid values as constants, the Refined Types compiler plugin fails before our program can compile and forces us to correct the input preventing a potential runtime exception.

// error: "$it should be in the closed range of 0..65535 to be a valid port number"

For cases where the input values are dynamic and not evaluable at compile-time, the plugin advises us to use a safe API based on nullable types.

fun f(n: Int) {
// error: Prefer a safe alternative such as Port.orNull(n) or for explicit use of exceptions `Port.require(n)`

By implementing the Refined interface we gain access to the following validation APIs.

Safe nullable construction.

// Port(value=5555)

Resolved constraints.

// [(false, 70000 should be in the closed range of 0..65535 to be a valid port number)]
// [(true, 5555 should be in the closed range of 0..65535 to be a valid port number)]
// false
// true

Fold validation.

Port.fold(70000, { "failed: $it" }, { "success: $it" })
// failed: [(false, 70000 should be in the closed range of 0..65535 to be a valid port number)]
Port.fold(5555, { "failed: $it" }, { "success: $it" })
// success: Port(value=5555)

Unsafe require.

// 5555
try {
} catch (e: IllegalArgumentException) { 
// 70000 should be in the closed range of 0..65535 to be a valid port number

Refined types library.

The refined-types plugin includes a runtime library that also works standalone independently of the compiler plugin and includes useful predicates for chars, chars, strings, collections, digests, network, and time that can be composed and used alongside user type definitions.


The refined-types plugin is inspired by libraries like scala’s refined , Haskell’s refined and Liquid Haskell.

In contrast to a type-level implementation as we find in these libraries in Scala and Haskell, the refined-types plugin takes a different approach and leverages the Kotlin compiler APIs and its ability to interpret IR code to offer similar compile-time guarantees.

Refined Types were originally introduced in ML by Freeman and Pfenning


Constraints Constrains are represented by a list of pairs of Boolean and String where the boolean value represents if the predicate succeeded and the string value represent the error message if it failed.typealias Constraints = List<Pair<Boolean, String>>
Refined Abstract class for all Refined predicates.abstract class Refined<A, out B>


Law Annotation to flag ad-hoc refinements over third party functionsannotation class Law
Post annotation class Post
Pre annotation class Pre
Subject Annotation to flag ad-hoc refinements over third party functionsannotation class Subject

Extensions for External Classes



constraints Validates value obtaining the resolved Constraints for the refined predicatefun <A> constraints(refined: Refined<A, *>, value: A): Constraints
ensure Builds Constraints from a variadic array of predicate checks and failure messages. The arrow-refined compiler plugin uses this function to analyze and validate compile time calls to refined types constructorsfun ensure(vararg constraints: Pair<Boolean, String>): Constraints
post fun <A> String, predicate: (A) -> Boolean): A
fun <A> (A) -> Boolean, msg: () -> String): A
pre fun pre(msg: String, predicate: () -> Boolean): Unit
fun pre(predicate: Boolean, msg: () -> String): Unit
renderMessages Renders in a comma separated String all messages associated to failures validating resultsfun renderMessages(results: Constraints): String

``` kotlin:ank import arrow.refinement.ensure import arrow.refinement.require

val constraints = ensure(true to “passed”, false to “failed”) try { require(constraints) } catch (e: IllegalArgumentException) { e.message }
fun require(constraints: Constraints): Unit | | toEither | fun <V, R> Refined<V, R>.toEither(value: V): Either<String, R> | | toValidated | fun <V, R> Refined<V, R>.toValidated(value: V): Validated<String, R> | | toValidatedNel | fun <V, R> Refined<V, R>.toValidatedNel(value: V): ValidatedNel<String, R> |

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