Awesome
Portable validations for Kotlin
- ✅ Type-safe DSL
- 🔗 Multi-platform support (JVM, JS, Native, Wasm)
- 🐥 Zero dependencies
Installation
For multiplatform projects:
kotlin {
sourceSets {
commonMain {
dependencies {
implementation("io.konform:konform:0.10.0")
}
}
}
}
For jvm-only projects add:
dependencies {
implementation("io.konform:konform-jvm:0.10.0")
}
Use
Suppose you have a data class like this:
data class UserProfile(
val fullName: String,
val age: Int?
)
Using the Konform type-safe DSL you can quickly write up a validation
val validateUser = Validation<UserProfile> {
UserProfile::fullName {
minLength(2)
maxLength(100)
}
UserProfile::age ifPresent {
minimum(0)
maximum(150)
}
}
and apply it to your data
val invalidUser = UserProfile("A", -1)
val validationResult = validateUser(invalidUser)
since the validation fails the validationResult will be of type Invalid and you can get a list of validation errors by indexed access:
validationResult.errors.messagesAtPath(UserProfile::fullName)
// yields listOf("must have at least 2 characters")
validationResult.errors.messagesAtPath(UserProfile::age)
// yields listOf("must be at least '0'")
or you can get all validation errors with details as a list:
validationResult.errors
// yields listOf(
// ValidationError(path=ValidationPath(Prop(fullName)), message=must have at least 2 characters),
// ValidationError(path=ValidationPath(Prop(age)), message=must be at least '0')
// )
In case the validation went through successfully you get a result of type Valid with the validated value in the value field.
val validUser = UserProfile("Alice", 25)
val validationResult = validateUser(validUser)
// yields Valid(UserProfile("Alice", 25))
Detailed usage
Hints
You can add custom hints to validations
val validateUser = Validation<UserProfile> {
UserProfile::age ifPresent {
minimum(0) hint "Registering before birth is not supported"
}
}
You can use {value} to include the .toString()-ed data in the hint
val validateUser = Validation<UserProfile> {
UserProfile::fullName {
minLength(2) hint "'{value}' is too short a name, must be at least 2 characters long."
}
}
Custom context
You can add customs context to validation errors
val validateUser = Validation<UserProfile> {
UserProfile::age {
minimum(0) userContext Severity.ERROR
// You can also set multiple things at once
minimum(0).replace(
hint = "Registering before birth is not supported",
userContext = Severity.ERROR,
)
}
}
Custom validations
You can add custom validations on properties by using constrain
val validateUser = Validation<UserProfile> {
UserProfile::fullName {
constrain("Name cannot contain a tab") { !it.contains("\t") }
// Set a custom path for the error
constrain("Name must have a non-whitespace character", path = ValidationPath.of("trimmedName")) {
it.trim().isNotEmpty()
}
// Set custom context
constrain("Must have 5 characters", userContext = Severity.ERROR) {
it.size >= 5
}
}
}
You can transform data and then add a validation on the result
val validateUser = Validation<UserProfile> {
validate("trimmedName", { it.fullName.trim() }) {
minLength(5)
}
// This also required and ifPresent for nullable values
required("yourName", /* ...*/) {
// your validations, giving an error out if the result is null
}
ifPresent("yourName", /* ... */) {
// your validations, only running if the result is not null
}
// You can use a more extensive path, for example
// the path will be ".fullName.trimmed" here:
validate(ValidationPath.of(UserProfile::fullName, "trimmed"), { /* ... */ }) {
/* ... */
}
}
Split validations
You can define validations separately and run them from other validations
val ageCheck = Validation<Int?> {
required {
minimum(21)
}
}
val validateUser = Validation<UserProfile> {
UserProfile::age {
run(ageCheck)
}
// You can also transform the data and then run a validation against the result
validate("ageMinus10", { it.age?.let { age -> age - 10 } }) {
run(ageCheck)
}
}
Collections
It is also possible to validate nested data classes and properties that are collections (List, Map, etc...)
data class Person(val name: String, val email: String?, val age: Int)
data class Event(
val organizer: Person,
val attendees: List<Person>,
val ticketPrices: Map<String, Double?>
)
val validateEvent = Validation<Event> {
Event::organizer {
// even though the email is nullable you can force it to be set in the validation
Person::email required {
// Optionally set a hint, default hint is "is required"
hint = "Email address must be given"
pattern(".+@bigcorp.com") hint "Organizers must have a BigCorp email address"
}
}
// validation on the attendees list
Event::attendees {
maxItems(100)
}
// validation on individual attendees
Event::attendees onEach {
Person::name {
minLength(2)
}
Person::age {
minimum(18) hint "Attendees must be 18 years or older"
}
// Email is optional but if it is set it must be valid
Person::email ifPresent {
pattern(".+@.+\..+") hint "Please provide a valid email address (optional)"
}
}
// validation on the ticketPrices Map as a whole
Event::ticketPrices {
minItems(1) hint "Provide at least one ticket price"
}
// validations for the individual entries
Event::ticketPrices onEach {
// Tickets may be free in which case they are null
Entry<String, Double?>::value ifPresent {
minimum(0.01)
}
}
}
Errors in the ValidationResult can also be accessed using the index access method. In case of Iterables and Arrays you use the
numerical index and in case of Maps you use the key as string.
// get the error messages for the first attendees age if any
result.errors.messagesAtPath(Event::attendees, 0, Person::age)
// get the error messages for the free ticket if any
result.errors.messagesAtPath(Event::ticketPrices, "free")
Dynamic Validations
Sometimes you want to create validations that depend on the context of the actual value being validated, or define validations for fields that depend on other fields. Note that this will generally have worse performance than using static validations.
Validation<Address> {
Address::postalCode dynamic { address ->
when (address.countryCode) {
"US" -> pattern("[0-9]{5}")
else -> pattern("[A-Z]+")
}
}
}
if you need to use a value further in, you can capture an earlier value with dynamic.
data class Numbers(val minimum: Int, val numbers: List<Int>)
Validation<Numbers> {
dynamic { numbers ->
Numbers::numbers onEach {
minimum(numbers.minimum)
}
}
}
Subtypes
You can run validations only if the value is of a specific subtype, or require it to be specific subtype.
sealed interface Animal {
val name: String
}
data class Cat(override val name: String, val favoritePrey: String) : Animal
data class Dog(override val name: String) : Animal
val validateAnimal = Validation<Animal> {
Animal::name {
notBlank()
}
// Only run this validation if the current Animal is a Cat and not null
ifInstanceOf<Cat> {
Cat::favoritePrey {
notBlank()
}
}
}
val requireCat = Validation<Animal> {
// This will return an invalid result is the current Animal is not a Cat or null
requireInstanceOf<Cat> {
Cat::favoritePrey {
// ...
}
}
}
Recursive validation
If you have a recursive type that you can validate, this requires
- an extra getter to get a self-reference to the validation, and
- dynamic to create an extra instance of the validation as-needed to avoid an infinite loop
data class Node(val children: List<Node>)
val validation = Validation<Node> {
// Use dynamic and a function to get the current validation again
Node::children onEach {
runDynamic { validationRef() }
}
}
// Type must be explicitly specified on either this or the val
private val validationRef get(): Validation<Node> = validation
Other validation libraries for Kotlin
- Akkurate: https://akkurate.dev/docs/overview.html
- Kalidation: https://github.com/rcapraro/kalidation
- Valikator: https://github.com/valiktor/valiktor
Integration with testing libraries
- Kotest provides various matchers for use with Konform. They can be used in your tests to assert that a given object is validated successfully or fails validation with specific error messages. See documentation.
Maintainer
David Hoepelman (Current maintainer) Niklas Lochschmidt (Original author, co-maintainer)