In the class

When the class is yours, its rules belong next to its fields. Implement Validatable and return the rules from validation(), written with the validation { } block:

Account.kt

data class Account(
    val username: String?,
    val password: String?,
    val bio: String? = null
) : Validatable {
    override fun validation() = validation {
        ::username required {
            lengthBetween(min = 3, max = 20)
            lettersOrDigits()
        }
        ::password required {
            minLength(8)
            containsDigit()
        }
        ::bio ifPresent { maxLength(160) }
    }
}

Inside the block, ::username is the property of the object being validated. Each rule starts from a property, says what null means for it (required, ifPresent or forbidden), and lists the constraints of its value in the block that follows. Several constraints in one block are all checked, in order.

Checking the rules

A Validatable has four ways to run its rules. They run the same rules and differ only in how they wait and how they answer:

	Returns the violations	Throws when a rule is broken
Inside a coroutine	`validate()`	`validateOrThrow()`
Anywhere else	`validateBlocking()`	`validateOrThrowBlocking()`

val account = Account(username = "a!", password = "secret")

account.validate().violations           // suspends while the rules run
account.validateBlocking().violations   // blocks the thread instead

Path	Message	Code
`username`	must have between 3 and 20 characters	`lengthBetween`
`username`	must contain only letters or digits	`lettersOrDigits`
`password`	must have at least 8 characters	`minLength`
`password`	must contain a digit	`containsDigit`

Every rule runs: one object reports every broken rule at once, in the order the rules are declared. When a rule only makes sense after another, failFast stops at the first failure.

The suspending functions are the main path: rules can call suspend functions. The blocking ones exist for code that cannot suspend, such as a plain servlet or a main function.

Conditions are plain Kotlin

The block is ordinary Kotlin code that runs when the object is validated, so the values of the object are there to read. A rule that applies only sometimes is an if or a when:

override fun validation() = validation {
    ::country required { isoCountryCode() }

    if (country == "BR") ::document required { cpf() }
    if (newsletter == true) ::email required { email() }
}

There are no validation groups and no class-level annotations: the condition is written where it applies.

The configuration of one call

Each of the four functions takes an optional Validity, a configuration for that call only: other messages, or another clock.

account.validate(portuguese)

Beside the class

For a class that is not yours, or rules that belong elsewhere.

Reading the result

ValidationResult, Violation and what each field holds.

Introduction

Declaring a validation

Inside a validation

Your own constraints

Running a validation

Configuration

For AI agents

Checking the rules

Conditions are plain Kotlin

The configuration of one call

Beside the class

Reading the result

​Checking the rules

​Conditions are plain Kotlin

​The configuration of one call

Beside the class

Reading the result

Checking the rules

Conditions are plain Kotlin

The configuration of one call