Results and violations

validate() returns a ValidationResult. It holds every violation, in the order the rules are declared, and says whether there is any:

val result = user.validate()

result.isValid       // true when every rule holds
result.isInvalid     // true when at least one is broken
result.violations    // List<Violation>, empty when the object is valid
result.orThrow()     // throws a ValidationException when it is invalid

The parts of a violation

Path	Message	Code
`items[1].quantity`	must be at least 1	`min`

String

Where the value is. A property (email), a nested one (shipping.zipCode), an element (items[1]) or an entry of a map (prices[USD]). It is empty for a rule about the whole object, such as exactlyOneOf without a path.

String

The message for people, with its placeholders filled in: must be at least 1, not must be at least {min}. It follows the configured messages, so it can be in your users’ language.

String

The stable name of the rule, for programs. A built-in constraint uses its own name (min, email, cpf), and a custom one the code you give it, or an empty code when you give none. Codes do not change with the messages.

Map<String, Any?>

The entries attached with metadata, and the fields of the presence rules. Empty when there are none.

Violation has value equality, so two violations with the same four parts are equal, which keeps tests short.

Turning violations into a response

A violation is plain data, so mapping it to what your API returns is ordinary Kotlin. Grouped by path, the messages are ready for a form:

val errors: Map<String, List<String>> = result.violations.groupBy(
    keySelector = { it.path },
    valueTransform = { it.message }
)
// {name=[must have at least 2 characters], email=[must be a valid email], age=[must be at least 18]}

And with the codes, a front end can pick its own text, translated on the client:

data class FieldError(val field: String, val code: String, val message: String)

val body = result.violations.map { FieldError(field = it.path, code = it.code, message = it.message) }

Use the code, not the message, to decide anything in code: messages change with translations and wording, codes do not.

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Writing rules

Beyond the object

Reading the result

Configuration

For AI agents

Results and violations

The parts of a violation

Turning violations into a response

​The parts of a violation

​Turning violations into a response

The parts of a violation

Turning violations into a response