Fail fast

Every rule runs by default, so one request reports every error. Some rules, though, only make sense after another: a database lookup is wasted on an email that is not even an email, and its answer would only add noise. Put both in failFast { }: its rules run in order, and the ones after the first failure are not evaluated.

Newsletter.kt

data class Newsletter(
    val email: String?
) : Validatable {
    override fun validation() = validation {
        ::email required {
            failFast {
                email()
                constraint("is already subscribed", code = "subscribed") { !Subscribers.contains(it) }
            }
        }
    }
}

Newsletter(email = "ana@").validate().violations           // Subscribers.contains is never called
Newsletter(email = "ana@mail.com").validate().violations   // when it is already subscribed

Path	Message	Code
`email`	must be a valid email	`email`
`email`	is already subscribed	`subscribed`

The rules after the block, and the other properties, run as usual: failFast only shortens its own block.

What counts as one step

Each call in the block is one step. An each { } or a valid() counts once, with all its elements: when any element fails, the steps after it are skipped, and every violation of that step is reported.

::items required {
    failFast {
        notEmpty()
        each { valid() }                    // one step, however many items fail
        constraint("is out of stock") { stock.hasAll(it) }
    }
}

failFast also works inside a branch of or, where it makes the branch report only its first failure.

It skips rules, not code

The rules are declared first and checked later. Work written in the block but outside a rule runs every time, even when the rules before it fail:

failFast {
    email()
    val taken = Subscribers.contains(email)          // runs even for "ana@"
    constraint("is already subscribed") { !taken }
}

Put the work inside constraint { }, after the rule it depends on.

Get started

Writing rules

Beyond the object

Reading the result

Configuration

For AI agents

What counts as one step

It skips rules, not code

​What counts as one step

​It skips rules, not code

What counts as one step

It skips rules, not code