Context

Some rules depend on who is asking: the credit limit of the logged-in customer, the catalog of a tenant, a repository to look things up in. That data is not part of the object, so the object cannot reach it on its own. Implement ContextualValidatable<C>, and validation(context) receives it:

Shopper.kt

data class Shopper(
    val creditLimit: BigDecimal,
    val catalog: Set<String>
)

PurchaseLine.kt

data class PurchaseLine(
    val sku: String?
) : ContextualValidatable<Shopper> {
    override fun validation(
        context: Shopper
    ) = validation {
        ::sku required { constraint("is not sold in your region", code = "catalog") { it in context.catalog } }
    }
}

Purchase.kt

data class Purchase(
    val total: BigDecimal?,
    val lines: List<PurchaseLine>?
) : ContextualValidatable<Shopper> {
    override fun validation(
        context: Shopper
    ) = validation {
        ::total required { max(context.creditLimit) }
        ::lines required { each { valid(context) } }
    }
}

The caller passes the context to validate:

val shopper = Shopper(creditLimit = "100".toBigDecimal(), catalog = setOf("BOOK-1"))

Purchase(
    total = "150".toBigDecimal(),
    lines = listOf(PurchaseLine(sku = "BOOK-1"), PurchaseLine(sku = "TV-9"))
).validate(shopper).violations

Path	Message	Code
`total`	must be at most 100	`max`
`lines[1].sku`	is not sold in your region	`catalog`

The type of the context is checked by the compiler: a Purchase cannot be validated without a Shopper, and there is no map of attributes to read and cast.

Passing the context down

Nested objects receive the context explicitly, with valid(context). That keeps the flow visible, and lets a child take only a part of it:

::lines required { each { valid(context) } }          // the whole shopper
::store required { valid(context.tenant) }            // only the part the store needs

A child that needs no context is a plain Validatable, checked with valid(), even inside a contextual parent.

The four ways to run

ContextualValidatable has the same four functions as Validatable, with the context first:

purchase.validate(shopper)
purchase.validateBlocking(shopper)
purchase.validateOrThrow(shopper)
purchase.validateOrThrowBlocking(shopper)

Each also takes a Validity after the context, for other messages or another clock.

The context can be anything: a data class, a repository, a client. When it is a repository with suspend functions, the rules can call them directly. See Coroutines.

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Writing rules

Beyond the object

Reading the result

Configuration

For AI agents

Passing the context down

The four ways to run

​Passing the context down

​The four ways to run

Passing the context down

The four ways to run