Turning one failure into another

A failure can come from something the compiler knows can go wrong, such as a read that finds nothing, a write that breaks a unique rule or a construction, or from another call. You turn it into your own failure with condition: Failure in the operation's block. The failure's kind must fit the condition:

A read that finds nothing: missing:

A query names the failure for a record that isn't there (Named queries):

query todo_by_id(todo_id: Todo.id) fails TodoNotFound -> TodoView {    from: Todo    where: Todo.id == todo_id    missing: TodoNotFound {        todo_id: todo_id    }}

A value built at run time: rule keys and invalid:

A construction is keyed by the rule that can break (Construction failures by rule):

var quantity: Quantity = attempt Quantity(count) {    max: QuantityTooLarge    invalid: QuantityOutOfRange}

invalid: is the catch-all only for a construction's rules. A read's, a write's or an integration call's conditions each need their own arm, and invalid: in their block is the error failure.invalid_outside_construction.

A write that breaks a unique rule: conflict:

A write names a failure for each unique field it could clash on (When a write is refused):

var account: Account = attempt Account.create {    set: {        ...    }    conflict: {        Account.handle: HandleTaken    }}

A failure from a call: match

A match on the call gives each of its failures an arm (Handling a failure in match):

var receipt: Receipt = match payments.charge(order) {    success(receipt) -> receipt    CardDeclined -> reject PaymentDeclined}

Decoding JSON

json.read fails with mismatch when the JSON doesn't fit the type (json). When the failure you map mismatch to declares the field problem: DecodeProblem, Jadpo fills it with the first problem found. So the arm gives every other field of the failure, and never problem. No other mapping fills a DecodeProblem field.

The original failure is kept

The failure that was mapped is kept as the internal cause of the new one, so the log shows both:

PaymentDeclined rejected at domains/checkout/actions/pay.jadpo:14  caused by CardDeclined from payments.charge (internal: decline_code)  propagated through route POST /checkout