check

check envelope::author_id(envelope) == device

check evaluates a boolean expression and terminates policy execution with a check failure if it evaluates to false.

check statements are meant to check policy invariants. For example, if you need to make sure that the author of a command has the correct permissions, check is the appropriate tool to enforce that. A failed check statement causes the runtime to execute the recall block, which allows a policy to take corrective measures after a command is no longer valid. This could for example, cascade to deleting a Fact or emitting an Effect that the application can use to take further action.

command ActivateFoo {
    ...

    policy {
        let author = envelope::author_id(envelope)
        let perms = unwrap query Permissions[user: author]=>{level: ?}
        check perms.level == Permission::WRITE
        finish {
            ...
        }
    }

    recall {
        let author = envelope::author_id(envelope)
        finish {
            // oopsie doopsie
            emit ActivationFailed {
                author: author
            }
        }
    }
}

You could alternatively think of check like a kind of exception return and recall is a global catch block.

See the Errors section for more information on check failures.