update

update FooCounter[deviceID: myId] to {count: 1}
update FooCounter[deviceID: myId]=>{count: 0} to {count: 1}

update takes an existing fact and updates its value fields. It has two forms. In both forms, new values are fully specified in a value struct specified after to, and all value fields must be specified.

In the first form, only the key fields are specified, and any value fields are updated unconditionally for the fact that matches the keys. Bind values cannot be used.

In the second form, all fields must be specified. If a fact does not exist matching key and value fields, policy evaluation terminates with a runtime exception. This is conceptually similar to:

let unused = unwrap query FooCounter[deviceId: myId]
check unused.count == 0
finish {
    update FooCounter[deviceID: myId] to {count: 1}
}

Except that this will produce a check error if the count is not as expected.

In either case, attempting to update a non-existent fact is an error and will terminate policy execution with a runtime exception. Updating a fact that has not been defined by schema should fail with a compile error.