UML22 — Section: Actions - Output of read actions for no values

Output of read actions for no values. In the various read actions (links, structural features, variables), what is the output when there are no values read? Is it a null token or no tokens at all?

Suppose the result output pin of a read action is connected by an object flow to an action with an input pin with a multiplicity lower bound of zero. Assuming the second action has no other inputs or incoming control flows, it still will not fire unless it receives some token on its input pin (note that this semantic interpretation of enabled actions is explicit in the Foundational UML specification). Thus, if the read action produces no token in the case that no values are read, then the second action will not fire, even though its input is optional. In order to ensure that the second action can actually fire with no input values, it would be necessary to also model an explicit control flow from the read action to the second action, which is inconvenient and can lead to ordering and sequencing issues between control and object tokens in the case when the read action does produce values.
On the other hand, if the read action produces a null token when no values are read, then this will be offered to the second action, which can accept it, since its input multiplicity allows the case of no values. The second action will thus fire with an empty input, as would be intuitively expected. Note that if the second action's input pin had a multiplicity lower bound greater than zero, the semantics would not be effected by the offering of a null token, since this still would not provide the minimum number of values required for the action to fire.
Therefore, in the framework of the token offer semantics of activities, it makes the most sense for a read action to produce a null token when there are no values to read. Note that this is also consistent with the semantics for call actions implied by the statement in Subclause 12.3.41 that if, at the time an activity finishes execution, "some output parameter nodes are empty…they are assigned null tokens", in which case one would expect the null tokens to be offered by the corresponding output pins of a call action for the activity. That is, call actions should also offer null tokens in the case that an output has no values.