FUML11 — ExpansionRegionActivation isReady condition is too strong

Specification: Semantics of a Foundational Subset for Executable UML Models (fUML) (formal/2011-02-01)

Subclause: 8.5.4.2.3 ExpansionRegionActivation

The specification for ExpansionRegionActivation::isReady() requires that each of the input expansion nodes for an expansion region have the same (non-zero) number of values offered to it before the expansion region can fire. In particular, this means that if an expansion region receives null tokens representing “empty collections”, then the expansion region will not fire. This may seem to be the same as having the expansion region fire and do nothing, except that, if the region does not actually fire, then it does not offer control tokens on any outgoing control flows and any node downstream of those flows will be inhibited from firing. This does not like a reasonable limiting case of expansion region semantics on “empty” collections, and, in fact, it seems to violate what would be expected from the semantics of expansion regions as described in the full UML spec.

The requirement for all input expansion nodes to have the same number of values also seems to strong, requiring careful sizing of all expansion inputs lest the region not be able to fire. Instead, the size of the smallest input collection could be used to govern the number of executions of the body of the expansion region, to insure there are parallel inputs available from each input expansion node for each body execution.

agreed