UMLR — The behavior of an OpaqueExpression should be allowed to have input parameters

The constraint OpaqueExpression::only_return_result_parameters requires that, if an OpaqueExpression has a behavior, then this behavior may not have any other parameters than a return parameter. In 8.3.3.3 it states, "Note that the behavior of an OpaqueExpression does not have Parameters other than its return and thus cannot be passed data upon invocation. It must therefore access any input data through elements of its behavioral description."

This constraint is too restrictive. In particular, when an OpaqueExpression is used as a guard on an ActivityEdge or as the specification of a guard Constraint on a Transition, it is often desirable to pass data into the OpaqueExpression, such as variables within an Activity or data obtained from the Event occurrence triggering a Transition. In the body text of an OpaqueExpression, this is often specified by simply using a variable name or parameter name. However, if such a body is to be formalized using, say, an Activity as the behavior for the OpaqueExpression, there is no currently way to specify access to such data as part of the "behavioral description" of the Activity. (Only attribute data of the context object can be accessed within such an Activity. Even accessing variables in an enclosing Activity is not possible.)

If the behavior of an OpaqueExpression was allowed to have input parameters, then, for example, local names in a body expression could be mapped to parameters of the behavior, such that, however the values of those names are to be resolved at runtime, those values could be passed to the invoked behavior. Of course, the actual resolution of local names and the semantics of what values are passed to behavior parameters would still be specific to the body language and/or the evaluating tool. However, at least there would be an allowance for the possibility of passing such data into the behavior.

(This issue came up during work on the Precise Semantics of State Machines. If OpaqueExpression behaviors were allowed to have input parameters, then PSSM will define a standard way, using this mechanism, in which Event occurrence data can be passed to the behavior of an OpaqueExpression used as the specification of a Transition guard Constraint, for tools conforming to the PSSM specification.)