Tuesday, 7 July 2009

Week 26/27 of 2009

During the first week, I defined a new type of operation called an operator (unary operator, binary operator or dot operator). Operators are always owned by data types. Unary operators include: not, +, -, empty, not_empty and cardinality. Binary operators include: or, and, ==, !=, <, <=, >, >=, +, -, *, / and %. There are no plans to allow custom operator symbols at present. Dot operators are like object instance processes except that they are invoked on a data value operand rather than an object instance reference. However, all dot operator names must be unique within a data type. OOA09 does not allow any operation name (excluding operator symbols) to be overloaded using different sets of input parameters.

Operators were introduced so that the semantics of predefined operators on core types could be specified within a model. However, they now also allow new data types to be defined with custom semantics for those same unary and binary operators. For example, the Extended Boolean data type mentioned in [DataType97] can now be specified with correct implementations for not, and and or. However, to ensure operators are side-effect free, severe restrictions have been placed on what can be done within an operator body, i.e. operators can't access objects at all, can't generate events and can only invoke other operators.

During the second week, I spent some time nailing down process modelling. Other Shlaer-Mellor and Executable UML tool vendors have taken the view that process modelling adds no value. However, I think that process modelling provides the perfect mechanism for defining concurrent data flows within composed operations. To better facilitate the work, I split the old Process Model subsystem in the OOA Tool Model into a new Operation subsystem and a more focused Process Model subsystem. The Data Dictionary subsystem was also renamed as the Data Type subsystem at the same time. Process models in OOA09 allow concurrent data flows to be safely specified within a composed operation (action or synchronous service) since circular data flows are not allowed. However, most composed operations won't benefit from concurrent data flows and are more easily defined using an Action Language statement block.