Reflections on the Object Constraint Language

The Object Constraint Language (OCL), which forms part of the UML set of modelling notations, is a precise, textual language for expressing constraints that cannot be shown diagrammatically in UML. This paper reflects on a number of aspects of the syntax and semantics of the OCL, and makes proposals for clarification or extension. Specifically, the paper suggests that: the concept of flattening collections of collections is unnecessary, state models should be connectable to class models, defining object creation should be made more convenient, OCL should be based on a 2-valued logic, set subtraction should be covered more fully, and a ''let'' feature should be introduced.