The Use of Semantic Constraints on Diagram Editors

Current software tools supporting diagramming notations are not satisfactory. Editors for diagramming notations such as State Transition Networks, Petri Nets or the Entity-Relationship data model, are always confronted with a problem: how much guidance should be given to the user throughout the editing task? Not enough guidance allows the diagram to evolve to non plausible configurations and may provoke the user to feel lost in the editing process. At the other extreme, if too much guidance is provided the user feels like being shepherded through the diagram drawing; this results in an obtrusive and unfriendly system. Current tools normally offer a trade-off solution based on the introduction of some semantic constraints in the diagram editor to forbid a number of operations. To assert the correctness of the diagram, the user must explicitly request it to be checked. I believe this solution is not satisfactory. All the semantic constraints should be embedded in the editor in order to allow automatic diagram validation. The challenge is: how to do it without limiting the user's freedom during the editing task? I propose an approach that provides a solution to this problem.