UML: An Evaluation of the Visual Syntax of the Language

Examination of the UML indicates weaknesses in its graphic syntax which undermine its structure as a visual language. Although the UML Notation claims to provide a "canonical notation", there are insufficient rules governing the graphic constructs used to produce the essential 'signifiers' of this visual language and to define their permissible combinations. The nature and composition of the graphical elements actually shown is a fundamental consideration, separate from the underlying constructs that they may signify. A much earlier formulation for notational systems, that provided by Nelson Goodman, clarifies the issues involved and makes it possible to set basic tests for a notational scheme, such as the UML, which require syntactic disjointness and differentiability. Application of these tests (plus others) to graphical primitives, simple characters and diagrams shows a variety of failures that lead to a fundamental questioning of the graphical syntax which forms part of the UML structure as a language.