A graphical form of EXPRESS

EXPRESS-G is a graphical notation for the display of information models. Using the EXPRESS language, an information model is represented by sentences in the language. In EXPRESS-G, an information model is represented by graphic symbols forming a diagram. Although EXPRESS-G has been specifically developed for the graphical rendition of information models defined in the EXPRESS language, it may be used as a modeling technology in its own right. EXPRESS-G supports the notions of entity, type, relationship and cardinality. It also separately supports the notion of schema. The notation only supports a subset of the EXPRESS language as it does not provide any support for the complex constraints which can be represented in the EXPRESS lexical language. The design goals for the notation are: • The diagrams should be intuitively understandable. • The diagrams should support levels of model abstraction. • A diagram must be able to span more than one sheet of paper. • The pictures should be definable using minimal computer graphics capabilities. Further, it should be possible to print the diagrams using only non-graphic symbols, for example on a line printer. • It should be possible to develop a processor that automatically converts from EXPRESS source to the graphical description. EXPRESS-G requires almost minimal graphical capabilities, namely the ability to draw straight lines of three kinds, to draw rectangular and rounded boxes, to draw small circles, and to put text onto a drawing. Two kinds of boxes are used as symbols: Definition These symbols denote the things (i.e., concepts, ideas, etc.) which form the basis of the information model. Rectangular boxes are used for these symbols. Composition These symbols enable a model diagram to be displayed on more than one sheet of paper. Boxes with rounded corners are used for these. Three styles of lines are used by EXPRESS-G — a thin solid line, a thick solid line, and a dashed line — each of which should be readily distinguishable. For computer displays that support graphics there should be no problems in choosing suitable line styles. For displays that only support a single line width, thick lines can be drawn as two closely spaced parallel lines. For line printer type displays, the lines have to be drawn using characters rather than graphics.