: A definition of types in an information system is given from real-world abstractions through data constructs, schema and definitions to physical data values. Category theory suggests that four levels are sufficient to provide ultimate closure for computational types to construct information systems. Examples of information systems are examined in terms of the four-level architecture including IRDS, the Grid, the semantic web and MOF/MDA.
B. Nick Rossiter, David A. Nelson, Michael A. Heat