Persistent programming is concerned with the construction of large and long lived systems of data. In designing and building persistent object systems, we are attempting to regularise the activities that are performed on data by programming languages, operating systems, database management systems and file systems. We have identified the following areas of research which we are investigating in the context of persistent systems. They are: controlling complexity, protection of data, orthogonal persistence, controlled system evolution and concurrent computation. In this paper, we describe the data modelling facilities of the Napier type system. We also demonstrate the flexible and incremental nature of the type checking mechanism that is required for persistent programming. The type system is central to the nature of the Napier language and we will demonstrate how it has been designed to solve problems in the five areas identified above
