Simulating file operations: an exercise in calculational data refinement

This note was written as an exercise in calculating with abstract data type specifications, both from the user’s point of view and from the implementer’s. We have chosen an example that is well-known to anyone who has ever struggled to convert programs involving ISO Pascal file operations to some nonstandard Pascal version – an experience by now familiar to a sizable proportion of the earth’s population. In order to derive a solution to this problem, we regard ISO Pascal files as an abstract data type, disregarding the fact that Pascal compilers will not support this. We specify this abstract data type by giving a model, i.e. we define a number of variables of mathematically well-understood types in terms of which the operations on ISO Pascal files can be described. Let a component type T be given. Our model consists of four variables, namely