Data Renement and Algebraic Structure

We recall Hoare's formulation of data re nement in terms of upward, downward and total simulations between locally ordered functors from the structured locally ordered category generated by a programming language with an abstract data type to a semantic locally ordered category: we use a simple imperative language with a data type for stacks as leading example. We give a uni ed category theoretic account of the sort of structures on a category that allow upward simulation to extend from ground types and ground programs to all types and programs of the language. This answers a question of Hoare about the category theory underlying his constructions. It involves a careful study of algebraic structure on the category of small locally ordered categories, and a new de nition of sketch of such structure. This is accompanied by a range of detailed examples. We extend that analysis to total simulations for modelling constructors of mixed variance such as higher order types.