A functional approach to database updates

摘要

PFL is a functional database language in which functions are defined equationally and bulk data is stored using a special class of functions called selectors. It is a lazy language, supports higher-order functions, has a strong polymorphic type inference system, and allows new user-defined data types and values to be declared. All functions, types and values persist in a database. Functions can be written which update all aspects of the database: by adding data to selectors, by defining new equations, and by introducing new data types and values. PFL is “semi-referentially transparent”, in the sense that whilst updates are referentially opaque and are executed destructively, all evaluation is referentially transparent. Similarly, type checking is “semi-static” in the sense that whilst updates are dynamically type checked at run time, expressions are type checked before they are evaluated and no type errors can occur during their evaluation.In this paper we examine the expressiveness of PFL with respect to updates, and illustrate the language by developing a number of general purpose update functions, including functions for restructuring selectors, for memoisation, and for generating unique system identifiers. We also provide a translation mechanism between Datalog programs and equations, and show how different Datalog evaluation strategies can be supported.