Persistent Functional Language

functional language, database (PFL) A functional database

language developed by Carol Small at Birkbeck College, London,

UK and Alexandra Poulovassilis (now at King's CollegeLondon).

In PFL, functions are defined equationally and bulk data is

stored using a special class of functions called selectors.

PFL is a lazy language, supports higher-order functions,

has a strong polymorphic type inference system, and allows

new user-defined data types and values. 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.

["A Functional Approach to Database Updates",

C. Small, Information Systems 18(8), 1993, pp. 581-95].

(1995-04-27)