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 College London).
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
["A Functional Approach to Database Updates", C. Small, Information
Systems 18(8), 1993, pp. 581-95].
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