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Computing (FOLDOC) dictionary
Persistent Functional Language
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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.