Source : Free On-Line Dictionary of Computing

Persistent Functional Language
     
         (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
        {destructive}ly, 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
        (http://web.dcs.bbk.ac.uk/CS/Research/DBPL/papers/INFSYS93.abs.html)}",
        C. Small, Information Systems 18(8), 1993, pp. 581-95].
     
        (1995-04-27)