The Free On-line Dictionary of Computing (30 December 2018):

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
   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

(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)