We introduce a persistentfunctional languagecalled PFL which adapts functional programming to the area of deductive databases,much as logic-based deductive databaselanguagesadapt logic programming. PFL inherits the advantagesof functional programming languages, including higher-order functions, static type checking, lazy evaluation, and support for user-definedtypes and constants. However, PFL allows functions to be definedincrementally by the insertion and deletion of equations, and stores theseequationsin thedatabase.PFL also supportsa class of extensionally defined updateablefunctions called selectorswhich allow the storageof arbitrarily nestedvalues. Further functions canbe written over selectorswhich act asderivation rules and which are “invertible” in the sense that they simulate predicates. We begin the paper by motivating the development of PFL. We review the respective advantages of functional and logic programming, particularly with respect to databasemodelling and manip...