functional database
     
         A database which uses a functional
        language as its query language.
     
        Databases would seem to be an inappropriate application for
        functional languages since, a purely functional language
        would have to return a new copy of the entire database every
        time (part of) it was updated.  To be practically scalable,
        the update mechanism must clearly be destructive rather than
        functional; however it is quite feasible for the query
        language to be purely functional so long as the database is
        considered as an argument.
     
        One approach to the update problem would use a monad to
        encapsulate database access and ensure it was single
        threaded.  Alternative approaches have been suggested by
        Trinder, who suggests non-destructive updating with shared
        data structures, and Sutton who uses a variant of a Phil
        Wadler's linear type system.
     
        There are two main classes of functional database languages.
        The first is based upon Backus' FP language, of which
        FQL is probably the best known example.  Adaplan is a more
        recent language which falls into this category.
     
        More recently, people have been working on languages which are
        syntactically very similar to modern functional programming
        languages, but which also provide all of the features of a
        database language, e.g. bulk data structures which can be
        incrementally updated, type systems which can be incrementally
        updated, and all data persisting in a database.  Examples are
        PFL [Poulovassilis&Small, VLDB-91], and Machiavelli [Ohori
        et al, ACM SIGMOD Conference, 1998].
     
        Query optimisation is very important for database languages
        in general and the referential transparency of functional
        languages allows optimisations which would be harder to verify
        in presence of side-effects.
     
        [Trinder, P., "Referentially transparent database languages",
        1989 Glasgow Workshop on Functional programming]
     
        [Breazu-Tannen et al., DBPL-91].
     
        [Poulovassilis, VLDB-94].
     
        (1995-05-09)