<HTML> <HEAD> <!-- Created by texi2html 1.56k + clip patches and <A href="http://www.clip.dia.fi.upm.es/Software">lpdoc</A> from ciao.texi on 28 January 2007 --> <LINK rel="stylesheet" href="ciao.css" type="text/css"> <TITLE>The Ciao Prolog System - Using the persdb library</TITLE> </HEAD> <BODY> Go to the <A HREF="ciao_1.html">first</A>, <A HREF="ciao_148.html">previous</A>, <A HREF="ciao_150.html">next</A>, <A HREF="ciao_241.html">last</A> section, <A HREF="ciao_toc.html">table of contents</A>. <P><HR><P> <H1><A NAME="SEC622" HREF="ciao_toc.html#TOC622">Using the persdb library</A></H1> <P> <STRONG>Author(s):</STRONG> The CLIP Group. <P> <STRONG>Version:</STRONG> 1.10#7 (2006/4/26, 19:22:13 CEST) <P> <STRONG>Version of last change:</STRONG> 1.9#232 (2003/12/22, 18:4:0 CET) <P> Through the following examples we will try to illustrate the two mains ways of declaring and using persistent predicates: statically (the preferred method) and dynamically (necessary when the new persistent predicates have to be defined at run-time). The final example is a small application implementing a simple persistent queue. <UL> <LI><A HREF="ciao_149.html#SEC623">An example of persistent predicates (static version)</A> <LI><A HREF="ciao_149.html#SEC624">An example of persistent predicates (dynamic version)</A> <LI><A HREF="ciao_149.html#SEC625">A simple application / a persistent queue</A> </UL> <H2><A NAME="SEC623" HREF="ciao_toc.html#TOC623">An example of persistent predicates (static version)</A></H2> <PRE> :- use_package(iso). :- use_package(persdb). %% Declare the directory associated to the key "db" where the %% persistence sets of the persistent predicates are stored: persistent_dir(db,'./'). %% Declare a persistent predicate: :- persistent(bar/1, db). %% Read a term, storing it in a new fact of the persistent predicate %% and list all the current facts of that predicate main:- read(X), assertz_fact(bar(X)), findall(Y,bar(Y),L), write(L). erase_one :- retract_fact(bar(_)). erase_all :- retractall_fact(bar(_)). u(X) :- update_files(X). </PRE> <H2><A NAME="SEC624" HREF="ciao_toc.html#TOC624">An example of persistent predicates (dynamic version)</A></H2> <PRE> :- use_package(iso). :- use_package(persdb). main([X]):- %% Declare the directory associated to the key "db" asserta_fact(persistent_dir(db,'./')), %% Declare the predicate bar/1 as dynamic (and data) at run-time data(bar/1), %% Declare the predicate bar/1 as persistent at run-time make_persistent(bar/1, db), assertz_fact(bar(X)), findall(Y, bar(Y), L), write(L). </PRE> <H2><A NAME="SEC625" HREF="ciao_toc.html#TOC625">A simple application / a persistent queue</A></H2> <PRE> :- module(queue, [main/0],[persdb]). :- use_package(iso). :- use_module(library(read)). :- use_module(library(write)). :- use_module(library(aggregates)). persistent_dir(queue_dir,'./pers'). :- persistent(queue/1, queue_dir). queue(first). queue(second). main:- write('Action ( in(Term). | slip(Term) | out. | list. | halt. ): '), read(A), ( handle_action(A) -> true ; write('Unknown command.'), nl ), main. handle_action(end_of_file) :- halt. handle_action(halt) :- halt. handle_action(in(Term)) :- assertz_fact(queue(Term)), main. handle_action(slip(Term)) :- asserta_fact(queue(Term)), main. handle_action(out) :- ( retract_fact(queue(Term)) -> write('Out '), write(Term) ; write('FIFO empty.') ), nl, main. handle_action(list) :- findall(Term,queue(Term),Terms), write('Contents: '), write(Terms), nl, main. </PRE> <P><HR><P> Go to the <A HREF="ciao_1.html">first</A>, <A HREF="ciao_148.html">previous</A>, <A HREF="ciao_150.html">next</A>, <A HREF="ciao_241.html">last</A> section, <A HREF="ciao_toc.html">table of contents</A>. </BODY> </HTML>