java-prolog-connectivity.github.io/mapquery.html at master · java-prolog-connectivity/java-prolog-connectivity.github.io · GitHub

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>The London Underground Case Study</title>

	<link rel="stylesheet" href="/styles/publication/sc.tutorial.css" />
	<link rel="stylesheet" href="/styles/publication/sc.tooglejs.css" />

	<script type="text/javascript" src="/js/jquery.min.js" charset="utf-8"></script>
	<script type="text/javascript" src="/js/ace-builds/src-noconflict/ace.js" charset="utf-8"></script>
    <script type="text/javascript" src="/js/publication/jquery.tableofcontents.min.js" charset="utf-8"></script>
	<script type="text/javascript" src="/js/publication/jquery.sc.publication.js" charset="utf-8"></script>

    <script type="text/javascript">//<![CDATA[
  (function ($) {
    $(document).ready(function(){

      $('#table_of_content_entries').tableOfContents('#content', {startLevel:2, depth: 3});
      $.prettifyCodeSnippets("eclipse");
      //$.prettifyCodeSnippets("solarized_light");
      $.configureFootnotes();

    })

  })(jQuery);


//]]></script>


<script type="text/javascript">

  var _gaq = _gaq || [];
  _gaq.push(['_setAccount', 'UA-47515052-1']);
  _gaq.push(['_trackPageview']);

  (function() {
    var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
    ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
    var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
  })();

</script>

</head>


<body>


<h1>The MapQuery Case Study</h1>


<div id="table_of_content">
  <strong>Contents</strong>
  <ol id="table_of_content_entries"></ol>
</div>


<div id="content">

<h2>Before Starting</h2>

<p>
This section illustrates a case study showing how to interact with Java from Prolog.
It requires a basic understanding of <a href="http://logtalk.org/manuals/index.html" title="Logtalk Manuaks">Logtalk</a>.

In order to download and install this example please refer to the <a href="/installation_guide.html#installingJpcExamples" title="JPC installation guide">JPC installation guide</a>.
</p>


<h2>Introduction</h2>

<p>
Our case study addresses a typical problem that can be implemented easily with a logic language: a querying system about subway lines and stations.
It has been adapted from the <a href="https://www.cs.bris.ac.uk/~flach/SimplyLogical.html" title="Simply Logical. Intelligent Reasoning by Example.">Simply Logical</a> book by <a href="https://www.cs.bris.ac.uk/~flach/index.html" title="Peter Flach">Peter Flach</a>.
This document describes both a concrete implementation in the Prolog side (using Logtalk) and a set of Java interfaces that model the domain artifacts in the Java side.
</p>

<p>
The main entities of our example are shown in figure 1:
These are: subway stations, subway lines connecting those stations and the subway itself (referred as <code>metro</code> in the figure).
Relevant relations between these entities are:
</p>

<div class="divList">
<ul>
	<li><code>Connected stations</code>: Stations are <code>connected</code> to other stations by subway lines.</li>
	<li><code>Nearby stations</code>: A station is <code>nearby</code> another one if there is at most one station in between.</li>
	<li><code>Reachable stations</code>: A station <code>A</code> is <code>reachable</code> from another station <code>B</code> if there exists a list of stations <code>L</code> that form a path going from <code>B</code> to <code>A</code>.</li>
</ul>
</div>


<div class="divImg">
  <img src="img/examples/london_underground.png" alt="The London Underground." style="padding-bottom:0.5em;" height="400" />
  <div id="imglabel:london_underground" >Fig 1. - The London Underground (adapted from the <a href="https://www.cs.bris.ac.uk/~flach/SimplyLogical.html" title="Simply Logical book">Simply Logical</a> book).</div>
</div>

<h2>The Logtalk Entities</h2>

<p>
The first stage of the problem consists in expressing our knowledge about the London Underground as a set of logic statements.
Instead of implementing it in plain Prolog as in the <a href="https://www.cs.bris.ac.uk/~flach/SimplyLogical.html" title="Simply Logical. Intelligent Reasoning by Example.">original example</a>, we introduce an interesting variation: we use Logtalk, a portable object-oriented layer on top of Prolog.
</p>

<p>
In the rest of this section we overview how the <a href="/jpc.examples/logtalkdocs/underground_example/" title="The Underground System Entities in Logtalk">artifacts in our example</a> are modelled as <a href="http://logtalk.org/manuals/userman/objects.html" title="Logtalk objects">Logtalk objects</a>.
Figure 2 shows an overview of such objects and their dependencies.
</p>


<div class="divImg">
  <img src="/jpc.examples/logtalkdocs/underground_example/metro_lib_entity_diagram.svg" alt="The Logtalk entity diagram." style="padding-bottom:0.5em;" height="800" />
  <div id="imglabel:london_underground" >Fig 2. - The Logtalk entities diagram for the London underground example.</div>
</div>


<h3>The <code>metro</code> Logtalk Object</h3>

<p>
The <a href="/jpc.examples/logtalkdocs/underground_example/metro_0.html" title="The metro/0 Logtalk object"><code>metro/0</code></a> object encapsulates the knowledge about how stations are connected (e.g., the <code>connected/3</code> predicate).
Note that the <code>connected/3</code> predicate is declared as <a href="http://logtalk.org/manuals/refman/directives/dynamic_1.html" title="The dynamic/1 directive">dynamic</a> (line 7) since new connections can be configured at runtime.
Plus the rules for the logic predicates <code>nearby/2</code>, <code>reachable/3</code>, <code>line/1</code> and <code>remove_all/0</code>.
The messages (queries) that the metro object can respond to are specified by the <code>public/1</code> directive (lines 3-5). <a href="http://logtalk.org/manuals/userman/messages.html" title="Logtalk messages">Messages in Logtalk</a> are sent using the <code>::/2</code> operator, as illustrated on line 23 for the <a href="http://logtalk.org/library/listp_0.html#member/2" title="The member/2 predicate at the Listp protocol"><code>member/2</code></a> method.
</p>


<p><pre data-editor="prolog" data-show-gutter="true">
:- object(metro).

	:- public([
		connected/3, nearby/2, reachable/3, line/1, remove_all/0
	]).

    :- dynamic(connected/3).

	nearby(Station1, Station2) :-
		connected(Station1, Station2, _).
	nearby(Station1, Station2) :-
		connected(Station1, IntermediateStation, Line),
		connected(IntermediateStation, Station2, Line).

	reachable(Station1, Station2, []) :-
		connected(Station1, Station2, _).
	reachable(Station1, Station2, [IntermediateStation| IntermediateStations]) :-
		connected(Station1, IntermediateStation, _),
		reachable(IntermediateStation, Station2, IntermediateStations).

	line(Name) :-
		setof(Line, Station1^Station2^connected(Station1,Station2,Line), AllLines),
		list::member(line(Name), AllLines).

    remove_all :-
		metro::retractall(connected(_,_,_)).

:- end_object.</pre></p>


<h3>The <code>line</code> Logtalk Object</h3>

<p>
A subway line is modelled by the Logtalk <a href="http://logtalk.org/manuals/userman/features.html#features_parametric" title="Logtalk parametric objects"> parametric object</a> <a href="/jpc.examples/logtalkdocs/underground_example/line_1.html" title="The line/1 Logtalk object"><code>line/1</code></a>.
The object parameter denotes the name of the line.
It provides predicates answering connected stations by means of the line object (e.g., <code>connects/2</code>).
In addition, it allows a programmer to declare new connections between stations by means of the line object (e.g., <code>add_connection/2</code>).
These predicates delegate to the <code>metro</code> Logtalk object.
</p>

<p><pre data-editor="prolog" data-show-gutter="true">
:- object(line(_Name)).

	:- public([
		name/1, connects/2, add_connection/2
	]).

	name(Name) :-
		parameter(1, Name).

	connects(Station1, Station2) :-
		self(Self),
		metro::connected(Station1, Station2, Self).

	add_connection(Station1, Station2) :-
		self(Self),
		metro::assertz(connected(Station1, Station2, Self)).

:- end_object.</pre></p>


<h3>The <code>station</code> Logtalk Object</h3>

<p>
A subway station is modelled by the Logtalk <a href="http://logtalk.org/manuals/userman/features.html#features_parametric" title="Logtalk parametric objects">parametric object</a> <a href="/jpc.examples/logtalkdocs/underground_example/station_1.html" title="The station/1 Logtalk object"><code>station/1</code></a>.
The object parameter denotes the name of the station.
It provides predicates answering stations connected to this line (e.g., <code>connected/1</code> and <code>connected/2</code>), stations that are nearby this station (e.g., <code>nearby/1</code>) and stations that are reachable from this station (e.g., <code>reachable/1</code> and <code>reachable/2</code>).
These predicates delegate to the <code>metro</code> Logtalk object.
</p>

<p><pre data-editor="prolog" data-show-gutter="true">
:- object(station(_Name)).

	:- public([
		name/1, connected/1, connected/2, nearby/1, reachable/1, reachable/2
	]).

	name(Name) :-
		parameter(1, Name).

	connected(Station) :-
		connected(Station, _).

	connected(Station, L) :-
		self(Self),
		metro::connected(Self, Station, L).

	nearby(Station) :-
		self(Self),
		metro::nearby(Self, Station).

	reachable(Station) :-
		reachable(Station, _).

	reachable(Station, IntermediateStations) :-
		self(Self),
		metro::reachable(Self, Station, IntermediateStations).

:- end_object.</pre></p>


<h3>The Logtalk Loader File</h3>

<p>
Typically Logtalk libraries are loaded by means of a loader file.
This file contains one or more <a href="http://logtalk.org/manuals/refman/predicates/logtalk_load_1.html" title="The logtalk_load/1 predicate"><code>logtalk_load/1</code></a> predicates loading the distinct required entities.
Assuming these entities are defined in files named <code>metro.lgt</code>, <code>station.lgt</code> and <code>line.lgt</code>, the following script will load our library:
</p>

<p><pre data-editor="prolog" data-show-gutter="true">
:- initialization((
	logtalk_load(library(types_loader)),
	logtalk_load([metro, station, line])
)).</pre></p>

<p>
In addition to load the main entities of the library, the loader also loads depending entities (e.g., the <code>types</code> library is loaded in line 2).
</p>

<h3 id="configuringUndergroundStructure">Configuring the Underground Structure</h3>

<p>
The previous Logtalk entities implements the logic of our application.
However, they do not define concrete stations, lines and their relations.
For this example, we make use of a separate file that contains directives loading some test data.
This file should be loaded at runtime by means of the <a href="http://logtalk.org/manuals/refman/predicates/logtalk_load_1.html" title="The logtalk_load/1 predicate"><code>logtalk_load/1</code></a> predicate previously discussed.
The content of this data-loading file is shown below:
</p>


<p><pre data-editor="prolog" data-show-gutter="true">
:- initialization((
	line(central)::add_connection(station(bond_street),station(oxford_circus)),
	line(central)::add_connection(station(oxford_circus),station(tottenham_court_road)),
	line(jubilee)::add_connection(station(bond_street),station(green_park)),
	line(jubilee)::add_connection(station(green_park),station(charing_cross)),
	line(piccadilly)::add_connection(station(green_park),station(piccadilly_circus)),
	line(piccadilly)::add_connection(station(piccadilly_circus),station(leicester_square)),
	line(victoria)::add_connection(station(green_park),station(oxford_circus)),
	line(bakerloo)::add_connection(station(oxford_circus),station(piccadilly_circus)),
	line(bakerloo)::add_connection(station(piccadilly_circus),station(charing_cross)),
	line(northern)::add_connection(station(tottenham_court_road),station(leicester_square)),
	line(northern)::add_connection(station(leicester_square),station(charing_cross))
)).</pre></p>


<h2>The Java Entities</h2>

<p>
In the <a href="/jpc.examples/javadocs/org/jpc/examples/metro/model/package-summary.html" title="Java side documentation">Java side</a>, we define the
<a href="/jpc.examples/javadocs/org/jpc/examples/metro/model/Metro.html" title="The Metro class"><code>Metro</code></a>,
<a href="/jpc.examples/javadocs/org/jpc/examples/metro/model/Line.html" title="The Line class"><code>Line</code></a> and
<a href="/jpc.examples/javadocs/org/jpc/examples/metro/model/Station.html" title="The Station class"><code>Station</code></a>
interfaces in correspondence with the <a href="/jpc.examples/logtalkdocs/underground_example/" title="The Underground System Entities in Logtalk">Logtalk entities</a> described before, as illustrated by figure 3 (not all methods are shown).
</p>


<div class="divImg">
  <img src="img/examples/interaction.svg" alt="Java-Prolog interaction." style="padding-bottom:0.5em;" height="300" />
  <div id="imglabel:london_underground" >Fig 3. - Java Prolog interaction.</div>
</div>


</div> <!-- end of content -->


</body>
</html>