In the first article in this series, I explained some of the semantics of RDF via Prolog (and vice versa). In this article, I'll explore some of the nitty-gritty of working with RDF in Prolog using the SWI-Prolog system.

Using RDF with SWI-Prolog

SWI-Prolog is a fast, robust, and free open-source Prolog system with great support for XML and RDF. It's being used as an inference engine for Mozilla and in several other RDF projects. With the release of SWI-Prolog 4.0.0 the graphics, GUI, and object library XPCE have been bundled in. XPCE is a rich and powerful cross-platform (Windows and Unixen) toolkit with a number of immediately useful demos, including various drawing systems, a partial HTML renderer, a help system, and an Emacs clone.

All in all, a wonderful system both for play and for serious work, and for play that becomes serious work!

A good example application of SWI-Prolog's RDF support is the online RDF parser demo. Here you can upload or paste RDF documents in RDF/XML syntax and get an HTML table presentation of the statements. Even with its few quirks, I find it handy for debugging RDF documents (indeed, somewhat handier than the oft-recommended RDFviz, especially for anything other than small documents).

First Steps with SWI-Prolog's RDF Tools

SWI-Prolog comes bundled with several modules for parsing RDF/XML and manipulating the result at various levels of detail and convenience. Assuming you have a SWI-Prolog 4.0.x installation running, you can load the bundled modules by typing at the "command", (ie. query), prompt:

?-[library(rdf)].

and hitting enter. On my machine, it prints back:

  % library(sgml) compiled into sgml 0.27 sec, 12,536 bytes
  % uri compiled into uri 0.00 sec, 1,804 bytes
  % rewrite compiled into rewrite 0.00 sec, 5,716 bytes
  % rdf_parser compiled into rdf_parser 0.17 sec, 24,168 bytes
  % library(gensym) compiled into gensym 0.00 sec, 2,096 bytes
  % rdf_triple compiled into rdf_triple 0.06 sec, 10,924 bytes
  % library(rdf) compiled into rdf 0.56 sec, 56,080 bytes

In other words, all the modules on which the RDF library depends get loaded. The library(sgml) module handles SGML, HTML, and XML parsing, and it's quite nifty in its own right.

In Prolog lingo, to load a file that contains a knowledge base/list-of-definitions is to consult it. The square brackets are just a bit of syntactic sugar for this process. The consulting involves reading each clause, perhaps doing a bit of processing on the read terms, and then asserting the clause into the Prolog knowledge base. Consulting a file puts all of its predicates right into the global knowledge base, which can be a problem if there are conflicts. Fortunately, SWI-Prolog has a module system and, as one can see above, facilities for indicating and managing dependencies. Thus, it might have been better to have used use_module(library(rdf)). instead of (the syntactic sugar for) consult(library(rdf). above, but in the current context it doesn't really matter.

Prolog predicates are distinguished by name and arity (i.e., number of permissible arguments). So you'll see things like cool_predicate/1, cool_predicate/2, etc. Each of these is a completely different predicate. This may feel odd to folks used to the possibility of having optional and defaulted arguments (e.g., as in Python), but it works out very well in practice.

At this point, I can parse an RDF/XML document into a list of Prolog terms using the (now consulted) predicate load_rdf/2. I'm going to use as my example an RSS 1.0 file from XMLhack.com.

?- load_rdf('bijan/xmlhack.rss', List_of_RDF_statements).

List_of_RDF_statements = [rdf('http://xmlhack.com/rss10.php', rdf:type, 'http://purl.org/rss/1.0/channel'), rdf('http://xmlhack.com/rss10.php', 'http://purl.org/rss/1.0/':title, literal('XMLhack')), rdf('http://xmlhack.com/rss10.php', 'http://purl.org/rss/1.0/':link, literal('http://xmlhack.com/')), rdf('Description__1', 'http://purl.org/dc/elements/1.1/':resource, literal('http://xmlhack.com/')), rdf('http://xmlhack.com/rss10.php', 'http://purl.org/dc/elements/1.1/':source, 'Description__1'), rdf('http://xmlhack.com/rss10.php', 'http://purl.org/rss/1.0/':description, literal('Developer news from the XML community')), rdf('http://xmlhack.com/rss10.php', 'http://purl.org/dc/elements/1.1/':language, literal('en-us')), rdf('http://xmlhack.com/rss10.php', ... :..., literal(...)), rdf(..., ..., ...)|...]

Yes

I've not yet "consulted" the RDF file, I've merely parsed it into a list of terms. What's the difference? For one, none of these statements can be queried, e.g.,:

?- rdf(X,Y,Z).

ERROR: Undefined procedure: rdf/3

Thus far, I haven't ever (in this session) asserted any clause of an rdf/3 predicate, so the system has no special understanding of terms like rdf('http://xmlhack.com/rss10.php', rdf:type, 'http://purl.org/rss/1.0/channel). But, in the symbolic processing language tradition, that term is a well-formed data structure (indeed, it's a "term"), and it can be manipulated in various ways -- including being asserted as a clause of a predicate. In other words, Prolog has a uniform representation of code and data and facilities for manipulating a term from either perspective. Since rdf_load/3 returned a list of terms that are perfectly legal Prolog clauses, it's no problem to assert them into the current knowledge base, simply using the assert/1 predicate.

?- load_rdf('bijan/xmlhack.rss', [H|_]), assert(H).	Both the file at that relative path must be parseable into a list of terms with at least one item. The first item is bound to Hwhile the tail of the list is bound to the "anonymous", "throw away", or "don't care" variable_ and (remember a comma between predicates in a query is read as "and") that first term must be entered in our knowledge base.
H = rdf('http://xmlhack.com/rss10.php', rdf:type, 'http://purl.org/rss/1.0/channel') Yes	The query succeeded with the first parsed item from the document is bound to H.
?- rdf(X,Y,Z).	Are there any X, Y, and Z that exist in an rdf relation?
X = 'http://xmlhack.com/rss10.php' Y = rdf:type Z = 'http://purl.org/rss/1.0/channel' ; No	Yes, there's one with the listed bindings, but if I ask for more (e.g., by entering the semicolon) Prolog doesn't find any.

Two points. First, using rdf_load/3, I can transform a certain serialization of a set of RDF statements into a form that Prolog can handle well (i.e. a list of rdf/3 based terms). Second, given the appropriate output predicates, I can use that form to compose RDF as well. After all, I can assert any rdf/3 statement that I'd like at the query prompt, e.g.,

?- assert(rdf(a, b, c)).

Yes

And I can test to see if it's in fact in the knowledge base:

?- rdf(a, b c).

Yes

Since that's not a particularly useful RDF statement, I'll just retract it:

?- retract(rdf(a, b, c)).

Yes

?- rdf(a, b c).

no

I really don't want to assert these statements tediously one at a time at the query prompt. But I can get a list of the rdf/3 terms, which suggests that I could use some sort of "mapping" predicate to assert all the items in the statement list. checklist/2 takes a predicate and applies it to each member of a supplied list:

?- load_rdf('bijan/xmlhack.rss', List_of_RDF_statements),
| checklist(assert, List_of_RDF_statements).

(Note, the | is the "continuation" prompt in SWI-Prolog and not a bit of syntax.)

After this query, I have quite a few RDF statements in my Prolog knowledge base:

?- rdf(X,Y,Z).

X = 'http://xmlhack.com/rss10.php'
Y = rdf:type
Z = 'http://purl.org/rss/1.0/channel' ;

X = 'http://xmlhack.com/rss10.php'
Y = rdf:type
Z = 'http://purl.org/rss/1.0/channel' ;

X = 'http://xmlhack.com/rss10.php'
Y = rdf:type
Z = 'http://purl.org/rss/1.0/channel' ;

X = 'http://xmlhack.com/rss10.php'
Y = 'http://purl.org/rss/1.0/':title
Z = literal('XMLhack') ;

...etc.

This is equivalent to consulting a file containing all the rdf/3 items, written just as they appear in the list. Thus, one could use the rdf/3 notation to compose RDF documents (although this notation isn't all that much more convenient than the dreaded RDF/XML).

The rdf_db Module

In the previous section, I gave some examples of reading into the Prolog knowledge base a set of RDF statements. Jan Wielemaker (the author of SWI-Prolog) put together a little module of convenience predicates for doing this sort of thing, called rdf_db. One very nice feature of rdf_db is that it allows you to maintain multiple, disjoint RDF "databases". You can have the advantages of dealing with RDF documents as separate lists of statements while having a Prologesque interface. rdf_db is also a good example of a simple Prolog-based RDF application.

To get going with rdf_db, first download and pop into a convenient spot (for me, that's the "library" directory) rdf_db.pl and xml_write.pl. Now a simple [library(rdf_db)]. at the query prompt will load all the relevant stuff. The slightly confusingly named rdf_load/1 will open the file at the supplied path, parse the RDF/XML into rdf/3 based terms, and then asserts them into a special RDF database named, by default, "user".

?- rdf_load('bijan/xmlhack.rss').

Yes

?- rdf(X,Y,Z).

X = 'http://xmlhack.com/rss10.php'
Y = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#type'
Z = 'http://purl.org/rss/1.0/channel' ;

X = 'http://xmlhack.com/rss10.php'
Y = 'http://purl.org/rss/1.0/title'
Z = literal('XMLhack')

Yes

(Prolog said "Yes" because I didn't ask it for any more answers. It says "No" when it can't find an answer, and in this case it had already found one.)

Most of the predicates in rdf_db have two variants, the basic one and one with an extra argument. The extra argument specifies the particular RDF database which is to be the context of the predicate. Without that argument, the predicate is evaluated in the context of the current RDF database, which may be specified by the rdf_db/2 predicate. For example,

?-rdf_load('bijan/rdfiglog-2001-03-22.rdf', rdfiglog).

Yes

?-rdf_db(CurrentDB,rdfiglog).

CurrentDB = user

Yes

?-rdf_db(CurrentDB).

CurrentDB = rdfiglog

Yes

?-rdf(X, Y, Z).

X = 'irc://irc.openprojects.net/rdfig'
Y = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#type'
Z = 'http://xmlns.com/foaf/0.1/ChatChannel'

Yes

?-rdf(X,Y,Z, user).

X = 'http://xmlhack.com/rss10.php'
Y = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#type'
Z = 'http://purl.org/rss/1.0/channel'

Yes

In the previous section, when I asserted the terms returned by load_rdf/2, those terms were of the form rdf(Subject,Predicate,Object), and that was how they were entered into the Prolog database. With rdf_db, the rdf/3 predicate is, in fact, a rule! Each ground level rdf statement is asserted as rdf/4 so that each statement's "provenance" can be determined (by the fourth argument). This is, of course, merely an implementation detail. One could replace rdf_db's use of the builtin Prolog knowledge base with something else.

The rdf_db library also has convenience predicates for asserting and retracting RDF statments, either from the current RDF database or from a named one. Perhaps more importantly, rdf_register_ns/2assigns prefixes to URIs in the standard XML namespace way but also permits the use of prefixed names ("QNames") in Prolog statements:

?-rdf_register_ns(dc, 'http://purl.org/dc/elements/1.1/').

Yes

?-rdf(X,dc:date,Y).

X = 'Description__5'
Y = literal('2001-03-22T00:00:01Z') ;
X = 'Description__7'
Y = literal('2001-03-22T00:00:18Z')

Yes

?-rdf_assert('http://www.xmlhack.com/a/spurious/article', dc:creator, literal('A spurious author!')).

Yes

?- rdf('http://www.xmlhack.com/a/spurious/article', dc:creator, Who).

Who = literal('A spurious author!')

Yes

?-rdf_save('bijan/current-rdf-db-dumped-to-XML-format.rdf').

Yes

The rdf_save/1 and /2 predicates will use the namespace declarations established by rdf_register_ns/2.

A Transformation Example

The XML serialization of RDF (known as RDF/XML) is both a boon and a bane. Part of the bane is common to many XML-based languages: it's just plain ol' nasty to read and write. Part of the boon is, of course, that there is an ever-growing pool of tools and techniques for dealing with XML-based languages. One of the foundational techniques for dealing with XML is the structural transformation of XML documents. The standard (if not the best) tool is XSLT. Thanks to RDF/XML, one can manage RDF documents/knowledge bases using XSLT stylesheets.

Prolog, as a symbolic processing language, is perfectly capable of doing straightforward tree transformations, and this a perfectly reasonable way to use it. However, it's not quite in the spirit of RDF. Structural transformation is syntactically focused (albeit on abstract syntax), whereas RDF is supposed to be the foundation of the "Semantic" Web. Given such, one might expect it to take an inference-based approach. With structural transformation, one concentrates on the shape of the tree. With inference based "transformation", one concentrates on the implications of the knowledge base.

What does this mean in practice? When I sit down to write an XSLT script to generate HTML from an RSS 1.0 document, there are two basic sorts of where-ish questions I ask: "Where do I find the elements I want in the source tree?" and "Where do they go in the result tree?" Answers to the first sort of question are typically going to be XPath queries, whereas answers to the second will be templates, element constructors, and the like. Though in an XSLT script, querying and result building are intertwined, clearly they are conceptually distinct aspects of the transform. But in each case, I'm still walking the trees. My script is sensitive to the position of the elements (and attributes, and...) in the tree, and not to their meaning.

Contrariwise, when writing a Prolog script to generate HTML from an RSS 1.0 document, I don't care about how the elements were arranged -- what was which's parent, and so on -- but rather on what the document expresses, and what conclusions I could draw from it. At least that's true for the querying part: since HTML isn't itself a knowledge-oriented system, a structural approach to its generation makes perfect sense. It would be different if I were "transforming" one knowledge base into another. In that case, I would conceive the problem in terms of how the second knowledge base should learn from the first. To help us with the problem of creating a structured result, Prolog (and SWI-Prolog in particular) has a wonderful facility for generating result trees, the Definite Clause Grammar (DCG). My use of DCGs to generate HTML is precisely analogous to using XSLT templates, with Prolog predicates serving the role of XPath expressions.

Definite Clause Grammars

Generally, one uses a declarative language when one has a problem that's well described by some formalism, and it is very tedious to generate, by hand, a computer program that will implement such descriptions. For example, the parser generator YACC implements a declarative language for describing context free grammars (CFGs), and it will generate the requisite nasty C code for a parser that will recognize sentences which adhere to those grammars. In essence, one tells YACC "write me a parser that can parse sentences conforming to this grammar". And one does this because it's relatively easy to understand and modify the formalistic description, especially when compared to hacking around with large symbol tables for finite state automata. An even more familiar formalism is that of regular expressions.

The formalism that Prolog implements is extremely expressive, so much so that it can pretty much directly express CFGs. That is, it's straightforward to write a Prolog program that looks very much like a standard BNFesque description of a CFG. Being a Prolog program, it is executable, and will happily parse sentences that conform to the grammar. It can even generate conformant sentences, given the right seeds. This suggests that in order to generate HTML, I imagine what I want the resultant HTML page (think of the page as a "sentence") to look like, write a grammar for that page just as if I were trying to parse some similar page to scrape it for data, and feed that Prolog grammar the bits it needs to write a such a page, only using the data I supply (in this case, via queries of my knowledge base created with the RSS document).

While I could write the grammar in straight Prolog, there are several bits of housekeeping a straight Prolog program would require that will just clutter things up. Thus, most Prolog systems add a bit of syntactic sugar to manage the repetitive stuff, the DCG notation. SWI-Prolog goes further and provides a DCG library for HTML generation, html_write.

To build the HTML page, I need to write a grammar describing the structure of that page (actually, that class of pages; the grammar is very much like a template). For our RSS->HTML renderer, the first step is to come up with a symbol which represents the page as a whole (the "start" non-terminal), in this case I'll use rss_html_list. Any rss_html_list is going to consist of a page of HTML, obviously, and html_write gives me a grammar rule for generating HTML pages, page/2. So, my template's basic form looks like:

rss_html_list -->
       page(...,
	    ...).

The "-->" is the DCG operator; it works somewhat analogously to the ":-" operator, except that the body of the rule is expecting DCG predicates (i.e., ones defined with "-->") instead of regular Prolog predicates. In the normal case. "-->" can be read as "expands to" or "consists of". The first for page/2 specifies the HTML <head> content and the second the <body> content. If I were constructing a full grammar for rss_html_list, I'd expect to find a rule in it with page as its head. This rule is already defined by html_write, so I don't have to do it myself. The arguments allow me to customize the page production rules without modifying any of its actual clauses. Instead, I pass it little chunks of grammar (encapsulated as DCGs) which page tucks into the right places in its definition:

rss_html_list -->
	      page(\rss_list_head,
                    \rss_list_body).

(The leading slash is a convention of the html_write library that indicates that the rest of the atom is the name of a grammar rule.)

The "head" of the final page needs a title element. To create HTML elements, html_write supplies a DCG predicate for the case, html/1 which takes a list of element "specifications" (the leading slash convention is one kind of specification that html/1 understands). The specification for a simple element with just textual content is of the form element_name(content).

rss_list_head -->
          html([title('XMLhack')]).

It's a little annoying to have the title hardcoded like that, so I'll add an argument to the predicate:

rss_list_head (PageTitle)-->
        html([title(PageTitle)]).

This will do for the head. I want the body of the page to have a header, centered, and then an unordered list of the RSS 1.0 items. Since I want to use the title of the page as the header of the body as well, I'll give rss_list_body an argument too.

rss_list_body (Header)-->
  html([h2([align=center],[Header]),
  %Note that the "h2" spec takes *two* lists as args,
  %the first being the attributes, which can be spec'ed
  %either as name=value, or name(value).
  ul(\rss_list_items(Items))] ).

Both the head and body clauses need to get a value passed to them. In this case, I'm going to pass the buck up to the caller of rss_html_list:

rss_html_list(ChannelTitle) -->
	      page(\rss_list_head(ChannelTitle),
                    \rss_list_body(ChannelTitle)).

That's fine for the page head, but the body clause still has that mysterious variable Items. To fill in that hole I need to make a query, and I intend to use rdf/3, a normal Prolog predicate, not a DCG rule, to make it. Thus, some sort of escaping device needs to be employed. For DCGs, stuff inside curly brackets {} is not treated as DCG clauses (and not expanded in the usual way -- see any Prolog text for details):

rss_list_body (Header)-->
  {...},
  %A query goes in there! Some predicates which bind Items.

  html([h2([align=center],[Header]),
  ul(\rss_list_items(Items))] ).

I expect that the query which binds Items will bind a list of items, so rss_list_items needs to handle it. A simple recursive rule will take care of that:

rss_list_items([First_item|Rest_of_items]) -->
  html([li([\list_item_content(First_item)])]),
  rss_list_items(Rest_of_items).

  rss_list_items([]) --> [].
  %The base case: If the list is empty, just return an empty list.

The final two production rules are simple, although both involve queries:

list_item_content(Item) -->
  {...},? %A query that fetches the Item's Description.
  html([\item_link(Item), br([]), Description]).

item_link(Item) --> {...}, %A query that fetches the item's Link and Title. html(i(a(href(Link),Title))).

That's it the template aspect of the grammar.

Back to the Queries

The grammar needs three queries to be complete. The content and link queries are quite straightforward. I only expect one result in each case, so simple calls to rdf/3 will do the job:

list_item_content(Item) -->
  {rdf(Item, dc:description, literal(Description))},
  html([\item_link(Item), br([]), Description]).

  item_link(Item) -->
  %This is a conjunctive query, though each conjunct is independent.

  {rdf(Item,rss:link,literal(Link)),
   rdf(Item,rss:title,literal(Title))},
  html(i(a(href(Link),Title))).

These queries are very simple. No need to walk up or down a tree. No need to think of how the information is encoded. Items have dc:descriptions, rss:links, and rss:titles -- to find out the description, link, and title for an item, we just ask.

Alas, this isn't entirely the case, as I'm sticking pretty close to the bare RDF metal here, and even to the particular representation of RDF given in rdf_db. I could encapsulate the ways of determining the title and links of an RSS item in more pleasing predicates, which would also allow us to change the way they were determined without affecting our template.

rss_title(Item, Title) :- 
    rdf(Item,rss:title,literal(Title)).
rss_link(Item, Link) :-
    rdf(Item,rss:link,literal(Link)).

The query for rss_list_body is more complex. The basic form is clear: I want the rss:items, so I ask for rdf(Item, rdf:type, rss:item). But this only gives me the first rss:item found. I need to say, "Give me, in a list, all the values that satisfy this query". There are several predicates that have this or similar meaning. In this case, I'll use set_of/3, which has the additional virtue of eliminating duplicates:

set_of(Item, %The variable which gets bound to the desired value. 
       rdf(Item, rdf:type, rss:item), %The query.
       Items) %The variable that gets bound to a list of the results.

This, when popped into rss_list_body, completes the transforming grammar.

DCGs Compared to XSLT

To use the transforming grammar requires a bit more infrastructure. In this case, I wrote a predicate, rss_to_html_list/3, which loads up an RSS 1.0 file, invokes the grammar to generate the HTML, then writes it all to a specified file. I could call it from a GUI front end or just package it up in a shell or CGI script. I backtranslated the grammar into an XSLT stylesheet. Comparing the two is instructive.

:- use_module(library(rdf_db)). 
:- use_module(library(html_write)).

rss_to_html_list(Resource, Source, Target) :-

  rdf_register_ns(rss, 'http://purl.org/rss/1.0/'),
  rdf_register_ns(dc, 
               'http://purl.org/dc/elements/1.1/'),

  rdf_load(Source),        
  rdf(Resource, rss:title, literal(Title)),        
        

  phrase(rss_html_list(Title), TargetHtml),
  tell(Target),

  print_html(TargetHtml),        
  told.

<xsl:output method="html"/> 

rss_html_list(ChannelTitle) --> 
    page(\rss_list_head(ChannelTitle), 
         \rss_list_body(ChannelTitle)).

<xsl:template match="/">
   <html>   
       ...
   </html>
</xsl:template>

rss_list_head(PageTitle) -->
     html([title([PageTitle])]).
        

rss_list_body(Header) --> 
    {setof(Item, 
           rdf(Item, rdf:type, rss:item), 
           Items)},

    html([h2(align(center),[Header]), 
          ul(\rss_list_items(Items))]).

<head>
 <title>
  <xsl:value-of select="$title"/>
 </title>
</head>

<body>
     <h2 align="center">
       <xsl:value-of select="$title"/>
     </h2>
     <ul>
       <xsl:apply-templates
           select="rdf:RDF/rss:item"/>
     </ul>     
</body>

rss_list_items([First_item|Rest_of_items]) -->
    html([li([\list_item_content(First_item)])]),
    rss_list_items(Rest_of_items).
rss_list_items([]) --> [].

<xsl:template match="rdf:RDF/rss:item">  
   <li>...</li>  
</xsl:template>

list_item_content(Item) --> 
    {rdf(Item,dc:description,literal(Description))}, 
    html([\rss_link(Item),br([]),Description]).

...
<br/>
<xsl:value-of select="./dc:description"/>

rss_link(Item) --> 
    {rdf(Item,rss:link,literal(Link)),  
    rdf(Item,rss:title,literal(Title))}, 
    html(i(a(href(Link),Title))).

<i>
   <a href="{./rss:link}">
      <xsl:value-of select="./rss:title"/>
   </a>
</i>

Standing back and looking at the XSLT sheet right after writing it, it seemed much clearer than the DCGs. A large part of this is that I'm more accustomed to writing HTML and HTML templates with embedded code than writing DCG templates. But I'm finding that the DCGs are more helpful for thinking through the problem.

Conclusion

Since the inferences in this transformation were trivial, it doesn't involve any particularly sophisticated Prolog, and yet this is precisely the kind of everyday task many of us find ourselves doing all the time. The Semantic Web, if it's to work out, will be made up as much of the ordinary and familiar as of the exotic.

Useful Links

The Transformation Scripts

• The Transformation DCG
• The XSLT Stylesheet
• Sample Source File
• Sample HTML output

SWI-Prolog

• Home page
• Documentation
  • XML/SGML support and RDF Demo
  • checklist/2
  • set_of/3
  • DCG Rules (and phrase/2)
  • File stuff (tell/1 and told)
  • The module system
  • html_write
• The Wiki (contains info about rdf_db)

SWI-Prolog Based RDF Applications

• The RDF Parser Demo
• RDF Schema Explorer and the associated paper

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