SWI-Prolog SGML/XML parser
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8 library(pwp): Prolog Well-formed Pages

author
Richard O'Keefe
To be done
Support compilation of PWP input files

PWP is an approach to server-side scripting using Prolog which is based on a simple key principle:

Especially when generating XML rather than HTML, this is such an obvious thing to do. We have many kinds of XML checking tools.

Having decided that the input should be well formed, that means NO NEW SYNTAX

None of the weird and horrible <% ... %> or whatever not-quite-XML stuff you see in other template systems, making checking so very hard (and therefore, making errors so distressingly common).

That in turns means that PWP "markup" must be based on special elements or special attributes. The fact that an XML parser must allow undeclared attributes on any element even when validating, but must not allow undeclared elements, suggests doing this through attributes. In particular, one should be able to take an existing DTD, such as an XHTML DTD, and just use that without modification. So the design reduces to

This description uses the following name space: 

xmlns:pwp='http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl'

The attributes are

Here's what they mean. Each element is expanded in the context of a set of variable bindings. After expansion, if the tag is not mapped to’-’, all attributes in the pwp: namespace are removed and the children elements are recursively expanded.

Examples:

  1. A "Hello World" like example 
    <html
  xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl"
  pwp:ask = "ensure_loaded(msg), once(msg(Greeting))">
  <head>
    <title pwp:use="Greeting"/>
  </head>
  <body>
    <p><span pwp:use="Greeting" pwp:tag='-'/></p>
  </body>
</html>

    where msg.pl contains 

    msg('Hello, World!').

    This example illustrates an important point. Prolog Well-Formed Pages provide NO way to physically incorporate Prolog clauses into a page template. Prolog clauses must be put in separate files which can be checked by a Prolog syntax checker, compiler, cross-referencer, &c WITHOUT the Prolog tool in question needing to know anything whatsoever about PWP. You load the files using pwp:ask on the root element.

  2. Binding some variables and using them 
    <html
  xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl">
  <head><title>Example 2</title></head>
  <body pwp:ask="Hello = 'Hello world', A = 20, B = 22">
    <h1 pwp:use="Hello"/>
    <p>The answer is <span pwp:use="C" pwp:ask="C is A+B"/>.</p>
  </body>
</html>

  3. Making a table We are given a Prolog database staff.pl defining staff(NickName, FullName, Office, Phone, E_Mail_Address). status(NickName, full_time | part_time). We want to make a phone list of full time staff. 
    <html
  xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl"
  pwp:ask='ensure_loaded(staff)'>
  <head>
    <title>Phone list for Full-Time staff.</title>
  </head>
  <body>
    <h1>Phone list for Full-Time staff.</h1>
    <table
      pwp:ask = "setof(FullName-Phone,
                       N^O^E^(
                         status(N, full_time),
                         staff(N, FullName, O, Phone, E)
                       ),
                       Staff_List)">
      <tr><th>Name</th><th>Phone</th></tr>
      <tr pwp:ask="member(FullName-Phone, Staff_List)">
        <td pwp:use="FullName"/>
        <td pwp:use="Phone"/>
      </tr>
    </table>
  </body>
</html>

  4. Substituting into an attribute Same data base as before, but now we want to make a mailing list page. 
    <html
  xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl"
  pwp:ask='ensure_loaded(staff)'>
  <head>
    <title>Phone list for Full-Time staff.</title>
  </head>
  <body>
    <h1>Phone list for Full-Time staff.</h1>
    <table
      pwp:ask = "setof(FullName-E_Mail,
                       N^O^P^staff(N, FullName, O, P, E_Mail),
                       Staff_List)">
      <tr><th>Name</th><th>Address</th></tr>
      <tr pwp:ask="member(FullName-E_Mail, Staff_List)">
        <td pwp:use="FullName"/>
        <td><a pwp:use="E_Mail"
               pwp:att='$' href="mailto:$(E_Mail)$"/></td>
      </tr>
    </table>
  </body>
</html>

  5. If-then-else effect A page that displays the value of the’SHELL’environment variable if it has one, otherwise displays’There is no default shell.’ 
    <html
  xmlns:pwp="http://www.cs.otago.ac.nz/staffpriv/ok/pwp.pl">
  <head><title>$SHELL</title></head>
  <body>
    <p pwp:ask="getenv('SHELL', Shell)"
    >The default shell is <span pwp:tag="-" pwp:use="Shell"/>.</p>
    <p pwp:ask="\+getenv('SHELL',_)">There is no default shell.</p>
  </body>
</html>

    There is one other criterion for a good server-side template language:

    It should be possible to compile templates so as to eliminate most if not all interpretation overhead.

    This particular notation satisfies that criterion with the limitation that the conversion of a term to character data requires run-time traversal of terms (because the terms are not known until run time).

[det]pwp_files(:In:atom, +Out:atom)
loads an Xml document from the file named In, transforms it using the PWP attributes, and writes the transformed version to the new file named Out.
[det]pwp_stream(:Input:input_stream, +Output:output_stream, +Context:list)
Loads an Xml document from the given Input stream, transforms it using the PWP attributes, and writes the transformed version to the given Output stream. Context provides initial contextual variables and is a list of Name=Value.
pwp_xml(:In:list(xml), -Out:list(xml), +Context)
maps down a list of XML items, acting specially on elements and copying everything else unchanged, including white space. The Context is a list of’VariableName’=CurrentValue bindings.

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