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pwp.pl -- Prolog Well-formed Pages
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PWP is an approach to server-side scripting using Prolog which is based on a simple key principle:

  • The source form of a PWP should be WELL-FORMED XML

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

  • We can tell whether an XML document is WELL FORMED (all the punctuation is right, all tags balance) using practically any decent parser, including SWI Prolog's 'sgml'.
  • If we can write a Document Type Description then we can check that a document is VALID using tools like Open SP (formerly James Clark's SP) or SWI Prolog's 'sgml'. This does not guarantee that the output will be valid, but it does catch a lot of errors very early.
  • If we can write an XML Schema then we can check that a document is schema-valid. (SWI Prolog's 'sgml' does not yet come with a schema validator, but who knows what the future holds?).
  • Since an XML document is just a data structure, we can use any checking tool that we can write in Prolog, IF the input is well-formed so that we can load a template as a Prolog data structure.

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

  • Allow dynamic data selection, insertion, and transformation just using a small number of extra attributes.

This description uses the following name space:


The attributes are

  • pwp:ask = Query
  • pwp:use = Term
  • pwp:how = text | xml
  • pwp:tag = QName or '-'
  • pwp:att = '' | 'one non-alphanumeric character'

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.

  • pwp:ask = Query
    • Query is a Prolog goal. For each solution of Query, the element is further processed with the new variables of Query added to the context.
    • If Query is not a well formed Prolog goal, or if execution of Query throws an exception, page transformation comes to a complete halt and no page is generated.
  • pwp:use = Term
  • pwp:how = text | xml | text-file | xml-file | pwp-file
    Term is a Prolog term; variables in Term are bound by the context.
    An empty Term is regarded as a missing value for this attribute.
    The Prolog variable CONTEXT refers to the entire context, a list
    of Name = Value, where Name is a Prolog atom holding the name of        the context variable and Value is an arbitrary Prolog term.
    • If pwp:how is text, The value of Term is used to define a sequence of characters.
      • A number produces the same characters that write/1 would.
      • An atom produces the same characters that write/1 would.
      • A string produces the same characters that write/1 would.
      • A list of character codes produces those characters.
      • The following terms produce the same sequence of characters that the corresponding goal would have sent to the current output stream:
        • write(Datum)
        • writeq(Datum)
        • write_canonical(Datum)
        • print(Datum)
        • print(Datum)
        • format(Format)
        • format(Format, Arguments)
      • A singleton list [X] defines the characters that X defines.
      • Any other term F(T1,...,Tn) defines the characters that T1 defines, followed by the characters that T2 defines, ..., followed by the characters that Tn defines.
    • If pwp:how is xml, The value of Term must be an XML term as defined in the SGML2PL documentation or a list of such terms. A single term is taken as if it had been [Term]. The resulting list of terms replaces the children of the current element and will not be further processed.
    • If pwp:how is text-file, The value of Term is used to define a sequence of characters. That sequence of characters is used as a file name. The file is read as a sequence of characters, and that sequence used as character data.
    • If pwp:how is xml-file, The value of Term is used to define a sequence of characters. That sequence of characters is used as a file name. The file is loaded as XML, and the sequence of XML items thus obtained used. This means that PWP provides XML inclusion without depending on the parser to support XInclude.
    • If pwp:how is pwp-file, Like xml-file, but PWP attributes are evaluated and processed. The current context variables are passed to the PWP processor.

      The default value for pwp:how is text.

  • pwp:tag = QName or '-'
    If pwp:tag is missing or the value is empty, the current element
    appears in the output (after further processing) with its present
    tag.  If pwp:tag is a QName, the current element appears (...)
    with that as its tag.  That option is most useful in DTDs, where
    an "authoring" DTD may use one tag and have it automatically mapped
    to another tag in the output, e.g., <item> -> <li>.  Finally, if
    pwp:tag is '-', the children of the current element (either the
    result of pwp:use or the transformed original children, whichever
    applies) appear in the output but there is no element around them.
    A missing or empty pwp:ask is just like pwp:ask = 'true'.
  • pwp:att = '' | 'one non-alphanumeric character'.
    Attributes in the pwp namespace are not affected by this attribute.
    Such attributes are always stripped out and never substituted into.
    If pwp:att is missing or empty, attributes of the current
    element are copied over to the output unchanged.
    If pwp:att = 'c' for some non-alphanumeric character c,
    each attribute is examined for occurrences of c(...)c and c[...]c
    which are as short as possible.
    There is no one character which could be used every time, so you
    have to explicitly choose a substitution marker which is safe
    for the data you do not want to be altered.  None of the pwp
    attributes are inherited, least of all this one.
    Text outside c(...)c groups is copied unchanged; text inside
    a c(...)c group is parsed as a Prolog term and treated as if by
    pwp:how = text. Text inside a c[...]c group is evaluated (in the
    current context), and if it fails, the entire attribute will be
    removed from the element.


  1. A "Hello World" like example
      pwp:ask = "ensure_loaded(msg), once(msg(Greeting))">
        <title pwp:use="Greeting"/>
        <p><span pwp:use="Greeting" pwp:tag='-'/></p>

    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
      <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>
  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.
        <title>Phone list for Full-Time staff.</title>
        <h1>Phone list for Full-Time staff.</h1>
          pwp:ask = "setof(FullName-Phone,
                             status(N, full_time),
                             staff(N, FullName, O, Phone, E)
          <tr pwp:ask="member(FullName-Phone, Staff_List)">
            <td pwp:use="FullName"/>
            <td pwp:use="Phone"/>
  4. Substituting into an attribute Same data base as before, but now we want to make a mailing list page.
        <title>Phone list for Full-Time staff.</title>
        <h1>Phone list for Full-Time staff.</h1>
          pwp:ask = "setof(FullName-E_Mail,
                           N^O^P^staff(N, FullName, O, P, E_Mail),
          <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>
  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.'
        <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>

    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).

- Richard O'Keefe
To be done
- Support compilation of PWP input files
Source pwp_files(:In:atom, +Out:atom) is det
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.
Source pwp_stream(:Input:input_stream, +Output:output_stream, +Context:list) is det
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.
Source 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.
Source pwp_attributes(+Atts0:list(atom=atom), -Ask:atom, -Use:atom, -How:atom, -Att:atom, -Tag:atom, -Atts1:list(atom=atom))[private]
Walks down a list of AttributeName=ItsValue pairs, stripping out those whose AttributeName begins with the 'pwp:' prefix, and copying the rest to Atts1. Along the way, Ask/Use/How/Att/Tag are bound to the values of the pwp:ask/pwp:use/pwp:how/pwp:att/pwp:tag attributes, if any. At the end, any of these variables that are still unbound REMAIN unbound; they are not bound to default values.
Source pwp_attr(+XMLAttr, -PWPLocal) is semidet[private]
True if PWPLocal is the local name of a pwp:Local expression in XML. This predicate deals with the three different XML representations: the form is returned of XML namespace processing is not enabled. The second if it is enabled and the namespace is properly defined and the last if the namespace is not defined.
Source pwp_unite(+Bindings, +Context0, -Context:list(atom=any))[private]
merges the new Bindings with the bindings in the outer Context0, constructing a new list of VariableName=CurrentValue bindings in Context1. This is only used when the CurrentValue parts of the new Bindings are known to be distinct new variables, so the Bindings cannot possibly conflict with any existing binding in Context0. This is O(|Bindings|.|Context0|), which is not that efficient, but since we do not expect there to be very many variables it doesn't matter much.
Source pwp_unite(+Bindings, +Context0:list(atom=any))[private]
looks up the bindings in Bindings in the outer Context0. This is only used for 'pwp:use' terms (and the related terms in $(...)$ attribute value substitutions), so that we have no interest in forming a new context. (If we did, we'd use pwp_unite/3 instead.) This is only used when the CurrentValue parts of the new Bindings are known to be distinct new variables, so the Bindings cannot possibly conflict with any existing binding in Context0. However, there could be new variables in Bindings, and that would cause problems. An XML term may not contain variables, and a term we want to convert to a list of character codes had better not contain variables either. One approach would be to just bind such variables to something, another is to throw some kind of exception. For the moment we call functor/3 so as to get an instantiation error.
Source pwp_attach(+Tree, ?Ys0:list(xml), ?Ys:list(xml))[private]
is a combination of "flatten" and "append". It unifies Ys0\Ys with the result of flattening Tree.
Source pwp_is_codes(+String:any)[private]
is true when String is a list of integers and each of those integers is a possible Unicode value (in the range U+0000..U+10FFFF). Back in the days of ISO Latin 1 we would have checked for 0..255, and way back in the days of ASCII for 0..127. Yes, there are more than a million possible characters in Unicode; currently about 100 000 of them are in use.