xmls

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<head>
  <title>XMLS: Common Lisp XML Parser</title>
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<h1>XMLS</h1>
<center>Manual For Version 3</center>

<h2>Summary</h2>
<p>
  Xmls is a small, simple, non-validating xml parser for Common Lisp.  It's
  designed to be a self-contained, easily embedded parser that recognizes a useful
  subset of the XML spec.  It provides a simple mapping from xml to lisp 
  structures or s-expressions and back.
</p>
<p>Since XMLS was first released it has gained some additional
complications/features.  In particular:</p>
<ul>
  <li><strong>Now XMLS by default parses XML documents into lisp structures,
  rather than s-expressions.</strong>  This makes accessing the structures
  simpler and more reliable. See <a href="#Compatibility" >section on backward compatibility</a>.</li>
  <li>We have added clearly named accessors to further improve
  extraction of information from parsed XML.</li>
  <li>Thanks to Max Rottenkolber, we now have the affiliated library,
  <a href="#octets"><code>xmls/octets</code></a> that will open streams for the XMLS parser,
  processing any content-type declarations in the process.</li>
</ul>


<h2>Features</h2>
<ul>
  <li>
    Free (BSD license).
  </li>
  <li>
    Understands enough of the xml spec to parse many common documents, including
    those occurring in common internet protocols like xml-rpc, webdav, and BEEP.
    Parses 85 out of the 98 valid documents in the oasis parser compliance suite.
  </li>
  <li>
    Small and easily embedded.  The entire parser is contained in one
    file and it's currently less than 600 lines of code.  Xmls is written in
    pure lisp and requires no external parsing tools or foreign libraries.
  </li>
  <li>
    Supports xml namespaces.
  </li>
  <li>
    Threadsafe.
  </li>
  <li>
    Serializes s-expr list structures back to xml as well as parsing xml.
  </li>
</ul>

<h2>Limitations</h2>
<ul>
  <li>
    Parses entire document into memory and consequently can't handle large
    documents.
  </li>
  <li>
    No detailed error reporting.
  </li>
  <li>
  Hand-built LR parser, meaning the parser structure is a little hard
  to understand, and can be hard to modify.  Use of CL-YACC or similar
  might be a preferable route for a rewrite.
  </li>
</ul>

<h2>XML Representation</h2>

<p>
  Parsed xml is represented as a nested lisp structure, unlike in the
  original version, where it was a lisp list.  The s-expression
  representation is still maintained, and there are <a href="#translators">functions to
  translate to and from this notation</a>. 
</p>

<h3>XML representation as lisp structures</h3>

<p>In the structure representation, a node, corresponding to an XML
element, is defined as follows:
</p>

<pre>(defstruct (node (:constructor %make-node))
  name
  ns
  attrs
  children)</pre>

<p>
  Xmls also includes a helper function, make-node for creating xml nodes
  of this form:
</p>

<pre>
(make-node &amp;key name ns attrs children)
</pre>

<p>
  Xmls provides the corresponding accessor functions node-name, node-ns
  node-attrs, and node-children.
</p>

<h3>XML representation as s-expressions</h3>

<p>In the s-expression representation, a node is represented as follows:</p>

<pre>
(name (attributes) children*)
</pre>

<p>
  A name is either a simple string, if the element does not belong to a namespace,
  or a list of (name namespace-url) if the element does belong to a namespace.
</p>

<p>
  Attributes are stored as (name value) lists.
</p>

<p>
  Children are stored as a list of either element nodes or text nodes.
</p>

<p>
  For example, the following xml document:
</p>

<pre>
&lt;?xml version="1.0"?&gt;
&lt;!-- test document --&gt;
&lt;book title='The Cyberiad'&gt;
  &lt;!-- comment in here --&gt;
  &lt;author xmlns='http://authors'&gt;Stanislaw Lem&lt;/author&gt;
  &lt;info:subject xmlns:info='http://bookinfo' rank='1'&gt;&amp;quot;Cybernetic Fables&amp;quot;&lt;/info:subject&gt;
&lt;/book&gt;
</pre>

Would parse as:

<pre>
("book" (("title" "The Cyberiad"))
 (("author" . "http://authors") NIL "Stanislaw Lem")
 (("subject" . "http://bookinfo") (("rank" "1")) "\"Cybernetic Fables\""))
</pre>

<a  name="Compatibility"><h3>Backward Compatibility</h3></a>

<p>To detect whether in this version of XMLS the return value of <code>PARSE</code> will be a
list or a structure, check for the feature <code>:XMLS-NODES-ARE-STRUCTS</code>.

<p>For old code that wants XML parsed into lists, instead of
structures, you may replace calls to <code>(parse str)</code> with
<code>(node-&gt;nodelist (parse str))</code>.</p>

<p>For greater convenience, we offer <code>PARSE-TO-LIST</code>, which
performs the same function.</p>

<h2>Usage</h2>

<p>
  The interface is straightforward.  The two main functions are
  <code>PARSE</code> and <code>TOXML</code>.
</p>

<pre>
(parse source &amp;key (compress-whitespace t) (quash-errors t)
</pre>

<p>
  Parse accepts either a string or an input stream and attempts to parse the xml
  document contained therein.  It will return the s-expr parse tree if it's
  successful or nil if parsing fails.</p>

<p>
  If <code>COMPRESS-WHITESPACE</code> is non-<code>NIL</code>, content nodes will be trimmed of whitespace and
  empty whitespace strings between nodes will be discarded.
</p>


<pre>
(parse-to-list source (&rest args))
</pre>

<p>Functions as <code>PARSE</code>, but returns a list representation
of the XML document, instead of a structure.</p>

<pre>
(write-prologue xml-decl doctype stream)
</pre>

<p>
  write-prologue writes the leading
  <code>&lt;?xml ... ?&gt;</code> and <code>&lt;!DOCTYPE ... &gt;</code>
  elements to <code>stream</code>.
  <code>xml-decl</code> is an alist of attribute name value pairs.
  Valid xml-decl attributes per the xml spec are "version", "encoding",
  and "standalone", though write-prologue does not verify this.
  <code>doctype</code> is a string containing the document type definition.
</p>

<pre>
(write-prolog xml-decl doctype stream)
</pre>
<p>
U.S. spelling alternative to <code>write-prologue</code>.
</p>

<pre>
(write-xml xml stream &amp;key (indent nil))
</pre>

<p>
  write-xml accepts a lisp list in the format described above and writes the
  equivalent xml string to stream.  Currently, if nodes use namespaces xmls will not 
  assign namespaces prefixes but will explicitly assign the namespace to each node.  This
  will be changed in a later release.

  Xmls will indent the generated xml output if indent is non-nil.
</p>

<pre>
(toxml node &amp;key (indent nil))
</pre>

<p>
  <code>TOXML</code> is a convenience wrapper around write-xml that returns the in a newly
  allocated string.
</p>

<a name="translators"><h3>Translating to and from s-expressions</h3></a>

<p>XMLS provides two exported functions to translate between the CL
structure representation of the XML tree and the s-expression
representation:</p>
<dl>
  <dt><code>node-&gt;nodelist</code></dt>
  <dd>Translate the structure representation into s-expressions.</dd>

  <dt><code>nodelist-&gt;nodes</code></dt>
  <dd>Translate the s-expression representation of an XMLS parse tree
  into lisp structures.</dd>
</dl>


<h3>Helper functions</h3>

<p>These are intended to allow programmers to avoid direct manipulation of the
 XMLS element representation.  If you use these, your code should be easier to
  read and you will avoid problems if there is a change in internal
  representation (such changes would be hard to even find, much less correct, if
  using the lists directly).</p>

<dl>
  <dt><code>
      make-xmlrep  (tag &amp;key attribs children)
  </code>
  </dt>
  <dd>Constructor function.</dd>

  <dt>
    <code>xmlrep-add-child! (xmlrep child)</code>
  </dt>
  <dd>Add a new child node to the XMLREP node.</dd>

  <dt>
    <code>xmlrep-tag (xmlrep)</code>
  </dt>
  <dd>Extract the tag from XMLREP.</dd>

  <dt>
    <code>xmlrep-tagmatch (tag treenode)</code>
  </dt>
  <dd>Returns true if TAG is the tag of TREENODE.  Match is
    case <em>insensitive</em> (quite possibly this is the Wrong Thing). </dd>

  <dt>
    <code>xmlrep-attribs (xmlrep)</code>
  </dt>
  <dd>Extract the attributes from an XMLREP node.</dd>

  <dt>
    <code>xmlrep-children (xmlrep)</code>
  </dt>
  <dd>Extract the children from an XMLREP node.</dd>

  <dt>
    <code>xmlrep-find-child-tags (tag treenode)</code>
  </dt>
  <dd>
    Return all of the (direct) children of TREENODE whose tags are TAG.
    Matching done by <a href="#xmlrep-tagmatch"><code>xmlrep-tagmatch</code></a>.
  </dd>
  
  <dt>
    <code>xmlrep-find-child-tag (tag treenode &amp;optional (if-unfound :error))</code>
  </dt>
  <dd>
    Find a <em>single</em> child of TREENODE with TAG.  Returns an error
    if there is more or less than one such child.
  </dd>

  <dt>
    <code>xmlrep-string-child (treenode &amp;optional (if-unfound :error))</code>
  </dt>
  <dd>
    Returns the <em>single</em> string-valued child of TREENODE.
    If there is more than one child, or if a single child is not
    a simple value, returns IF-UNFOUND, which defaults to :ERROR.
  </dd>

  <dt>
    <code>xmlrep-integer-child (treenode)</code>
  </dt>
  <dd>
    Find the <em>single</em> child of TREENODE whose value is a string that
    can be parsed into an integer.  Returns an
    error if there is more than one child, or if a single child is not
    appropriately valued.
  </dd>

  <dt>
    <code>xmlrep-attrib-value (attrib treenode &amp;optional (if-undefined :error))</code>
  </dt>
  <dd>
    Find the value of ATTRIB, a string, in TREENODE.
    if there is no ATTRIB, will return the value of IF-UNDEFINED,
    which defaults to :ERROR.
  </dd>

  <dt>
    <code>xmlrep-boolean-attrib-value (attrib treenode &amp;optional (if-undefined :error))</code>
  </dt>
  <dd>
    Find the value of ATTRIB, a string, in TREENODE.
    The value should be either "true" or "false".  The
    function will return T or NIL, accordingly.  If there is no ATTRIB,
    will return the value of IF-UNDEFINED, which defaults to :ERROR.
  </dd>
</dl>

<a  name="octets"><h2>XMLS/Octets</h2></a>
<p>
XMLS itself simply processes strings or streams.  This means that it
does not provide native support for handling character encodings, as
declared in the XML headers.  The system <code>xmls/octets</code>,
which depends on <code>xmls</code> provides that support with the
exported function <code>make-xml-stream</code>, which takes an
octet-stream as argument, processes its header, choosing the
appropriate character encoding, and then returns a stream suitable for
passing to <code>xmls:parse</code>.</p>

<p>Probably <code>make-xml-stream</code> should be made generic, and support
arguments of other types (e.g., strings interpreted as filenames,
pathnames, etc.).</p>

<h2>Installation</h2>

<p>
  xmls can be installed as an asdf system.  An asdf
  system definition is provided with the distribution.
</p> 
<p>Previous versions of XMLS were single files, and could be installed simply by
  loading the file xmls.lisp.  This option is no longer supported.</p>
 
<h2>Contact Information</h2> 
 
<p> 
  Please contact Robert Goldman, rpgoldman AT sift.net with any 
  questions or bug reports.
</p>
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