_XML_ Library
=============

Basic XML Data Types
====================

Document:
       This structure represents an XML document.  The only useful part is
       the document-element, which contains all the content.  The rest of
       of the structure contains DTD information, which isn't supported,
       and processing-instructions.

Element:
       Each pair of start/end tags and everything in between is an element.
       It has the following pieces:
              a name
              attributes
              contents including sub-elements
Xexpr:
       S-expression representations of XML data.

The end of this document has more details.

Functions
=========

> read-xml : [Input-port] -> Document
       reads in an XML document from the given or current input port
       XML documents contain exactly one element.  It throws an xml-read:error
       if there isn't any element or if there are more than one element.

> write-xml : Document [Output-port] -> Void
       writes a document to the given or current output port, currently
       ignoring everything except the document's root element.

> write-xml/content : Content [Output-port] -> Void
       writes a document's contents to the given or current output port

> display-xml : Document [Output-port] -> Void
       just like write-xml, but newlines and indentation make the output more
       readable, though less technically correct when white space is
       significant.

> display-xml/content : Content [Output-port] -> Void
       just like write-xml/content, but with indentation and newlines

> xml->xexpr : Content -> Xexpr
       converts the interesting part of an XML document into an Xexpression

> xexpr->xml : Xexpr -> Content
       converts an Xexpression into the interesting part of an XML document

> xexpr->string : Xexpression -> String
       converts an Xexpression into a string representation
       
> eliminate-whitespace : (listof Symbol) (Bool -> Bool) -> Element -> Element
       Some elements should not contain any text, only other tags, except they
       often contain whitespace for formating purposes.  Given a list of tag names
       and the identity function, eliminate-whitespace produces a function that
       filters out pcdata consisting solely of whitespace from those elements and
       raises and error if any non-whitespace text appears.  Passing in the function
       called "not" instead of the identity function filters all elements which are not
       named in the list.  Using void filters all elements regardless of the list.

Parameters
==========

> empty-tag-shorthand : 'always | 'never | (listof Symbol)
       Default: 'always
       This determines if the output functions should use the <empty/> tag
       notation instead of writing <empty></empty>.  The first form is the
       preferred XML notation.  However, most browsers designed for HTML
       will only properly render XHTML if the document uses a mixture of the
       two formats.  _html-empty-tags_ contains the W3 consortium's
       recommended list of XHTML tags that should use the shorthand.

> collapse-whitespace : Bool
       Default: #f
       All consecutive whitespace is replaced by a single space.
       CDATA sections are not affected.

> trim-whitespace : Bool
       This parameter no longer exists.  Consider using collapse-whitespace
       and eliminate-whitespace instead.

> read-comments : Bool
       Default: #f
       Comments, by definition, should be ignored by programs.  However,
       interoperating with ad hoc extentions to other languages sometimes
       requires processing comments anyway.

> xexpr-drop-empty-attributes : Bool
       Default: #f
       It's easier to write functions processing Xexpressions, if they always
       have a list of attributes.  On the other hand, it's less cumbersome to
       write Xexpresssions by hand without empty lists of attributes
       everywhere.  Normally xml->xexpr leaves in empty attribute lists.
       Setting this parameter to #t drops them, so further editing the
       Xexpression by hand is less annoying.

Examples
========

Reading an Xexpression:
       (xml->xexpr (document-element (read-xml input-port)))

Writing an Xexpression:
       (empty-tag-shorthand html-empty-tags)
       (write-xml/content (xexpr->xml `(html (head (title ,banner))
					     (body ((bgcolor "white"))
						   ,text)))
			  output-port)

What this Library Doesn't Provide
=================================

       Document Type Declaration (DTD) processing
       Validation
       Expanding user-defined entites
       Reading user-defined entites in attributes
       Unicode support

XML Datatype Details
====================

Note: Users of the XML collection don't need to know most of these definitions.

Note: Xexpr is the only important one to understand.  Even then,
      Processing-instructions may be ignored.

> Xexpr ::= String
         |  (list* Symbol (listof (list Symbol String)) (list Xexpr))
         |  (cons Symbol (listof Xexpr)) ;; an element with no attributes
         |  Symbol ;; symbolic entities such as &nbsp;
         |  Number ;; numeric entities like &#20;
         |  Misc

> Document ::= (make-document Prolog Element (listof Processing-instruction))
  (define-struct document (prolog element misc))

> Prolog ::= (make-prolog (listof Misc) #f)
  (define-struct prolog (misc dtd))

> Element ::= (make-element Location Location
			    Symbol
			    (listof Attribute)
			    (listof Content))
  (define-struct (element struct:source) (name attributes content))

> Attribute ::= (make-attribute Location Location Symbol String)
  (define-struct (attribute struct:source) (name value))

> Content ::= Pcdata
           |  Element
           |  Entity
           |  Misc

  Misc ::= Comment
        |  Processing-instruction

> Pcdata ::= (make-pcdata Location Location String)
  (define-struct (pcdata struct:source) (string))

> Entity ::= (make-entity (U Nat Symbol))
  (define-struct entity (text))

> Processing-instruction ::= (make-pi Location Location String (list String))
  (define-struct (pi struct:source) (target-name instruction))

> Comment ::= (make-comment String)
  (define-struct comment (text))

  Source ::= (make-source Location Location)
  (define-struct source (start stop))

  Location ::= Nat
            |  Symbol
