The Ada95 Unicode and XML Library
*********************************

The Ada95 XML Library

   Version 4.1w

   Date: $Date: 2010-10-14 11:27:54 +0200 (Thu, 14 Oct 2010) $

   Copyright (C) 2000-2002, Emmanuel Briot This document may be copied,
in whole or in part, in any form or by any means, as is or with
alterations, provided that (1) alterations are clearly marked as
alterations and (2) this copyright notice is included unmodified in any
copy.

1 Introduction
**************

The Extensible Markup Language (XML) is a subset of SGML Its goal is to
enable generic SGML to be served, received, and processed on the Web in
the way that is now possible with HTML. XML has been designed for ease
of implementation and for interoperability with both SGML and HTML.

   This library includes a set of Ada95 packages to manipulate XML
input.  It implements the XML 1.0 standard (see the references at the
end of this document), as well as support for namespaces and a number of
other optional standards related to XML.

   We have tried to follow as closely as possible the XML standard, so
that you can easily analyze and reuse languages produced for other
languages.

   This document isn't a tutorial on what XML is, nor on the various
standards like DOM and SAX. Although we will try and give a few
examples, we refer the reader to the standards themselves, which are all
easily readable.

2 The Unicode module
********************

Unicode provides a unique number for every character, no matter what the
platform, no matter what the program, no matter what the language.

   Fundamentally, computers just deal with numbers.  They store letters
and other characters by assigning a number for each one.  Before Unicode
was invented, there were hundreds of different encoding systems for
assigning these numbers.  No single encoding could contain enough
characters: for example, the European Union alone requires several
different encodings to cover all its languages.  Even for a single
language like English no single encoding was adequate for all the
letters, punctuation, and technical symbols in common use.

   These encoding systems also conflict with one another.  That is, two
encodings can use the same number for two different characters, or use
different numbers for the same character.  Any given computer
(especially servers) needs to support many different encodings; yet
whenever data is passed between different encodings or platforms, that
data always runs the risk of corruption.

   Unicode provides a unique number for every character, no matter what
the platform, no matter what the program, no matter what the language.
The Unicode Standard has been adopted by such industry leaders as Apple,
HP, IBM, JustSystem, Microsoft, Oracle, SAP, Sun, Sybase, Unisys and
many others.  Unicode is required by modern standards such as XML, Java,
ECMAScript (JavaScript), LDAP, CORBA 3.0, WML, etc., and is the official
way to implement ISO/IEC 10646.  It is supported in many operating
systems, all modern browsers, and many other products.  The emergence of
the Unicode Standard, and the availability of tools supporting it, are
among the most significant recent global software technology trends.

   The following sections explain the basic vocabulary and concepts
associated with Unicode and encodings.

   Most of the information comes from the official Unicode Web site, at
<http://www.unicode.org/unicode/reports/tr17>.

   Part of this documentation comes from <http://www.unicode.org>, the
official web site for Unicode.

   Some information was also extracted from the "UTF-8 and Unicode FAQ"
by M. Kuhn, available at <???>.

2.1 Glyphs
==========

A glyph is a particular representation of a character or part of a
character.

   Several representations are possible, mostly depending on the exact
font used at that time.  A single glyph can correspond to a sequence of
characters, or a single character to a sequence of glyphs.

   The Unicode standard doesn't deal with glyphs, although a suggested
representation is given for each character in the standard.  Likewise,
this module doesn't provide any graphical support for Unicode, and will
just deal with textual memory representation and encodings.

   Take a look at the GtkAda library that provides the graphical
interface for unicode in the upcoming 2.0 version.

2.2 Repertoires and subsets
===========================

A repertoire is a set of abstract characters to be encoded, normally a
familiar alphabet or symbol set.  For instance, the alphabet used to
spell English words, or the one used for the Russian alphabet are two
such repertoires.

   There exist two types of repertoires, close and open ones.  The
former is the most common one, and the two examples above are such
repertoires.  No character is ever added to them.

   Unicode is also a repertoire, but an open one.  New entries are added
to it.  However, it is guaranteed that none will ever be deleted from
it.  Unicode intends to be a universal repertoire, with all possible
characters currently used in the world.  It currently contains all the
alphabets, including a number of alphabets associated with dead
languages like hieroglyphs.  It also contains a number of often used
symbols, like mathematical signs.

   The goal of this Unicode module is to convert all characters to
entries in the Unicode repertoire, so that any applications can
communicate with each other in a portable manner.

   Given its size, most applications will only support a subset of
Unicode.  Some of the scripts, most notably Arabic and Asian languages,
require a special support in the application (right-to-left
writing,...), and thus will not be supported by some applications.

   The Unicode standard includes a set of internal catalogs, called
collections.  Each character in these collections is given a special
name, in addition to its code, to improve readability.

   Several child packages (Unicode.Names.*) define those names.  For
instance:

Unicode.Names.Basic_Latin
     This contains the basic characters used in most western European
     languages, including the standard ASCII subset.

Unicode.Names.Cyrillic
     This contains the Russian alphabet.

Unicode.Names.Mathematical_Operators
     This contains several mathematical symbols

   More than 80 such packages exist.

2.3 Character sets
==================

A character set is a mapping from a set of abstract characters to some
non-negative integers.  The integer associated with a character is
called its code point, and the character itself is called the encoded
character.

   There exist a number of standard character sets, unfortunately not
compatible with each other.  For instance, ASCII is one of these
character sets, and contains 128 characters.  A super-set of it is the
ISO/8859-1 character set.  Another character set is the JIS X 0208, used
to encode Japanese characters.

   Note that a character set is different from a repertoire.  For
instance, the same character C with cedilla doesn't have the same
integer value in the ISO/8859-1 character set and the ISO/8859-2
character set.

   Unicode is also such a character set, that contains all the possible
characters and associate a standard integer with them.  A similar and
fully compatible character set is ISO/10646.  The only addition that
Unicode does to ISO/10646 is that it also specifies algorithms for
rendering presentation forms of some scripts (say Arabic), handling of
bi-directional texts that mix for instance Latin and Hebrew, algorithms
for sorting and string comparison, and much more.

   Currently, our Unicode package doesn't include any support for these
algorithms.

   Unicode and ISO 10646 define formally a 31-bit character set.
However, of this huge code space, so far characters have been assigned
only to the first 65534 positions (0x0000 to 0xFFFD). The characters
that are expected to be encoded outside the 16-bit range belong all to
rather exotic scripts (e.g., Hieroglyphics) that are only used by
specialists for historic and scientific purposes

   The Unicode module contains a set of packages to provide conversion
from some of the most common character sets to and from Unicode.  These
are the Unicode.CCS.* packages.

   All these packages have a common structure:

  1. They define a global variable of type 'Character_Set' with two
     fields, ie the two conversion functions between the given character
     set and Unicode.

     These functions convert one character (actually its code point) at
     a time.

  2. They also define a number of standard names associated with this
     character set.  For instance, the ISO/8859-1 set is also known as
     Latin1.

     The function 'Unicode.CCS.Get_Character_Set' can be used to find a
     character set by its standard name.

   Currently, the following sets are supported:
ISO/8859-1 aka Latin1
     This is the standard character set used to represent most Western
     European languages including: Albanian, Catalan, Danish, Dutch,
     English, Faroese, Finnish, French, Galician, German, Irish,
     Icelandic, Italian, Norwegian, Portuguese, Spanish and Swedish.

ISO/8859-2 aka Latin2
     This character set supports the Slavic languages of Central Europe
     which use the Latin alphabet.  The ISO-8859-2 set is used for the
     following languages: Czech, Croat, German, Hungarian, Polish,
     Romanian, Slovak and Slovenian.

ISO/8859-3
     This character set is used for Esperanto, Galician, Maltese and
     Turkish

ISO/8859-4
     Some letters were added to the ISO-8859-4 to support languages such
     as Estonian, Latvian and Lithuanian.  It is an incomplete precursor
     of the Latin 6 set.

2.4 Character encoding schemes
==============================

We now know how each encoded character can be represented by an integer
value (code point) depending on the character set.

   Character encoding schemes deal with the representation of a sequence
of integers to a sequence of code units.  A code unit is a sequence of
bytes on a computer architecture.

   There exists a number of possible encoding schemes.  Some of them
encode all integers on the same number of bytes.  They are called
fixed-width encoding forms, and include the standard encoding for
Internet emails (7bits, but it can't encode all characters), as well as
the simple 8bits scheme, or the EBCDIC scheme.  Among them is also the
UTF-32 scheme which is defined in the Unicode standard.

   Another set of encoding schemes encode integers on a variable number
of bytes.  These include two schemes that are also defined in the
Unicode standard, namely Utf-8 and Utf-16.

   Unicode doesn't impose any specific encoding.  However, it is most
often associated with one of the Utf encodings.  They each have their
own properties and advantages:

Utf32
     This is the simplest of all these encodings.  It simply encodes all
     the characters on 32 bits (4 bytes).  This encodes all the possible
     characters in Unicode, and is obviously straightforward to
     manipulate.  However, given that the first 65535 characters in
     Unicode are enough to encode all known languages currently in use,
     Utf32 is also a waste of space in most cases.

Utf16
     For the above reason, Utf16 was defined.  Most characters are only
     encoded on two bytes (which is enough for the first 65535 and most
     current characters).  In addition, a number of special code points
     have been defined, known as surrogate pairs, that make the encoding
     of integers greater than 65535 possible.  The integers are then
     encoded on four bytes.  As a result, Utf16 is thus much more
     memory-efficient and requires less space than Utf32 to encode
     sequences of characters.  However, it is also more complex to
     decode.

Utf8
     This is an even more space-efficient encoding, but is also more
     complex to decode.  More important, it is compatible with the most
     currently used simple 8bit encoding.

     Utf8 has the following properties:
        * Characters 0 to 127 (ASCII) are encoded simply as a single
          byte.  This means that files and strings which contain only
          7-bit ASCII characters have the same encoding under both ASCII
          and UTF-8.

        * Characters greater than 127 are encoded as a sequence of
          several bytes, each of which has the most significant bit set.
          Therefore, no ASCII byte can appear as part of any other
          character.

        * The first byte of a multibyte sequence that represents a
          non-ASCII character is always in the range 0xC0 to 0xFD and it
          indicates how many bytes follow for this character.  All
          further bytes in a multibyte sequence are in the range 0x80 to
          0xBF. This allows easy resynchronization and makes the
          encoding stateless and robust against missing bytes.

        * UTF-8 encoded characters may theoretically be up to six bytes
          long, however the first 16-bit characters are only up to three
          bytes long.

   Note that the encodings above, except for Utf8, have two versions,
depending on the chosen byte order on the machine.

   The Ada95 Unicode module provides a set of packages that provide an
easy conversion between all the encoding schemes, as well as basic
manipulations of these byte sequences.  These are the Unicode.CES.*
packages.  Currently, four encoding schemes are supported, the three Utf
schemes and the basic 8bit encoding which corresponds to the standard
Ada strings.

   It also supports some routines to convert from one byte-order to
another.

   The following examples show a possible use of these packages:

     Converting a latin1 string coded on 8 bits to a Utf8 latin2 file
     involves the following steps:

        Latin1 string  (bytes associated with code points in Latin1)
          |    "use Unicode.CES.Basic_8bit.To_Utf32"
          v
        Utf32 latin1 string (contains code points in Latin1)
          |    "Convert argument to To_Utf32 should be
          v         Unicode.CCS.Iso_8859_1.Convert"
        Utf32 Unicode string (contains code points in Unicode)
          |    "use Unicode.CES.Utf8.From_Utf32"
          v
        Utf8 Unicode string (contains code points in Unicode)
          |    "Convert argument to From_Utf32 should be
          v         Unicode.CCS.Iso_8859_2.Convert"
        Utf8 Latin2 string (contains code points in Latin2)

2.5 Unicode_Encoding
====================

XML/Ada groups the two notions of character sets and encoding schemes
into a single type, 'Unicode.Encodings.Unicode_Encoding'.

   This package provides additional functions to manipulate these
encodings, for instance to retrieve them by the common name that is
associated with them (for instance "utf-8", "iso-8859-15",...), since
very often the encoding scheme is implicit.  If you are speaking of
utf-8 string, most people always assume you also use the unicode
character set.  Likewise, if you are speaking of "iso-8859-1", most
people will assume you string is encoded as 8 byte characters.

   The goal of the 'Unicode.Encodings' package is to make these implicit
associations more obvious.

   It also provides one additional function 'Convert', which can be used
to convert a sequence of bytes from one encoding to another.  This is a
convenience function that you can use when for instance creating DOM
trees directly through Ada calls, since XML/Ada excepts all its strings
to be in utf-8 by default.

2.6 Misc. functions
===================

The package Unicode contains a series of 'Is_*' functions, matching the
Unicode standard.

Is_White_Space
     Return True if the character argument is a space character, ie a
     space, horizontal tab, line feed or carriage return.

Is_Letter
     Return True if the character argument is a letter.  This includes
     the standard English letters, as well as some less current cases
     defined in the standard.

Is_Base_Char
     Return True if the character is a base character, ie a character
     whose meaning can be modified with a combining character.

Is_Digit
     Return True if the character is a digit (numeric character)

Is_Combining_Char
     Return True if the character is a combining character.  Combining
     characters are accents or other diacritical marks that are added to
     the previous character.

     The most important accented characters, like those used in the
     orthographies of common languages, have codes of their own in
     Unicode to ensure backwards compatibility with older character
     sets.  Accented characters that have their own code position, but
     could also be represented as a pair of another character followed
     by a combining character, are known as precomposed characters.
     Precomposed characters are available in Unicode for backwards
     compatibility with older encodings such as ISO 8859 that had no
     combining characters.  The combining character mechanism allows to
     add accents and other diacritical marks to any character

     Note however that your application must provide specific support
     for combining characters, at least if you want to represent them
     visually.

Is_Extender
     True if Char is an extender character.

Is_Ideographic
     True if Char is an ideographic character.  This is defined only for
     Asian languages.

3 The Input module
******************

This module provides a set of packages with a common interface to access
the characters contained in a stream.  Various implementations are
provided to access files and manipulate standard Ada strings.

   A top-level tagged type is provided that must be extended for the
various streams.  It is assumed that the pointer to the current
character in the stream can only go forward, and never backward.  As a
result, it is possible to implement this package for sockets or other
strings where it isn't even possible to go backward.  This also means
that one doesn't have to provide buffers in such cases, and thus that it
is possible to provide memory-efficient readers.

   Two predefined readers are available, namely 'String_Input' to read
characters from a standard Ada string, and 'File_Input' to read
characters from a standard text file.

   They all provide the following primite operations:

'Open'
     Although this operation isn't exactly overriden, since its
     parameters depend on the type of stream you want to read from, it
     is nice to use a standard name for this constructor.

'Close'
     This terminates the stream reader and free any associated memory.
     It is no longer possible to read from the stream afterwards.

'Next_Char'
     Return the next Unicode character in the stream.  Note this
     character doesn't have to be associated specifically with a single
     byte, but that it depends on the encoding chosen for the stream
     (see the unicode module documentation for more information).

     The next time this function is called, it returns the following
     character from the stream.

'Eof'
     This function should return True when the reader has already
     returned the last character from the stream.  Note that it is not
     guarantee that a second call to Eof will also return True.

   It is the responsability of this stream reader to correctly call the
decoding functions in the unicode module so as to return one single
valid unicode character.  No further processing is done on the result of
'Next_Char'.  Note that the standard 'File_Input' and 'String_Input'
streams can automatically detect the encoding to use for a file, based
on a header read directly from the file.

   Based on the first four bytes of the stream (assuming this is valid
XML), they will automatically detect whether the file was encoded as
Utf8, Utf16,...  If you are writing your own input streams, consider
adding this automatic detection as well.

   However, it is always possible to override the default through a call
to 'Set_Encoding'.  This allows you to specify both the character set
(Latin1, ...)  and the character encoding scheme (Utf8,...).

   The user is also encouraged to set the identifiers for the stream
they are parsing, through calls to 'Set_System_Id' and 'Set_Public_Id'.
These are used when reporting error messages.

4 The SAX module
****************

4.1 Description
===============

Parsing XML streams can be done with two different methods.  They each
have their pros and cons.  Although the simplest, and probably most
usual way to manipulate XML files is to represent them in a tree and
manipulate it through the DOM interface (see next chapter).

   The Simple API for XML is an other method that can be used for
parsing.  It is based on a callbacks mechanism, and doesn't store any
data in memory (unless of course you choose to do so in your callbacks).
It can thus be more efficient to use SAX than DOM for some specialized
algorithms.  In fact, this whole Ada XML library is based on such a SAX
parser, then creates the DOM tree through callbacks.

   Note that this module supports the second release of SAX (SAX2), that
fully supports namespaces as defined in the XML standard.

   SAX can also be used in cases where a tree would not be the most
efficient representation for your data.  There is no point in building a
tree with DOM, then extracting the data and freeing the tree occupied by
the tree.  It is much more efficient to directly store your data through
SAX callbacks.

   With SAX, you register a number of callback routines that the parser
will call them when certain conditions occur.

   This documentation is in no way a full documentation on SAX. Instead,
you should refer to the standard itself, available at
<http://sax.sourceforge.net>.

   Some of the more useful callbacks are 'Start_Document',
'End_Document', 'Start_Element', 'End_Element', 'Get_Entity' and
'Characters'.  Most of these are quite self explanatory.  The characters
callback is called when characters outside a tag are parsed.

   Consider the following XML file:

     <?xml version="1.0"?>
     <body>
       <h1>Title</h1>
     </body>

   The following events would then be generated when this file is
parsed:

     Start_Document           Start parsing the file
     Start_Prefix_Mapping     (handling of namespaces for "xml")
     Start_Prefix_Mapping     Parameter is "xmlns"
     Processing_Instruction   Parameters are "xml" and "version="1.0""
     Start_Element            Parameter is "body"
     Characters               Parameter is ASCII.LF & "  "
     Start_Element            Parameter is "h1"
     Characters               Parameter is "Title"
     End_Element              Parameter is "h1"
     Characters               Parameter is ASCII.LF & "  "
     End_Element              Parameter is "body"
     End_Prefix_Mapping       Parameter is "xmlns"
     End_Prefix_Mapping       Parameter is "xml"
     End_Document             End of parsing

   As you can see, there is a number of events even for a very small
file.  However, you can easily choose to ignore the events you don't
care about, for instance the ones related to namespace handling.

4.2 Examples
============

There are several cases where using a SAX parser rather than a DOM
parser would make sense.  Here are some examples, although obvisouly
this doesn't include all the possible cases.  These examples are taken
from the documentation of libxml, a GPL C toolkit for manipulating XML
files.

   * Using XML files as a database

     One of the common usage for XML files is to use them as a kind of
     basic database, They obviously provide a strongly structured
     format, and you could for instance store a series of numbers with
     the following format.

          <array> <value>1</value> <value>2</value> ....</array>

     In this case, rather than reading this file into a tree, it would
     obviously be easier to manipulate it through a SAX parser, that
     would directly create a standard Ada array while reading the
     values.

     This can be extended to much more complex cases that would map to
     Ada records for instance.

   * Large repetitive XML files

     Sometimes we have XML files with many subtrees of the same format
     describing different things.  An example of this is an index file
     for a documentation similar to this one.  This contains a lot
     (maybe thousands) of similar entries, each containing for instance
     the name of the symbol and a list of locations.

     If the user is looking for a specific entry, there is no point in
     loading the whole file in memory and then traverse the resulting
     tree.  The memory usage increases very fast with the size of the
     file, and this might even be unfeasible for a 35 megabytes file.

   * Simple XML files

     Even for simple XML files, it might make sense to use a SAX parser.
     For instance, if there are some known constraints in the input
     file, say there are no attributes for elements, you can save quite
     a lot of memory, and maybe time, by rebuilding your own tree rather
     than using the full DOM tree.

   However, there are also a number of drawbacks to using SAX:

   * SAX parsers generally require you to write a little bit more code
     than the DOM interface
   * There is no easy way to write the XML data back to a file, unless
     you build your own internal tree to save the XML. As a result, SAX
     is probably not the best interface if you want to load, modify and
     dump back an XML file.

     Note however that in this Ada implementation, the DOM tree is built
     through a set of SAX callbacks anyway, so you do not lose any power
     or speed by using DOM instead of SAX.

4.3 The SAX parser
==================

The basic type in the SAX module is the SAX.Readers package.  It defines
a tagged type, called 'Reader', that represents the SAX parser itself.

   Several features are define in the SAX standard for the parsers.
They indicate which behavior can be expected from the parser.  The
package 'SAX.Readers' defines a number of constant strings for each of
these features.  Some of these features are read-only, whereas others
can be modified by the user to adapt the parser.  See the 'Set_Feature'
and 'Get_Feature' subprograms for how to manipulate them.

   The main primitive operation for the parser is 'Parse'.  It takes an
input stream for argument, associated with some XML data, and then
parses it and calls the appropriate callbacks.  It returns once there
are no more characters left in the stream.

   Several other primitive subprograms are defined for the parser, that
are called the callbacks.  They get called automatically by the 'Parse'
procedure when some events are seen.

   As a result, you should always override at least some of these
subprograms to get something done.  The default implementation for these
is to do nothing, except for the error handler that raises Ada
exceptions appropriately.

   An example of such an implementation of a SAX parser is available in
the DOM module, and it creates a tree in memory.  As you will see if you
look at the code, the callbacks are actually very short.

   Note that internally, all the strings are encoded with a unique
character encoding scheme, that is defined in the file
'sax-encodings.ads'.  The input stream is converted on the fly to this
internal encoding, and all the subprograms from then on will receive and
pass parameters with this new encoding.  You can of course freely change
the encoding defined in the file 'sax-encodings.ads'.

   The encoding used for the input stream is either automatically
detected by the stream itself (*note The Input module::), or by parsing
the
        <?xml version='1.0' encoding='UTF-8' ?>
   processing instruction at the beginning of the document.  The list of
supported encodings is the same as for the Unicode module (*note The
Unicode module::).

4.4 The SAX handlers
====================

We do not intend to document the whole set of possible callbacks
associated with a SAX parser.  These are all fully documented in the
file 'sax-readers.ads'.

   here is a list of the most frequently used callbacks, that you will
probably need to override in most of your applications.

'Start_Document'
     This callback, that doesn't receive any parameter, is called once,
     just before parsing the document.  It should generally be used to
     initialize internal data needed later on.  It is also garanteed to
     be called only once per input stream.

'End_Document'
     This one is the reverse of the previous one, and will also be
     called only once per input stream.  It should be used to release
     the memory you have allocated in Start_Document.

'Start_Element'
     This callback is called every time the parser encounters the start
     of an element in the XML file.  It is passed the name of the
     element, as well as the relevant namespace information.  The
     attributes defined in this element are also passed as a list.
     Thus, you get all the required information for this element in a
     single function call.

'End_Element'
     This is the opposite of the previous callback, and will be called
     once per element.  Calls to 'Start_Element' and 'End_Element' are
     garanteed to be properly nested (ie you can't see the end of an
     element before seeing the end of all its nested children.

'Characters and Ignore_Whitespace'
     This procedure will be called every time some character not part of
     an element declaration are encounted.  The characters themselves
     are passed as an argument to the callback.  Note that the white
     spaces (and tabulations) are reported separately in the
     Ignorable_Spaces callback in case the XML attribute 'xml:space' was
     set to something else than 'preserve' for this element.

   You should compile and run the 'testsax' executable found in this
module to visualize the SAX events that are generated for a given XML
file.

4.5 Using SAX
=============

This section will guide you through the creation of a small SAX
application.  This application will read an XML file, assumed to be a
configuration file, and setup some preferences according to the contents
of the file.

   The XML file is the following:

     <?xml version="1.0" ?>
     <preferences>
        <pref name="pref1">Value1</pref>
        <pref name="pref2">Value2</pref>
     </preferences>

   This is a very simple example which doesn't use namespaces, and has a
very limited nesting of nodes.  However, that should help demonstrate
the basics of using SAX.

4.5.1 Parsing the file
----------------------

The first thing to do is to declare a parser, and parse the file.  No
callback is put in place in this first version, and as a result nothing
happens.

   The main program is the following:

     with Sax.Readers;        use Sax.Readers;
     with Input_Sources.File; use Input_Sources.File;
     with SaxExample;         use SaxExample;

     procedure SaxExample_Main is
        My_Reader : SaxExample.Reader;
        Input     : File_Input;
     begin
        Set_Public_Id (Input, "Preferences file");
        Set_System_Id (Input, "pref.xml");
        Open ("pref.xml", Input);

        Set_Feature (My_Reader, Namespace_Prefixes_Feature, False);
        Set_Feature (My_Reader, Namespace_Feature, False);
        Set_Feature (My_Reader, Validation_Feature, False);

        Parse (My_Reader, Input);

        Close (Input);
     end SaxExample_Main;

   A separate package is provided that contain our implementation of an
XML parser

     with Sax.Readers;

     package SaxExample is

        type Reader is new Sax.Readers.Reader with null record;

     end SaxExample;

   There are two steps in setting up an XML parser:

   * Create an input stream

     This input stream is in charge of providing the XML input to the
     parser.  Several input streams are provided by XML/Ada, including
     the one we use in this example to read the XML data from a file on
     the disk.  The file is called 'pref.xml'.

     It has two properties, that should generally be set: the public id
     will be used by XML/Ada in its error message to reference locations
     in that file; the system id should be the location of the file on
     the system.  It is used to resolve relative paths found in the XML
     document.

   * Setup the parser

     The behavior of an XML parser can be changed in several ways by
     activating or deactivating some features.  In the example above, we
     have specified that the XML document doesn't contain namespaces,
     and that we do not intend to validate the XML file against a
     grammar.

   Once the two steps above are done, we can simply call the procedure
'Parse' to perform the actual parsing.  Since we are using SAX, XML/Ada
will call the primitive operations of 'My_Reader', which, so far, are
inherited form the default ones provided by XML, and do nothing.

4.5.2 Reacting to events
------------------------

We are now going to enhance the example a little, and make it react to
the contents of the XML file.

   We are only interested in two particular type of events, which are
the opening and closing of an XML tag, and finding the value of each
preference.

   The way to react to these events is to override some of the primitive
subprograms in the package 'saxexample.ads' as follows:

     with Sax.Readers;
     with Unicode.CES;
     with Sax.Attributes;
     with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;

     package SaxExample is

        type String_Access is access String;

        type Reader is new Sax.Readers.Reader with record
            Current_Pref  : Unbounded_String;
            Current_Value : Unbounded_String;
        end record;

        procedure Start_Element
          (Handler       : in out Reader;
           Namespace_URI : Unicode.CES.Byte_Sequence := "";
           Local_Name    : Unicode.CES.Byte_Sequence := "";
           Qname         : Unicode.CES.Byte_Sequence := "";
           Atts          : Sax.Attributes.Attributes'Class);

        procedure End_Element
          (Handler : in out Reader;
           Namespace_URI : Unicode.CES.Byte_Sequence := "";
           Local_Name    : Unicode.CES.Byte_Sequence := "";
           Qname         : Unicode.CES.Byte_Sequence := "");

        procedure Characters
          (Handler : in out Reader;
           Ch      : Unicode.CES.Byte_Sequence);

     end SaxExample;

   The primitive operations will be called automatically when the
corresponding events are detected in the XML file.

   The implementation for these subprograms is detailed below.

4.5.2.1 Start of XML tags
.........................

When an XML tag is started, we need to check whether it corresponds to
the definition of a preference value.  If that is the case, we get the
value of the 'name' attribute, which specifies the name of a preference.

     with Unicode.CES;    use Unicode.CES;
     with Sax.Attributes; use Sax.Attributes;
     with Ada.Text_IO;    use Ada.Text_IO;

     package body SaxExample is

        procedure Start_Element
          (Handler       : in out Reader;
           Namespace_URI : Unicode.CES.Byte_Sequence := "";
           Local_Name    : Unicode.CES.Byte_Sequence := "";
           Qname         : Unicode.CES.Byte_Sequence := "";
           Atts          : Sax.Attributes.Attributes'Class)
        is
        begin
           Handler.Current_Pref  := Null_Unbounded_String;
           Handler.Current_Value := Null_Unbounded_String;

           if Local_Name = "pref" then
              Handler.Current_Pref :=
                To_Unbounded_String (Get_Value (Atts, "name"));
           end if;
        end Start_Element;

4.5.2.2 Characters
..................

XML/Ada will report the textual contents of an XML tag through one or
more calls to the 'Characters' primitive operation.  An XML parser is
free to divide the contents into as many calls to 'Characters' as it
needs, and we must be prepared to handle this properly.  Therefore, we
concatenate the characters with the current value.

        procedure Characters
          (Handler : in out Reader;
           Ch      : Unicode.CES.Byte_Sequence) is
        begin
           if Handler.Current_Pref /= Null_Unbounded_String then
              Handler.Current_Value := Handler.Current_Value & Ch;
           end if;
        end Characters;

4.5.2.3 End of tag
..................

Once we meet the end of a tag, we know there will be no more addition to
the value, and we can now set the value of the preference.  In this
example, we simply display the value on the standard output.

        procedure End_Element
          (Handler : in out Reader;
           Namespace_URI : Unicode.CES.Byte_Sequence := "";
           Local_Name    : Unicode.CES.Byte_Sequence := "";
           Qname         : Unicode.CES.Byte_Sequence := "")
        is
        begin
           if Local_Name = "pref" then
              Put_Line ("Value for """ & To_String (Handler.Current_Pref)
                        & """ is " & To_String (Handler.Current_Value));
           end if;
        end End_Element;

   In a real application, we would need to handle error cases in the XML
file.  Thankfully, most of the work is already done by XML/Ada, and the
errors will be reported as calls to the primitive operation
'Fatal_Error', which by default raises an exception.

4.6 Understanding SAX error messages
====================================

XML/Ada error messages try to be as explicit as possible.  They are not,
however, meant to be understood by someone who doesn't know XML.

   In addition to the location of the error (line and column in the
file), they might contain one of the following abbreviations:

   * '[WF]'

     This abbreviation indicates that the error message is related to a
     well-formedness issue, as defined in the XML standard.  Basically,
     the structure of the XML document is invalid, for instance because
     an open tag has never been closed.  Some of the error messages also
     indicate a more precise section in the XML standard.

   * '[VC]'

     This abbreviation indicates that the error message is related to an
     unsatisfied validity-constraint, as defined in the XML standard.
     The XML document is well formed, although it doesn't match the
     semantic rules that the grammar defines.  For instance, if you are
     trying to validate an XML document against a DTD, the document must
     contain a DTD that defines the name of the root element.

5 The DOM module
****************

DOM is another standard associated with XML, in which the XML stream is
represented as a tree in memory.  This tree can be manipulated at will,
to add new nodes, remove existing nodes, change attributes,...

   Since it contains the whole XML information, it can then in turn be
dump to a stream.

   As an example, most modern web browsers provide a DOM interface to
the document currently loaded in the browser.  Using javascript, one can
thus modify dynamically the document.  The calls to do so are similar to
the ones provided by XML/Ada for manipulating a DOM tree, and all are
defined in the DOM standard.

   The W3C commitee (<http://www.w3c.org>) has defined several version
of the DOM, each building on the previous one and adding several
enhancements.

   XML/Ada currently supports the second revision of DOM (DOM 2.0),
which mostly adds namespaces over the first revision.  The third
revision is not supported at this point, and it adds support for loading
and saving XML streams in a standardized fashion.

   Although it doesn't support DOM 3.0, XML/Ada provides subprograms for
doing similar things.

   Only the Core module of the DOM standard is currently implemented,
other modules will follow.

   Note that the 'encodings.ads' file specifies the encoding to use to
store the tree in memory.  Full compatibility with the XML standard
requires that this be UTF16, however, it is generally much more
memory-efficient for European languages to use UTF8.  You can freely
change this and recompile.

5.1 Using DOM
=============

In XML/Ada, the DOM tree is build through a special implementation of a
SAX parser, provided in the 'DOM.Readers' package.

   Using DOM to read an XML document is similar to using SAX: one must
setup an input stream, then parse the document and get the tree.  This
is done with a code similar to the following:

     with Input_Sources.File; use Input_Sources.File;
     with Sax.Readers;        use Sax.Readers;
     with DOM.Readers;        use DOM.Readers;
     with DOM.Core;           use DOM.Core;

     procedure DomExample is
        Input  : File_Input;
        Reader : Tree_Reader;
        Doc    : Document;
     begin
        Set_Public_Id (Input, "Preferences file");
        Open ("pref.xml", Input);

        Set_Feature (Reader, Validation_Feature, False);
        Set_Feature (Reader, Namespace_Feature, False);

        Parse (Reader, Input);
        Close (Input);

        Doc := Get_Tree (Reader);

        Free (Reader);
     end DomExample;

   This code is almost exactly the same as the code that was used when
demonstrating the use of SAX (*note Using SAX::).

   The main two differences are:

   * We no longer need to define our own XML reader, and we simply use
     the one provided in 'DOM.Readers'.
   * We therefore do not add our own callbacks to react to the XML
     events.  Instead, the last instruction of the program gets a handle
     on the tree that was created in memory.

   The tree can now be manipulated to get access to the value stored.
If we want to implement the same thing we did for SAX, the code would
look like:

     with Input_Sources.File; use Input_Sources.File;
     with Sax.Readers;        use Sax.Readers;
     with DOM.Readers;        use DOM.Readers;
     with DOM.Core;           use DOM.Core;
     with DOM.Core.Documents; use DOM.Core.Documents;
     with DOM.Core.Nodes;     use DOM.Core.Nodes;
     with DOM.Core.Attrs;     use DOM.Core.Attrs;
     with Ada.Text_IO;        use Ada.Text_IO;

     procedure DomExample2 is
        Input  : File_Input;
        Reader : Tree_Reader;
        Doc    : Document;
        List   : Node_List;
        N      : Node;
        A      : Attr;
        C      : Node;
     begin
        Set_Public_Id (Input, "Preferences file");
        Open ("pref.xml", Input);

        Set_Feature (Reader, Validation_Feature, False);
        Set_Feature (Reader, Namespace_Feature, False);

        Parse (Reader, Input);
        Close (Input);

        Doc := Get_Tree (Reader);

        List := Get_Elements_By_Tag_Name (Doc, "pref");

        for Index in 1 .. Length (List) loop
            N := Item (List, Index - 1);
            A := Get_Named_Item (Attributes (N), "name");
            Put_Line ("Value of """ & Value (A) & """ is "
                      & Node_Value (First_Child (N)));
        end loop;

        Free (List);

        Free (Reader);
     end DomExample2;

   The code is much simpler than with SAX, since most of the work is
done internally by XML/Ada.  In particular, for SAX we had to take into
account the fact that the textual contents of a node could be reported
in several events.  For DOM, the tree is initially normalized, ie all
text nodes are collapsed together when possible.

   This added simplicity has one drawback, which is the amount of memory
required to represent even a simple tree.

   XML/Ada optimizes the memory necessary to represent a tree by sharing
the strings as much as possible (this is under control of constants at
the beginning of 'dom-core.ads').  Still, DOM requires a significant
amount of information to be kept for each node.

   For really big XML streams, it might prove impossible to keep the
whole tree in memory, in which case ad hoc storage might be implemented
through the use of a SAX parser.  The implementation of
'dom-readers.adb' will prove helpful in creating such a parser.

5.2 Editing DOM trees
=====================

Once in memory, DOM trees can be manipulated through subprograms
provides by the DOM API.

   Each of these subprograms is fully documented both in the Ada specs
(the '*.ads' files) and in the DOM standard itself, which XML/Ada
follows fully.

   One important note however is related to the use of strings.  Various
subprograms allows you to set the textual content of a node, modify its
attributes,....  Such subprograms take a Byte_Sequence as a parameter.

   This Byte_Sequence must always be encoded in the encoding defined in
the package 'Sax.Encoding' (as described earlier, changing this package
requires recompiling XML/Ada).  By default, this is UTF-8.

   Therefore, if you need to set an attribute to a string encoded for
instance in iso-8859-15, you should use the subprogram
'Unicode.Encodings.Convert' to convert it appropriately.  The code would
thus look as follows:

       Set_Attribute (N, Convert ("å", From => Get_By_Name ("iso-8859-15")));

5.3 Printing DOM tress
======================

The standard DOM 2.0 does not define a common way to read DOM trees from
input sources, nor how to write them back to output sources.  This was
added in later revision of the standard (DOM 3.0), which is not yet
supported by XML/Ada.

   However, the package 'DOM.Core.Nodes' provides a 'Write' procedure
that can be used for that purpose.  It outputs a given DOM tree to an
Ada stream.  This stream can then be connected to a standard file on the
disk, to a socket, or be used to transform the tree into a string in
memory.

   An example is provided in the XML/Ada distribution, called
'dom/test/tostring.adb' which shows how you can create a stream to
convert the tree in memory, without going through a file on the disk

6 The Schema module
*******************

6.1 XML Grammars
================

There are several steps that applications must go through when they have
to use XML files:

   * Make sure the XML file is well-formed.

     This is a basic step where we ensure that XML tags are correctly
     nested, that closing tags have the same names as the matching
     opening tags, that attribute values are quoted,....  This
     corresponds to a syntactic parser in a compiler.

     This step does not depend on the application domain.  One file that
     is well-formed will always be so, no matter in what context you use
     it.

   * Make sure the contents of the XML file is semantically valid.

     Depending on the application domain, we must ensure that the
     content of the file makes sense.  This step is highly application
     dependent, and a file that is usable in one application might not
     be usable in another one.

     This is the phase in which the application needs to check whether a
     given XML file has all its required attributes, whether the
     children of an XML tag are the expected ones, whether the type of
     the attributes is valid,....

   * Use the XML file in the application.

     This is done through the already-described SAX or DOM parsers

   The first phase is mandatory, and necessarily enforced by XML/Ada.
You will not be able to access the contents of the XML file if it isn't
well-formed.

   The second phase is provided by the Schema module in XML/Ada.
Although such constraints can be checked at the application level, with
ad hoc code, it is generally easier to maintain a separate file that
describes the valid semantic contents of the file, that maintain
specific code when the semantic changes.  It is also difficult not to
forget special cases when doing the validating through a set of 'if'
statements in the Ada core.

   XML provides two ways to describe additional constraints that a file
must satisfy in order to be considered as valid.

   * DTD

     The Document Type Description is the original way to do this.  They
     come directly from the ancestor of XML, SGML. All XML parsers must
     parse the DTD, and report events if the user is using SAX. However,
     not all parsers are able to validate the document against a DTD
     (XML/Ada doesn't).

     Their use tend to greatly diminish.  Among their limitation are a
     limit capability to express constraints on the order of tag
     children, the fact they the DTD themselves are written in a
     separate language totally different from XML, and that users must
     learn as a result.

   * XML Schema

     The XML schemas are replacing the DTDs.  They are written in XML,
     and provide an extensive capability to describe what the XML
     document should look like.  In fact, almost all Ada types can be
     described in an XML schema, including range constraints, arrays,
     records, type inheritance, abstract types,....

     It is for instance possible to indicate that the value of a
     preference, in our example, must be a string of length 6.  Any
     other length will result in a validation error.

6.2 XML Schema Syntax
=====================

The Schema modules provides subprograms and types to parse an XML schema
and validate an XML document with this schema.

   This document does not provide a full documentation on the format of
XML Schemas.  This is extensive, has several obscure features, which,
although supported by XML/Ada, are of little use in most pratical uses.
We refer the reader to the first part of the XML Schema specification,
which is designed as a tutorial (<http://www.w3.org/TR/xmlschema-0/>).

   The typical extension for a schema file is '.xsd'.

   A schema file must be a valid XML file, and thus start with the usual
'<?xml version="1.0" ?>' line.  The root node must be named 'schema',
and belong to the namespace (<http://www.w3.org/2001/XMLSchema/>).  The
handling of namespaces is fairly powerful, but also complex.  A given
XML document might have nodes belonging to several namespaces, and thus
several schema files might have to be loaded, each defining one of the
namespaces.

   In the following simple example, we will not define our schema for a
specific namespace, and thus no special attribute is needed for the root
node.  Thus, our document will be organized as:

     <?xml version="1.0" ?>
     <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
         ... rest of the description goes here ...
     </xsd:schema>

   An XML schema does not enforce a specific root node in the XML
documents it validates.  However, it must define all the valid elements
that can be used in the XML file.  This is done through the '<element>'
tag, which takes one mandatory attribute, the name of the element we are
defining.

   The contents of the element is then defined in one of two ways:

   * Through a 'type' attribute.

     Schemas come with a number of predefined simple types.  A simple
     type is such that an element of that type accepts no child node,
     and that its contents must satisfy additional constraints (be an
     integer, a date, ...).

     Among the predefined simple type (which are all defined in the
     namespace <http://www.w3.org/2001/XMLSchema/>), one can find:
     'string', 'integer', 'byte', 'date', 'time', 'dateTime',
     'boolean',...

     If no additional constraint should be enforced on this simple type
     when applied to the element, the type of the element is given
     through a 'type' attribute, as in:

           <xsd:element name="tag1" type="xsd:string"  />
           <xsd:element name="tag2" type="xsd:boolean" />

     which would accept the following XML files

           <tag1>Any string is valid here</tag1>
          and
           <tag2>true</tag2>

     but not

            <tag2>String</tag2>

     As will be described later, it is possible to create new types in
     XML schema, which are created with a name.  Such new types can also
     be associated with the element through the 'type' attribute.

   * Through an inline type definition

     If the element must accept child elements, or if a further
     constraint needs to be enforced on the list of valid values, one
     must create the type.  As mentioned above, this can be done by
     creating a type separately and referencing it by name, or through
     an inline type definition.

     The syntax is mostly the same in both cases.  Schemas distinguish
     between the notion of simple types (that accept no child element)
     and complex types (that accept child elements, and possibly text
     value).

     To define a simple type, based on string, but that only allows a
     limited set of value (similar to an Ada enumeration), one would
     create a restriction of the standard string type, as in:

           <xsd:element name="tag3">
            <xsd:simpleType>
              <xsd:restriction base="xsd:string">
                <xsd:enumeration value="value1" />
                <xsd:enumeration value="value2" />
              </xsd:restriction>
            </xsd:simpleType>
           </xsd:element>

     Similarly, we could create an integer type whose valid range of
     values is between 10 and 20, as in:

           <xsd:element name="tag4">
            <xsd:simpleType>
              <xsd:restriction base="xsd:byte">
                <xsd:minInclusive value="10" />
                <xsd:maxInclusive value="20" />
              </xsd:restriction>
            </xsd:simpleType>
           </xsd:element>

     Complex types allow elements to have child nodes, as well as
     attributes.  The list of valid attributes is created by a set of
     '<xsd:attribute>' tags, and the list of valid child nodes is
     generally defined either through a '<xsd:choice>' or a
     '<xsd:sequence>' node (although it is possible to indicate that any
     child node is authorized, among other things).

     '<xsd:choice>' indicate the children can appear in any order,
     whereas '<xsd:sequence>' enforces a specific order on children.

     In both cases, extra attributes can be specified to indicate the
     number of times the sequence or choice itself can be repeated, or
     that each child node can appear.

     For instance, we can indicate that 'tag5' accepts between 1 and 4
     child nodes, chosen among 'tag6' and 'tag7', but that the latter,
     if present, can only appear once.  In addition, tag5 accepts one
     optional attribute.  Note that the type of tag6 and tag7 is here
     specified through a 'type' attribute, although it could in turn be
     defined inline.

           <xsd:element name="tag5">
             <xsd:complexType>
               <xsd:choice>
                 <xsd:element name="tag6" type="xsd:string"
                              minOccurs="1" maxOccurs="3/>
                 <xsd:element name="tag7" type="xsd:string" maxOccurs="1" />
               </xsd:choice>
               <xsd:attribute name="attr" type="xsd:boolean" use="optional" />
             </xsd:complexType>
           </xsd:element>

     In the example above, if 'tag6' was defined elsewhere in the
     schema, we could use a reference to it, instead of duplicating its
     type definition, as in:

            <xsd:element ref="tag6" />

     If you need an element with no child element (just a string value),
     but that accepts attributes, this also must be defined through a
     complex type, as in:

           <xsd:element name="tag8" />
             <xsd:complexType>
               <xsd:simpleContent>
                 <xsd:extension base="xsd:string">
                    <xsd:attribute name="attr" type="xsd:boolean" />
                 </xsd:extension>
               </xsd:simpleContent>
             </xsd:complexType>
           </xsd:element>

   As mentioned before, instead of defining inline types, we could
explicitly declare them, and reference them in the element declaration
later on.

      <xsd:simpleType name="string_of_length_10">
        <xsd:restriction base="xsd:string" />
          <xsd:length value="10"/>
        </xsd:restriction>
      </xsd:simpleType>
      <xsd:element name="tag9" type="string_of_length_10" />

6.3 Connecting XML documents and schemas
========================================

There are several ways that XML/Ada uses to find what schema to use when
validating a file.

   * Manually creating the grammar.

     The schema module contains the package 'Schema.Validators' which
     allows you to create a grammar by hand.  It is very low-level, and
     it is likely that you will never need to use it.  It is used
     internally mostly, and when creating the schema which is used to
     validate schema files themselves.

   * Explicitly parsing a schema file

     Parsing a schema file can be done through a call to parse for a
     reader derived from 'Schema.Schema_Readers.Schema_reader'.  As
     usual, you call 'Parse', and pass it an input source.  As output,
     you get access to a grammar, that can then be given to another
     instance of a 'Schema.Readers.Validating_Reader'.

     This technique will generally be used when you need to validate
     several XML files with the same grammar: you parse the grammar only
     once, and then reuse its instance, instead of reparsing the '.xsd'
     file every time.

          with Schema.Schema_Readers, Schema.Validators, Input_Sources.File;
          use  Schema.Schema_Readers, Schema.Validators, Input_Sources.File;
          with Schema.Schema_Grammar;  use  Schema.Schema_Grammar;

          procedure SchemaExample2 is
             Grammar : XML_Grammar;
             Schema  : Schema_Reader;
             Read    : File_Input;
          begin
               Open ("file.xsd", Read);
               Parse (Schema, Read);
               Close (Read);

               Grammar := Get_Grammar (Schema);
          end SchemaExample2;

     In the example above, the schema file itself is validated against
     the official schema for schema files.

     The resulting grammar object is in fact a collection of parsed
     schema files, each associated with its own namespace.  It can be
     kept as long as you need it in your application.  Memory will
     automatically be reclaimed when no longer needed.

     Every time you parse an XML file later on, you must associated the
     Grammar with the parser.

           declare
              Read      : File_Input;
              My_Reader : Validating_Reader;
           begin
              Set_Grammar (My_Reader, Grammar);
              Set_Feature (My_Reader, Schema_Validation_Feature, True);
              Open (Xml_File.all, Read);
              Parse (My_Reader, Read);
              Close (Read);
           end;

   * Implicitly parsing the schema

     Two special attributes, defined in the Schema standard, can be used
     to indicate, in an XML document itself, that it should be validated
     with a specific schema.

     These attributes are both defined in a special namespace,
     <http://www.w3.org/2001/XMLSchema-instance>.

        - 'xsi:noNamespaceSchemaLocation'

          The value of this attribute is the name of a file that
          contains the schema to use for elements that are not
          associated with a specific namespace.

        - 'xsi:schemaLocation'

          This attribute is a list of strings, alternatively the prefix
          of a namespace and the name of an xsd file to use for that
          namespace.  For instance, '"ns1 file1.xsd ns2 file2.xsd"'.

     When it encounters any of these two attributes, XML/Ada will
     automatically parse the corresponding schema files, and use the
     result to validate the file.

6.4 Validating documents with SAX
=================================

XML/Ada is quite unique in the category of XML parsers, since it allows
the validation of XML files when you are using an event-based parser
with SAX. Most other XML parsers only work on DOM trees.

   Basing the validation on SAX is more efficient, since there is no
need to read the whole XML stream (or even the grammar) in memory before
starting the validation, and errors can be reported immediatly.

   It also requires less memory to run, and thus can validate large XML
documents.

   It also means that even if you are using SAX, and not DOM, you still
have access to the validation features.

   Validating a XML document while parsing it is basically done the same
as when using SAX itself.  Instead of inheriting from
'Sax.Readers.Reader', your tagged type must inherit from
'Schema.Readers.Validating_Reader'.

   As usual, you can still override the predefined primitive operations
like 'Start_Element', 'End_Element', ...

   Note the activation of the 'Schema_Validation_Feature' feature,
without which no validation takes place.

     with Sax.Readers;        use Sax.Readers;
     with Schema.Readers;     use Schema.Readers;
     with Input_Sources.File; use Input_Sources.File;

     procedure SchemaExample is
        Input : File_Input;
        My_Reader : Validating_Reader;
     begin
        Set_Public_Id (Input, "Preferences file");
        Open ("pref.xml", Input);

        Set_Feature (My_Reader, Schema_Validation_Feature, True);
        Parse (My_Reader, Input);

        Close (Input);
     end SchemaExample;

6.5 Validating documents with DOM
=================================

This is very similar to using DOM itself, except the base class of your
reader should be 'Schema.Dom_Readers.Tree_Reader'.  Going back to the
example described in *Note Using DOM::, you would use the following to
validate XML streams before generating the DOM tree

     with Input_Sources.File; use Input_Sources.File;
     with Sax.Readers;        use Sax.Readers;
     with DOM.Core;           use DOM.Core;
     with Schema.Dom_Readers; use Schema.Dom_Readers;

     procedure DomSchemaExample is
        Input  : File_Input;
        Reader : Schema.Dom_Readers.Tree_Reader;
        Doc    : Document;
     begin
        Set_Public_Id (Input, "Preferences file");
        Open ("pref.xml", Input);

        Set_Feature (Reader, Validation_Feature, False);
        Set_Feature (Reader, Namespace_Feature, False);

        Parse (Reader, Input);
        Close (Input);

        Doc := Get_Tree (Reader);

        Free (Reader);
     end DomSchemaExample;

6.6 Unsupported schema elements
===============================

Not all aspects of XML schemas are supported by XML/Ada.  In particular,
it does not currently support XPath, so any part of the schema that is
related to XPath expressions (for instance '<xsd:key>' and
'<xsd:unique>') are not supported currently.

7 Using the library
*******************

XML/Ada is a library.  When compiling an application that uses it, you
thus need to specify where the spec files are to be found, as well as
where the libraries are installed.

   There are several ways to do it:

   * The simplest is to use the 'xmlada-config' script, and let it
     provide the list of switches for 'gnatmake'.  This is more
     convenient on Unix systems, where you can simply compile your
     application with

          gnatmake main.adb `xmlada-config`

     Note the use of backticks.  This means that 'xmlada-config' is
     first executed, and then the command line is replaced with the
     output of the script, thus finally executing something like:

          gnatmake main.adb -Iprefix/include/xmlada -largs -Lprefix/lib \
            -lxmlada_input_sources -lxmlada_sax -lxmlada_unicode -lxmlada_dom

     Unfortunately, this behavior is not available on Windows (unless of
     course you use a Unix shell).  The simplest in that case is to
     create a 'Makefile', to be used with the 'make' command, and
     copy-paste the output of 'xmlada-config' into it.

     'xmlada-config' has several switches that might be useful:

       1. '--sax': If you this flag, your application will not be linked
          against the DOM module.  This might save some space,
          particularly if linking statically.

       2. '--static': Return the list of flags to use to link your
          application statically against Xml/Ada.  Your application is
          then standalone, and you don't need to distribute XMl/Ada at
          the same time.

       3. '--static_sax': Combines both of the above flags.

     If you are working on a big project, particularly one that includes
     sources in languages other than Ada, you generally have to run the
     three steps of the compilation process separately (compile, bind
     and then link).  'xmlada-config' can also be used, provided you use
     one of the following switches:

       1. '--cflags': This returns the compiler flags only, to be used
          for instance with 'gcc'.

       2. '--libs': This returns the linker flags only, to be used for
          instance with 'gnatlink'.

     This 'xmlada-config' method doesn't provide access to the 'xml_gtk'
     module, which is only available when using project files (see
     below).

   * The preferred method, however, is to use the GNAT project files.
     See the GNAT user's guide for more information on the project files
     and how to create them for your application.

     Basically, a project file contains the description of your build
     environment (source directories, object directories,
     libraries,...).

     The very simple case is when you have all your sources in the same
     directory (say 'src/'), and the object files are all generated in
     the 'obj/' directory.

     In this case, your project file would look like:

             with "xmlada";
             project Default is
                for Source_Dirs use ("src/");
                for Object_Dir use "obj/";
             end Default;
   

     and you build your application with

             gnatmake -Pdefault main.adb
   

     Note in the project file the first line, which indicates that your
     application requires XML/Ada to build.  This will automatically set
     the appropriate compiler and linker switches to use XML/Ada.  Your
     application will be linker against all modules of XML/Ada (DOM,
     SAX, ...).

     If your application doesn't use DOM, you can replace the first line
     with something like:

             with "xmlada_sax";
   

     which will reduce the number of libraries that your application is
     linked with.

     WHen you are using project files, you need to let GNAT know where
     to find the project files.  This is done by setting the
     'ADA_PROJECT_PATH' environment variable, by adding to it the
     installation directory of XML/Ada, ie the one that contains
     xmlada.gpr

     If the installation prefix is the same as your GNAT installation,
     and you are using GNAT more recent than 5.03a, then it will
     automatically find XML/Ada's project files.

     Check the 'dom/test' directory in the XML/Ada package, which
     contains both code examples and project files that you can use as a
     basic for your own code.

   The default type of library depends on the way you installed XML/Ada.
In all cases, and assuming you installed both static and shared
libraries, you can choose among the two by setting the environment
variable: LIBRARY_TYPE=static or LIBRARY_TYPE=relocatable

   Whatever method you used to build your application, you might have to
change, at least one UNIX systems, the environment variable
'LD_LIBRARY_PATH' so that it contains the 'lib/' directory in the
XML/Ada installation, so that the dynamic libraries are correctly found.

   This is not needed if you build XML/Ada as a static directory.

7.1 Running on VxWorks
======================

On VxWorks, XML Ada processing might require more stack space than what
is typically available from the VxWorks shell, the tasks spawned from
there with "sp", or Ada tasks with no or a too small Storage_Size value
attached.

   Such stack overflow conditions are typically characterized by
non-deterministic erratic behavior and can be cured by allocating more
stack space for the tasks involved.

