<chapter id="ch-parse">
<?dbhtml filename="ch03.html"?>
<chapterinfo
><pubdate
>$Date$</pubdate
> <releaseinfo
>$Revision$</releaseinfo
> </chapterinfo>
<title
>Обработка документов DocBook</title>
<para>
<indexterm id="parsDocch4" class="startofrange"
><primary
>обработка</primary>
  <secondary
>документы DocBook</secondary
></indexterm>
<indexterm id="docDTDparsch4" class="startofrange"
><primary
>DocBook DTD</primary>
  <secondary
>синтаксический анализ документов</secondary
></indexterm>
Ключевое отличие разметок &SGML; и &XML; состоит в их <emphasis
>строгости</emphasis
>. DocBook &DTD; - это точное описание действительной вложенности, порядка элементов и их содержания. Все документы DocBook должны строго отвечать его описанию. </para>
<para>
<indexterm
><primary
>анализаторы</primary>
  <secondary
>проверка</secondary
></indexterm>
<indexterm
><primary
>validation</primary>
  <secondary
>анализаторы</secondary
></indexterm>
<firstterm
>Анализатор</firstterm
> (<firstterm
>validating parser</firstterm
>) - это программа, проверяющая соответствие документа указаной &DTD; (вложенность, порядок элементов). </para>
<para>
<indexterm
><primary
>текстовые редакторы</primary>
  <secondary
>неструктурированный анализ</secondary
></indexterm>
If you are not using a structured editor that can enforce the markup as you type, validation with an external parser is a particularly important step in the document creation process. You cannot expect to get rational results from subsequent processing (such as document publishing) if your documents are not valid. </para>
<para>
<indexterm
><primary
>SP nsgmls parser</primary
><see
>nsgmls parser</see
></indexterm>
The most popular free &SGML; parser is <acronym
>SP</acronym
> by James Clark, available at <ulink url="http://www.jclark.com/"
>http://www.jclark.com/</ulink
>. </para>
<para>
<indexterm
><primary
>nsgmls parser</primary
></indexterm>
<indexterm
><primary
>анализаторы</primary>
  <secondary
>free</secondary
></indexterm>
<indexterm
><primary
>XML</primary>
  <secondary
>parsers, free</secondary
></indexterm>
<indexterm
><primary
>xp parser</primary
></indexterm
> <acronym
>SP</acronym
> includes <command
>nsgmls</command
>, a fast command-line parser. In the world of free validating &XML; parsers, James Clark's <command
>xp</command
> is a popular choice. </para>
<note>
<para>
<indexterm
><primary
>анализаторы</primary>
  <secondary
>non-validating</secondary
></indexterm>
Not all &XML; parsers are validating, and although a non-validating parser may have many uses, it cannot ensure that your documents are valid according to the &DTD;.</para>
</note>
<sect1 id="ch03-validate"
><title
>Validating Your Documents</title>
<para
>The exact way in which the parser is executed varies according to the parser in use, naturally. For information about your particular parser, consult the documentation that came with it. </para>

<sect2
><title
>Using nsgmls</title>
<para>
<indexterm
><primary
>nsgmls parser</primary>
  <secondary
>SGML documents, validating</secondary
></indexterm>
The <command
>nsgmls</command
> command from <acronym
>SP</acronym
> is a validating &SGML; parser. The options used in the example below suppress the normal output (<option
>-s</option
>), except for error messages, print the version number (<option
>-v</option
>), and specify the catalog file that should be used to map public identifiers to system identifiers. Printing the version number guarantees that you always get <emphasis
>some</emphasis
> output, so that you know the command ran successfully: </para>
<screen
>[n:\dbtdg] nsgmls -sv -c \share\sgml\catalog test.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
</screen>
<para>
<indexterm
><primary
>error messages</primary>
  <secondary
>nsgmls parser</secondary
></indexterm>
<indexterm
><primary
>options</primary>
  <secondary
>error messages (parser), directing to file</secondary
></indexterm>
Because no error messages were printed, we know our document is valid. If you're working with a document that you discover has many errors, the <option
>-f</option
> option offers a handy way to direct the errors to a file so they don't all scroll off your screen. </para>
<para>
<indexterm
><primary
>nsgmls parser</primary>
  <secondary
>XML documents, validating</secondary
></indexterm>
<indexterm
><primary
>declarations</primary>
  <secondary
>nsgmls parser</secondary
></indexterm>
<indexterm
><primary
>xml.dcl declaration (nsgmls parser)</primary
></indexterm>
<indexterm
><primary
>XML</primary>
  <secondary
>nsgmls parser</secondary
></indexterm>
If you want to validate an &XML; document with <acronym
>SP</acronym
>, you must make sure that <acronym
>SP</acronym
> uses the correct declaration. An &XML; declaration called <filename
>xml.dcl</filename
> is included with <acronym
>SP</acronym
>. </para>

<para
>The easiest way to make sure that <acronym
>SP</acronym
> uses <filename
>xml.dcl</filename
> is to include the declaration explicitly on the command line when you run <command
>nsgmls</command
> (or Jade, or other <acronym
>SP</acronym
> tools): </para>
<screen
>[n:\dbtdg] nsgmls -sv -c \share\sgml\catalog m:\jade\xml.dcl test.xml
m:\jade\nsgmls.exe:I: SP version "1.3.2"
</screen>
</sect2>

<sect2
><title
>Using xp</title>
<para>
<indexterm
><primary
>xp parser</primary
></indexterm>
<indexterm
><primary
>Time program (xp parser)</primary
></indexterm
> The <application
>xp</application
> distribution includes several sample programs. One of these programs, <command
>Time</command
>, performs a validating parse of the document and prints the amount of time required to parse the &DTD; and the document. This program makes an excellent validity checker: </para>
<screen
>java com.jclark.xml.apps.Time examples\simple.xml
6.639
</screen>
<para
>The result states that it took 6.639 seconds to parse the &DTD; and the document. This indicates that the document is valid. If the document is invalid, additional error messages are displayed. </para>
</sect2>
</sect1>
<sect1 id="ch03-parseerr"
><title
>Understanding Parse Errors</title>
<para>
<indexterm id="errorparsech4" class="startofrange"
><primary
>errors</primary>
  <secondary
>parsing</secondary
></indexterm>

<indexterm
><primary
>обработка</primary>
  <secondary
>errors</secondary
></indexterm>
<indexterm
><primary
>error messages</primary>
  <secondary
>анализаторы</secondary
></indexterm>
Every parser produces slightly different error messages, but most indicate exactly (at least technically)<footnote
> <para
> It is often the case that you can correct an error in the document in several ways. The parser suggests one possible fix, but this is not always the right fix. For example, the parser may suggest that you can correct out of context data by adding another element, when in fact it's &ldquo;obvious&rdquo; to human eyes that the problem is a missing end tag. </para
> </footnote
> what is wrong and where the error occurred. With a little experience, this information is all you'll need to quickly identify what's wrong. </para>
<para>

<indexterm
><primary
>Jade</primary>
  <secondary
>SP parser, using</secondary
></indexterm>
In the rest of this section, we'll look at a number of common errors and the messages they produce in <acronym
>SP</acronym
>. We've chosen <acronym
>SP</acronym
> for the rest of these examples because that is the same parser used by Jade, which we'll be discussing further in <xref linkend="ch-publish"/>. </para>
<sect2
><title
>&DTD; Cannot Be Found</title>
<para>
<indexterm
><primary
>cannot be found errors</primary
></indexterm>
<indexterm
><primary
>DTDs</primary>
  <secondary
>cannot be found errors</secondary
></indexterm>
<indexterm
><primary
>public identifiers</primary>
  <secondary
>errors</secondary
></indexterm>
The telltale sign that SP could not find the &DTD;, or some module of the &DTD;, is the error message: "cannot generate system identifier for public text &hellip;". Generally, the errors that occur after this are spurious; if SP couldn't find some part of the &DTD;, it's likely to think that <emphasis
>everything</emphasis
> is wrong. </para>
<para
>Careful examination of the following document will show that we've introduced a simple typographic error into the public identifier (the word <quote
>DocBook</quote
> is misspelled with a lowercase <quote
>b</quote
>): </para>
<programlisting
><inlinegraphic format="linespecific" fileref="examples/errs/nodtd.sgm"/>
</programlisting>
<para
>SP responds dramatically to this error: </para>
<screen
>hermes:/documents/books/tdg/examples/errs$ nsgmls -sv -c cat1 /usr/lib/sgml/declaration/xml.dcl nodtd.sgm
nsgmls:I: SP version "1.3.4"
nsgmls:nodtd.sgm:2:76:E: could not resolve host "www.oasis-open.org" (try again later)
nsgmls:nodtd.sgm:2:76:E: DTD did not contain element declaration for document type name
nsgmls:nodtd.sgm:3:8:E: element "chapter" undefined
nsgmls:nodtd.sgm:3:15:E: element "title" undefined
nsgmls:nodtd.sgm:4:5:E: element "para" undefined
nsgmls:nodtd.sgm:10:5:E: element "para" undefined
nsgmls:nodtd.sgm:11:15:E: there is no attribute "role"
nsgmls:nodtd.sgm:11:21:E: element "emphasis" undefined
nsgmls:nodtd.sgm:12:9:E: element "emphasis" undefined
nsgmls:nodtd.sgm:12:24:E: element "emphasis" undefined
nsgmls:nodtd.sgm:13:18:E: element "superscript" undefined
nsgmls:nodtd.sgm:14:16:E: element "subscript" undefined
nsgmls:nodtd.sgm:16:5:E: element "para" undefined
</screen>
<para>
<indexterm
><primary
>catalog files</primary>
  <secondary
>errors</secondary
></indexterm>
Other things to look for, if you haven't misspelled the public identifier, are typos in the catalog or failure to specify a catalog that resolves the public identifier that can't be found. </para>
</sect2>
<sect2
><title
><acronym
>ISO</acronym
> Entity Set Missing</title>
<para>

<indexterm
><primary
>entity sets</primary>
  <secondary
>missing</secondary
></indexterm>
A missing entity set is another example of either a misspelled public identifier, or a missing catalog or catalog entry. </para>
<para
>In this case, there's nothing wrong with the document, but the catalog that's been specified is missing the public identifiers for the <acronym
>ISO</acronym
> entity sets: </para>
<screen
>[n:\dbtdg]nsgmls -sv -c examples\errs\cat2 examples\simple.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:53:65:W: cannot generate system identifier for public text "ISO 8879:1986//ENTITIES Added Math Symbols:Arrow Relations//EN"
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:54:8:E: reference to entity "ISOamsa" for which no system identifier could be generated
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:52:0: entity was defined here
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:60:66:W: cannot generate system identifier for public text "ISO 8879:1986//ENTITIES Added Math Symbols:Binary Operators//EN"
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:61:8:E: reference to entity "ISOamsb" for which no system identifier could be generated
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:59:0: entity was defined here
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:67:60:W: cannot generate system identifier for public text "ISO 8879:1986//ENTITIES Added Math Symbols:Delimiters//EN"
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:68:8:E: reference to entity "ISOamsc" for which no system identifier could be generated
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:66:0: entity was defined here
m:\jade\nsgmls.exe:n:/share/sgml/docbook/3.1/dbcent.mod:74:67:W: cannot generate system identifier for public text "ISO 8879:1986//ENTITIES Added Math Symbols:Negated Relations//EN"
...
</screen>
<para>
<indexterm
><primary
>entity sets</primary>
  <secondary
>locating</secondary
></indexterm>
The <acronym
>ISO</acronym
> entity sets are required by the DocBook &DTD;, but they are not distributed with it. That's because they aren't maintained by <acronym
>OASIS</acronym
>.<footnote
> <para
>If you need to locate the entity sets, consult <ulink url="http://www.oasis-open.org/cover/topics.html#entities"
>http://www.oasis-open.org/cover/topics.html#entities</ulink
>. </para>
</footnote>
</para>
</sect2>
<sect2
><title
>Character Data Not Allowed Here</title>
<para>
<indexterm
><primary
>error messages</primary>
  <secondary
>character data out of context</secondary
></indexterm>
<indexterm
><primary
>characters</primary>
  <secondary
>errors (out of context)</secondary
></indexterm>
<indexterm
><primary
>start tags</primary>
  <secondary
>errors</secondary
></indexterm>
Out of context character data is frequently caused by a missing start tag, but sometimes it's just the result of typing in the wrong place! </para>
<programlisting
><inlinegraphic format="linespecific" fileref="examples/errs/badpcdata.sgm"/>
</programlisting>
<screen
>[n:\dbtdg] nsgmls -sv -c \share\sgml\catalog examples\errs\badpcdata.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\badpcdata.sgm:9:0:E: character data is not allowed here
</screen>
<para>
<indexterm
><primary
>Chapter element</primary>
  <secondary
>errors, character data</secondary
></indexterm>
<sgmltag
>Chapter</sgmltag
>s aren't allowed to contain character data directly. Here, a wrapper element, such as <sgmltag
>Para</sgmltag
>, is missing around the sentence between the first two paragraphs. </para>
</sect2>
<sect2
><title
>Misspelled Start Tag</title>
<para>
<indexterm
><primary
>start tags</primary>
  <secondary
>errors</secondary>
    <tertiary
>misspelling</tertiary
></indexterm>
If you spell it wrong, the parser gets confused. </para>
<programlisting
><inlinegraphic format="linespecific" fileref="examples/errs/misspell.sgm"/>
</programlisting>
<screen
>[n:\documents\books\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\misspe
ll.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\misspell.sgm:9:5:E: element "PAAR" undefined
m:\jade\nsgmls.exe:examples\errs\misspell.sgm:14:6:E: end tag for element "PARA" which is not open
m:\jade\nsgmls.exe:examples\errs\misspell.sgm:21:9:E: end tag for "PAAR" omitted, but OMITTAG NO was specified
m:\jade\nsgmls.exe:examples\errs\misspell.sgm:9:0: start tag was here
</screen>
<para
>Luckily, these are pretty easy to spot, unless you accidentally spell the name of another element. In that case, your error might appear to be out of context. </para>
</sect2>
<sect2
><title
>Misspelled End Tag</title>
<para>
<indexterm
><primary
>end tags</primary>
  <secondary
>errors, misspelling</secondary
></indexterm>
Spelling the end tag wrong is just as confusing. </para>
<programlisting
><inlinegraphic format="linespecific" fileref="examples/errs/misspell2.sgm"/>
</programlisting>
<screen
>[n:\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\misspell2.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:2:35:E: end tag for element "TITEL" which is not open
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:3:5:E: document type does not allow element "PARA" here; missing one of "FOOTNOTE", "MSGTEXT" start-tag
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:9:5:E: document type does not allow element "PARA" here; missing one of "FOOTNOTE", "MSGTEXT" start-tag
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:15:5:E: document type does not allow element "PARA" here; missing one of "FOOTNOTE", "MSGTEXT" start-tag
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:21:9:E: end tag for "TITLE" omitted, but OMITTAG NO was specified
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:2:9: start tag was here
m:\jade\nsgmls.exe:examples\errs\misspell2.sgm:21:9:E: end tag for "CHAPTER" which is not finished
</screen>
<para
>These are pretty easy to spot as well, but look at how confused the parser became. From the parser's point of view, failure to close the open <sgmltag
>Title</sgmltag
> element means that all the following elements appear out of context. </para>
</sect2>
<sect2
><title
>Out of Context Start Tag</title>
<para>
<indexterm
><primary
>start tags</primary>
  <secondary
>errors</secondary>
    <tertiary
>out of context</tertiary
></indexterm>
<indexterm
><primary
>tags</primary>
  <secondary
>context errors</secondary
></indexterm>
Sometimes the problem isn't spelling, but placing a tag in the wrong context. When this happens, the parser tries to figure out what it can add to your document to make it valid. Then it proceeds as if it had seen what was added in order to recover from the error seen, which can cause future errors. </para>
<programlisting
><inlinegraphic format="linespecific" fileref="examples/errs/badstarttag.sgm"/>
</programlisting>
<screen
>[n:\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\badstarttag.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\badstarttag.sgm:9:12:E: document type does not allow element "TITLE" here; missing one of "CALLOUTLIST", "SEGMENTEDLIST", "VARIABLELIST", "CAUTION", "IMPORTANT", "NOTE", "TIP", "WARNING", "BLOCKQUOTE", "EQUATION", "EXAMPLE", "FIGURE", "TABLE" start-tag
</screen>
<para>
<indexterm
><primary
>titles</primary>
  <secondary
>formal elements</secondary>
    <tertiary
>errors</tertiary
></indexterm>
<indexterm
><primary
>paragraphs</primary>
  <secondary
>titles</secondary
></indexterm>
In this example, we probably wanted a <sgmltag
>FormalPara</sgmltag
>, so that we could have a title on the paragraph. But note that the parser didn't suggest this alternative. The parser only tries to add additional elements, rather than rename elements that it's already seen. </para>
</sect2>
<sect2
><title
>Missing End Tag</title>
<para>
<indexterm
><primary
>end tags</primary>
  <secondary
>missing, errors</secondary
></indexterm>
Leaving out an end tag is a lot like an out of context start tag. In fact, they're really the same error. The problem is never caused by the missing end tag per se, rather it's caused by the fact that something following it is now out of context. </para>
<programlisting
><inlinegraphic format="linespecific" fileref="examples/errs/noendtag.sgm"/>
</programlisting>
<screen
>[n:\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\noendtag.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\noendtag.sgm:14:5:E: document type does not allow element "PARA" here; missing one of "FOOTNOTE", "MSGTEXT", "CAUTION", "IMPORTANT", "NOTE", "TIP", "WARNING", "BLOCKQUOTE", "INFORMALEXAMPLE" start-tag
m:\jade\nsgmls.exe:examples\errs\noendtag.sgm:20:9:E: end tag for "PARA" omitted, but OMITTAG NO was specified
m:\jade\nsgmls.exe:examples\errs\noendtag.sgm:9:0: start tag was here
</screen>
<para
>In this case, the parser figured out that the best thing it could do is end the paragraph. </para>
</sect2>
<sect2
><title
>Bad Entity Reference</title>
<para>
<indexterm
><primary
>entities</primary>
  <secondary
>errors, spelling</secondary
></indexterm>
If you spell an entity name wrong, the parser will catch it. </para>
<programlisting
><inlinegraphic format="linespecific" fileref="examples/errs/badent.sgm"/>
</programlisting>
<screen
>[n:\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\badent.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\badent.sgm:10:26:E: general entity "xyzzy" not defined and no default entity
</screen>
<para
>More often than not, you'll see this when you misspell a character entity name. For example, this happens when you type <literal
>&amp;ldqou;</literal
> instead of <literal
>&amp;ldquo;</literal
>. </para>
</sect2>
<sect2
><title
>Invalid 8-Bit Character</title>
<para>
<indexterm
><primary
>characters</primary>
  <secondary
>character sets</secondary>
    <tertiary
>Unicode character set (SGML errors)</tertiary
></indexterm>
<indexterm
><primary
>8-bit characters, errors (SGML)</primary
></indexterm>
<indexterm
><primary
>errors</primary>
  <secondary
>8-bit characters (SGML)</secondary
></indexterm>
In &XML;, the entire range of Unicode characters is available to you, but in &SGML;, the declaration indicates what characters are valid. The distributed DocBook declaration doesn't allow a bunch of fairly common 8-bit characters. </para>
<programlisting
><inlinegraphic format="linespecific" fileref="examples/errs/badchar.sgm"/>
</programlisting>
<screen
>[n:\dbtdg]nsgmls -sv -c \share\sgml\catalog examples\errs\badchar.sgm
m:\jade\nsgmls.exe:I: SP version "1.3.2"
m:\jade\nsgmls.exe:examples\errs\badchar.sgm:11:0:E: non SGML character number 147
m:\jade\nsgmls.exe:examples\errs\badchar.sgm:11:5:E: non SGML character number 148
</screen>
<para>
<indexterm
><primary
>declarations</primary>
  <secondary
>character sets, errors</secondary
></indexterm>
<indexterm
><primary
>entities</primary>
  <secondary
>characters</secondary>
    <tertiary
>errors</tertiary
></indexterm>
In this example, the Windows code page values for curly left and right quotes have been used, but they aren't in the declared character set. Fix this by converting them to character entities. </para>
<para>
<indexterm
><primary
>interchange partners</primary
></indexterm
> You can also fix them by changing the declaration, but if you do that, make sure all your interchange partners are aware of, and have a copy of, the modified declaration. See <xref linkend="app-interchange"/>. <indexterm startref="errorparsech4" class="endofrange"
></indexterm>
<indexterm startref="parsDocch4" class="endofrange"
></indexterm>
<indexterm startref="docDTDparsch4" class="endofrange"
></indexterm>
</para>
</sect2>
</sect1>

<sect1 id="ch03-otherschema"
><title
>Considering Other Schema Languages</title>

<para
>Historically, &DTD;s were the only way to describe the valid stricture of SGML and XML documents, but that is no longer the case. At the time of this writing (January, 2001), DocBook is experimentally available in three other schema languages:</para>

<variablelist>
<varlistentry>
<term
><ulink url="http://www.w3.org/XML/Schema"
>XML Schema</ulink
></term>
<listitem>
<para
>The schema language being defined by the <ulink url="http://www.w3.org/"
>W3C</ulink
> as the successor to &DTD;s for describing the structure of &XML;. XML Schema are likely to become a <ulink url="http://www.w3.org/"
>W3C</ulink
> Recommendation in 2001. </para
></listitem>
</varlistentry>
<varlistentry>
<term
><ulink url="http://www.xml.gr.jp/relax/"
>RELAX</ulink
></term>
<listitem>
<para
>RELAX, the Regular Language description for XML) is a less complex alternative to XML Schemas. The RELAX Core module is defined by ISO in <citetitle
>ISO/IEC DTR 22250-1, Document Description and Processing Languages -- Regular Language Description for XML (RELAX) -- Part 1: RELAX Core, 2000</citetitle
>. The RELAX Namespaces module is currently under development. </para>
</listitem>
</varlistentry>
<varlistentry>
<term
><ulink url="http://www.thaiopensource.com/trex/"
>TREX</ulink
></term>
<listitem>
<para
>TREX, Tree Regular Expressions for XML, is another less complex alternative to XML Schemas. It is concise, powerful, and datatype neutral.</para>
</listitem>
</varlistentry>
</variablelist>

<sect2
><title
>Parsing and Validation</title>

<para
>Before we look closer at these new schema languages, there's one significant difference between &DTD;s and all of them that we should get out of the way: &XML; parsers (which may understand &DTD;s) build an &XML; information set out of a stream of characters, all of these other schema languages begin with an information set and perform validation on it.</para>

<para
>What I mean by that is that an &XML; parser reads a stream of bytes: <screen
>"&lt;" "?" "x" "m" "l" " " "v" "e" ...
"&lt;" "!" "D" "O" "C" "T" "Y" "P" "E" " " "b" "o" "o" "k" ...
"&lt;" "b" "o" "o" "k" " " "i" "d" "=" "'" "f" "o" "o" "'" ">"
...
"&lt;" "/" "b" "o" "o" "k" ">"</screen
> interprets them as a stream of characters (which may change the interpretation of some sequences of bytes) and constructs some representation of the &XML; document. This representation is the set of all the &XML; information items encountered: the information set of the document. The <ulink url="http://www.w3.org/"
>W3C</ulink
> <ulink url="http://www.w3.org/XML/"
>XML Core Working Group</ulink
> is in the process of defining what an <ulink url="http://www.w3.org/TR/xml-infoset"
>&XML; Information Set</ulink
> contains.</para>

<para
>The other schema languages are defined not in terms of the sequence of characters in the file but in terms of the information set of the &XML; document. They have to work this way because the <ulink url="http://www.w3.org/TR/REC-xml"
>&XML; Recommendation</ulink
> says what an &XML; document is and they all want to work on top of &XML;.</para>

<para
>So what, you might ask? Well, it turns out that this has at least one very significant implication: there's no way for these languages to provide support for entity declarations.</para>

<para
>An entity, like <quote
><literal
>&amp;ora;</literal
></quote
> as a shortcut for <quote
>O'Reilly &amp; Associates</quote
> or <quote
><literal
>&amp;eacute;</literal
></quote
> as a mnemonic for <quote
>&eacute;</quote
>, is a feature of the character stream seen by the &XML; parser, it doesn't exist in the information set of valid &XML; documents. More importantly, this means that even if the schema language had a syntax for declaring entities, it wouldn't help the &XML; parser that needs to know the definitions long before the schema language processor comes into play.</para>

<para
>There are a couple of other &XML; features that are impacted, though not necessarily as significantly: notations and default attribute values. One use for notations is on external entity declarations, and as we've already seen, the schema language is too late to be useful for anything entity related. Default attribute values are also problematic since you would like them to be in the information set produced by the parser so that the schema language sees them.</para>

</sect2>

<sect2
><title
>A Coarse Comparison of Three &XML; Schema Languages</title>

<para
>FIXME: write a short synopsis of how these languages compare.</para>

</sect2>

</sect1>
</chapter>

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