root/trunk/spec/null.dbk

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<!DOCTYPE article
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         "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<article>
  <articleinfo>
    <title>
      Null Language-Independent Type for
      <trademark>YAML</trademark>&nbsp;Version&nbsp;1.1
    </title>

    <subtitle>
      Working Draft 2005-01-18
    </subtitle>

    <authorgroup>
      <author>
        <firstname>Oren</firstname>
        <surname>Ben-Kiki</surname>
        <email>oren@ben-kiki.org</email>
      </author>

      <author>
        <firstname>Clark</firstname>
        <surname>Evans</surname>
        <email>cce+yaml@clarkevans.com</email>
      </author>

      <author>
        <firstname>Brian</firstname>
        <surname>Ingerson</surname>
        <email>ingy@ttul.org</email>
      </author>
    </authorgroup>

    <copyright>
      <year>2001-2005</year>
      <holder>Oren Ben-Kiki<fo>,&nbsp;</fo></holder>
      <holder>Clark Evans<fo>,&nbsp;</fo></holder>
      <holder>Brian Ingerson</holder>
    </copyright>

    <legalnotice>
      This document may be freely copied provided it is not modified.
    </legalnotice>

    <abstract>
      <title>Status</title>

      <para>
       This specification is a draft reflecting consensus reached by members of
       the <ulink
       url="http://lists.sourceforge.net/lists/listinfo/yaml-core">yaml-core
       mailing list</ulink>. Any questions regarding this draft should be
       raised on this list.
      </para>
    </abstract>
  </articleinfo>

  <section>
    <variablelist>
      <varlistentry>
        <term>
          <para>
            URI:
          </para>
        </term>

        <listitem>
          <para>
            <userinput>tag:yaml.org,2002:null</userinput>
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <para>
            Shorthand:
          </para>
        </term>

        <listitem>
          <para>
            <userinput>!!null</userinput>
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <para>
            Kind:
          </para>
        </term>

        <listitem>
          <para>
            Scalar.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <para>
            Canonical:
          </para>
        </term>

        <listitem>
          <para>
            <userinput>~</userinput>
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>Regexp:</term>
        <listitem>
<screen>
 ~ <lineannotation># (canonical)</lineannotation>
|null|Null|NULL <lineannotation># (English)</lineannotation>
| <lineannotation># (Empty)</lineannotation>
</screen>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>Definition:</term>
        <listitem>
          <para>
            Devoid of value.
          </para>

          <para>
            A null value is used to indicate the lack of a value. This is
            typically converted into any native null-like value (e.g.,
            <userinput>undef</userinput> in Perl, <userinput>None</userinput>
            in Python). Note that a null is different from an empty string and
            that a mapping entry with some key and a null value is valid and
            different from not having that key in the mapping.
          </para>
        </listitem>
      </varlistentry>
    </variablelist>

    <example>
      <title><userinput>!!null</userinput> Examples</title>
<programlisting>
# A document may be null.
---
---
# This mapping has four keys,
# one has a value.
empty:
canonical: ~
english: null
~: null key
---
# This sequence has five
# entries, two have values.
sparse:
  - ~
  - 2nd entry
  -
  - 4th entry
  - Null
</programlisting>
    </example>
  </section>
</article>