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<div class="section" id="yaml-syntax">
<h1>YAML syntax<a class="headerlink" href="#yaml-syntax" title="Permalink to this headline"></a></h1>
<p>This is an introduction to the most common YAML constructs. For more detailed
information, see <a class="reference external" href="http://pyyaml.org/wiki/PyYAMLDocumentation">PyYAML documentation</a>,
which this article is based on,
<a class="reference external" href="http://yaml.org/spec/1.1/#id857168">Chapter 2 of the YAML specification</a>
or the <a class="reference external" href="http://en.wikipedia.org/wiki/YAML">Wikipedia page</a>.</p>
<p>YAML is a data serialization format designed for human readability. YAML is a
recursive acronym for &#8220;YAML Ain&#8217;t Markup Language&#8221;.</p>
<p>YAML is similar to JSON, and in fact, JSON is a subset of YAML 1.2; but YAML has
some more advanced features and is easier to read. However, it is also more
difficult to parse (and probably somewhat slower). Data is stored in mappings
(associative arrays), sequences (lists) and scalars (single values). Data
structure hierarchy depends either on indentation (block context, similar to
Python code), or nesting of brackets and braces (flow context, similar to JSON).
YAML comments begin with <tt class="docutils literal"><span class="pre">#</span></tt> and continue until the end of line.</p>
<div class="section" id="documents">
<h2>Documents<a class="headerlink" href="#documents" title="Permalink to this headline"></a></h2>
<p>A YAML stream consists of one or more documents starting with <tt class="docutils literal"><span class="pre">---</span></tt> and
optionally ending with <tt class="docutils literal"><span class="pre">...</span></tt> . <tt class="docutils literal"><span class="pre">---</span></tt> can be left out for the first document.</p>
<p>Single document with no explicit start or end:</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Red</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Green</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Blue</span>
</pre></div>
</div>
<p>Same document with explicit start and end:</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="nn">---</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Red</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Green</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Blue</span>
<span class="nn">...</span>
</pre></div>
</div>
<p>A stream containing multiple documents:</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="nn">---</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Red</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Green</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Blue</span>
<span class="nn">---</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Linux</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">BSD</span>
<span class="nn">---</span>
<span class="l-Scalar-Plain">answer</span> <span class="p-Indicator">:</span> <span class="l-Scalar-Plain">42</span>
</pre></div>
</div>
</div>
<div class="section" id="sequences">
<h2>Sequences<a class="headerlink" href="#sequences" title="Permalink to this headline"></a></h2>
<p>Sequences are arrays of nodes of any type, similar e.g. to Python lists.
In block context, each item begins with hyphen+space &#8220;- &#8221;. In flow context,
sequences have syntax similar to D arrays.</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Block context</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Red</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Green</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Blue</span>
</pre></div>
</div>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Flow context</span>
<span class="p-Indicator">[</span><span class="nv">Red</span><span class="p-Indicator">,</span> <span class="nv">Green</span><span class="p-Indicator">,</span> <span class="nv">Blue</span><span class="p-Indicator">]</span>
</pre></div>
</div>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Nested</span>
<span class="p-Indicator">-</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Red</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Green</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Blue</span>
<span class="p-Indicator">-</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Linux</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">BSD</span>
</pre></div>
</div>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Nested flow</span>
<span class="p-Indicator">[[</span><span class="nv">Red</span><span class="p-Indicator">,</span> <span class="nv">Green</span><span class="p-Indicator">,</span> <span class="nv">Blue</span><span class="p-Indicator">],</span> <span class="p-Indicator">[</span><span class="nv">Linux</span><span class="p-Indicator">,</span> <span class="nv">BSD</span><span class="p-Indicator">]]</span>
</pre></div>
</div>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Nested in a mapping</span>
<span class="l-Scalar-Plain">Colors</span><span class="p-Indicator">:</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Red</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Green</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Blue</span>
<span class="l-Scalar-Plain">Operating systems</span><span class="p-Indicator">:</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">Linux</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">BSD</span>
</pre></div>
</div>
</div>
<div class="section" id="mappings">
<h2>Mappings<a class="headerlink" href="#mappings" title="Permalink to this headline"></a></h2>
<p>Mappings are associative arrays where each key and value can be of any type,
similar e.g. to Python dictionaries. In block context, keys and values are
separated by colon+space &#8221;: &#8221;. In flow context, mappings have syntax similar
to D associative arrays, but with braces instead of brackets:</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Block context</span>
<span class="l-Scalar-Plain">CPU</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Athlon</span>
<span class="l-Scalar-Plain">GPU</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Radeon</span>
<span class="l-Scalar-Plain">OS</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Linux</span>
</pre></div>
</div>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Flow context</span>
<span class="p-Indicator">{</span><span class="nv">CPU</span><span class="p-Indicator">:</span> <span class="nv">Athlon</span><span class="p-Indicator">,</span> <span class="nv">GPU</span><span class="p-Indicator">:</span> <span class="nv">Radeon</span><span class="p-Indicator">,</span> <span class="nv">OS</span><span class="p-Indicator">:</span> <span class="nv">Linux</span><span class="p-Indicator">}</span>
</pre></div>
</div>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Nested</span>
<span class="l-Scalar-Plain">PC</span><span class="p-Indicator">:</span>
<span class="l-Scalar-Plain">CPU</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Athlon</span>
<span class="l-Scalar-Plain">GPU</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Radeon</span>
<span class="l-Scalar-Plain">OS</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Debian</span>
<span class="l-Scalar-Plain">Phone</span><span class="p-Indicator">:</span>
<span class="l-Scalar-Plain">CPU</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Cortex</span>
<span class="l-Scalar-Plain">GPU</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">PowerVR</span>
<span class="l-Scalar-Plain">OS</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Android</span>
</pre></div>
</div>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Nested flow</span>
<span class="p-Indicator">{</span><span class="nv">PC</span><span class="p-Indicator">:</span> <span class="p-Indicator">{</span><span class="nv">CPU</span><span class="p-Indicator">:</span> <span class="nv">Athlon</span><span class="p-Indicator">,</span> <span class="nv">GPU</span><span class="p-Indicator">:</span> <span class="nv">Radeon</span><span class="p-Indicator">,</span> <span class="nv">OS</span><span class="p-Indicator">:</span> <span class="nv">Debian</span><span class="p-Indicator">},</span>
<span class="nv">Phone</span><span class="p-Indicator">:</span> <span class="p-Indicator">{</span><span class="nv">CPU</span><span class="p-Indicator">:</span> <span class="nv">Cortex</span><span class="p-Indicator">,</span> <span class="nv">GPU</span><span class="p-Indicator">:</span> <span class="nv">PowerVR</span><span class="p-Indicator">,</span> <span class="nv">OS</span><span class="p-Indicator">:</span> <span class="nv">Android</span><span class="p-Indicator">}}</span>
</pre></div>
</div>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Nested in a sequence</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">CPU</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Athlon</span>
<span class="l-Scalar-Plain">GPU</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Radeon</span>
<span class="l-Scalar-Plain">OS</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Debian</span>
<span class="p-Indicator">-</span> <span class="l-Scalar-Plain">CPU</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Cortex</span>
<span class="l-Scalar-Plain">GPU</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">PowerVR</span>
<span class="l-Scalar-Plain">OS</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Android</span>
</pre></div>
</div>
<p>Complex keys start with question mark+space &#8221;? &#8221;.</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="c1">#Nested in a sequence</span>
<span class="p-Indicator">?</span> <span class="p-Indicator">[</span><span class="nv">CPU</span><span class="p-Indicator">,</span> <span class="nv">GPU</span><span class="p-Indicator">]:</span> <span class="p-Indicator">[</span><span class="nv">Athlon</span><span class="p-Indicator">,</span> <span class="nv">Radeon</span><span class="p-Indicator">]</span>
<span class="l-Scalar-Plain">OS</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Debian</span>
</pre></div>
</div>
</div>
<div class="section" id="scalars">
<h2>Scalars<a class="headerlink" href="#scalars" title="Permalink to this headline"></a></h2>
<p>Scalars are simple values such as integers, strings, timestamps and so on.
There are multiple scalar styles.</p>
<p>Plain scalars use no quotes, start with the first non-space and end with the
last non-space character:</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="l-Scalar-Plain">scalar</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Plain scalar</span>
</pre></div>
</div>
<p>Single quoted scalars start and end with single quotes. A single quote is
represented by a pair of single quotes &#8216;&#8217;.</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="l-Scalar-Plain">scalar</span><span class="p-Indicator">:</span> <span class="s">&#39;Single</span><span class="nv"> </span><span class="s">quoted</span><span class="nv"> </span><span class="s">scalar</span><span class="nv"> </span><span class="s">ending</span><span class="nv"> </span><span class="s">with</span><span class="nv"> </span><span class="s">some</span><span class="nv"> </span><span class="s">spaces</span><span class="nv"> </span><span class="s">&#39;</span>
</pre></div>
</div>
<p>Double quoted scalars support C-style escape sequences.</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="l-Scalar-Plain">scalar</span><span class="p-Indicator">:</span> <span class="s">&quot;Double</span><span class="nv"> </span><span class="s">quoted</span><span class="nv"> </span><span class="s">scalar</span><span class="nv"> </span><span class="s">\n</span><span class="nv"> </span><span class="s">with</span><span class="nv"> </span><span class="s">some</span><span class="nv"> </span><span class="s">\\</span><span class="nv"> </span><span class="s">escape</span><span class="nv"> </span><span class="s">sequences&quot;</span>
</pre></div>
</div>
<p>Block scalars are convenient for multi-line values. They start either with
<tt class="docutils literal"><span class="pre">|</span></tt> or with <tt class="docutils literal"><span class="pre">&gt;</span></tt>. With <tt class="docutils literal"><span class="pre">|</span></tt>, the newlines in the scalar are preserved.
With <tt class="docutils literal"><span class="pre">&gt;</span></tt>, the newlines between two non-empty lines are removed.</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="l-Scalar-Plain">scalar</span><span class="p-Indicator">:</span> <span class="p-Indicator">|</span>
<span class="no">Newlines are preserved</span>
<span class="no">First line</span>
<span class="no">Second line</span>
</pre></div>
</div>
<div class="highlight-yaml"><div class="highlight"><pre><span class="l-Scalar-Plain">scalar</span><span class="p-Indicator">:</span> <span class="p-Indicator">&gt;</span>
<span class="no">Newlines are folded</span>
<span class="no">This is still the first paragraph</span>
<span class="no">This is the second</span>
<span class="no">paragraph</span>
</pre></div>
</div>
</div>
<div class="section" id="anchors-and-aliases">
<h2>Anchors and aliases<a class="headerlink" href="#anchors-and-aliases" title="Permalink to this headline"></a></h2>
<p>Anchors and aliases can reduce size of YAML code by allowing you to define a
value once, assign an anchor to it and use alias referring to that anchor
anywhere else you need that value. It is possible to use this to create
recursive data structures and some parsers support this; however, D:YAML does
not (this might change in the future, but it is unlikely).</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="l-Scalar-Plain">Person</span><span class="p-Indicator">:</span> <span class="nl">&amp;AD</span>
<span class="l-Scalar-Plain">gender</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">male</span>
<span class="l-Scalar-Plain">name</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Arthur Dent</span>
<span class="l-Scalar-Plain">Clone</span><span class="p-Indicator">:</span> <span class="nv">*AD</span>
</pre></div>
</div>
</div>
<div class="section" id="tags">
<h2>Tags<a class="headerlink" href="#tags" title="Permalink to this headline"></a></h2>
<p>Tags are identifiers that specify data types of YAML nodes. Most default YAML
tags are resolved implicitly, so there is no need to specify them. D:YAML also
supports implicit resolution for custom, user specified tags.</p>
<p>Explicitly specified tags:</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="l-Scalar-Plain">answer</span><span class="p-Indicator">:</span> <span class="kt">!!int</span> <span class="s">&quot;42&quot;</span>
<span class="l-Scalar-Plain">name</span><span class="p-Indicator">:</span> <span class="kt">!!str</span> <span class="s">&quot;Arthur</span><span class="nv"> </span><span class="s">Dent&quot;</span>
</pre></div>
</div>
<p>Implicit tags:</p>
<div class="highlight-yaml"><div class="highlight"><pre><span class="l-Scalar-Plain">answer</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">42</span> <span class="c1">#int</span>
<span class="l-Scalar-Plain">name</span><span class="p-Indicator">:</span> <span class="l-Scalar-Plain">Arthur Dent</span> <span class="c1">#string</span>
</pre></div>
</div>
<p>This table shows D types stored in <em>yaml.Node</em> default YAML tags are converted to.
Some of these might change in the future (especially !!map and !!set).</p>
<table border="1" class="docutils">
<colgroup>
<col width="52%" />
<col width="48%" />
</colgroup>
<thead valign="bottom">
<tr class="row-odd"><th class="head">YAML tag</th>
<th class="head">D type</th>
</tr>
</thead>
<tbody valign="top">
<tr class="row-even"><td>!!null</td>
<td>yaml.YAMLNull</td>
</tr>
<tr class="row-odd"><td>!!bool</td>
<td>bool</td>
</tr>
<tr class="row-even"><td>!!int</td>
<td>long</td>
</tr>
<tr class="row-odd"><td>!!float</td>
<td>real</td>
</tr>
<tr class="row-even"><td>!!binary</td>
<td>ubyte[]</td>
</tr>
<tr class="row-odd"><td>!!timestamp</td>
<td>std.datetime.SysTime</td>
</tr>
<tr class="row-even"><td>!!map, !!omap, !!pairs</td>
<td>yaml.Node.Pair[]</td>
</tr>
<tr class="row-odd"><td>!!seq, !!set</td>
<td>yaml.Node[]</td>
</tr>
<tr class="row-even"><td>!!str</td>
<td>string</td>
</tr>
</tbody>
</table>
</div>
</div>
</div>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">YAML syntax</a><ul>
<li><a class="reference internal" href="#documents">Documents</a></li>
<li><a class="reference internal" href="#sequences">Sequences</a></li>
<li><a class="reference internal" href="#mappings">Mappings</a></li>
<li><a class="reference internal" href="#scalars">Scalars</a></li>
<li><a class="reference internal" href="#anchors-and-aliases">Anchors and aliases</a></li>
<li><a class="reference internal" href="#tags">Tags</a></li>
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