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<h1 class="title">API Overview</h1>
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<h1 id="introduction">Introduction</h1><p>NimYAML advocates parsing YAML input into native Nim types. Basic Nim library types like integers, floats and strings, as well as all tuples, enums and objects without private fields are supported out-of-the-box. Reference types are also supported, and NimYAML is able to detect if a reference occurs more than once and will serialize it accordingly. This means that NimYAML is able to dump and load potentially cyclic objects.</p>
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<p>While loading into and dumping from native Nim types is the preferred way to use NimYAML, it also gives you complete control over each processing step, so that you can for example only use the parser and process its event stream yourself. The following diagram gives an overview of NimYAML's features based on the YAML processing pipeline. The items and terminology YAML defines is shown in <em>italic</em>, NimYAML's implementation name is shown in <strong>bold</strong>.</p>
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<object data="processing.svg" type="image/svg+xml" ></object>
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<h1 id="intermediate-representation">Intermediate Representation</h1><p>The base of all YAML processing with NimYAML is the <a class="reference external" href="yaml.stream.html#YamlStream">YamlStream</a>. This is basically an iterator over <a class="reference external" href="yaml.stream.html#YamlStreamEvent">YamlStreamEvent</a> objects. Every proc that represents a single stage of the loading or dumping process will either take a <tt class="docutils literal"><span class="pre">YamlStream</span></tt> as input or return a <tt class="docutils literal"><span class="pre">YamlStream</span></tt>. Procs that implement the whole process in one step hide the <tt class="docutils literal"><span class="pre">YamlStream</span></tt> from the user. Every proc that returns a <tt class="docutils literal"><span class="pre">YamlStream</span></tt> guarantees that this stream is well-formed according to the YAML specification.</p>
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<p>This stream-oriented API can efficiently be used to parse large amounts of data. The drawback is that errors in the input are only discovered while processing the <tt class="docutils literal"><span class="pre">YamlStream</span></tt>. If the <tt class="docutils literal"><span class="pre">YamlStream</span></tt> encounters an exception while producing the next event, it will throw a <tt class="docutils literal"><span class="pre">YamlStreamError</span></tt> which contains the original exception as <tt class="docutils literal"><span class="pre">parent</span></tt>. The caller should know which exceptions are possible as parents of <tt class="docutils literal"><span class="pre">YamlStream</span></tt> because they know the source of the <tt class="docutils literal"><span class="pre">YamlStream</span></tt> they provided.</p>
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<h1 id="loading-yaml">Loading YAML</h1><p>If you want to load YAML character data directly into a native Nim variable, you can use <a class="reference external" href="yaml.serialization.html#load,,K">load</a>. This is the easiest and recommended way to load YAML data. This section gives an overview about how <tt class="docutils literal"><span class="pre">load</span></tt> is implemented. It is absolutely possible to reimplement the loading step using the low-level API.</p>
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<p>For parsing, a <a class="reference external" href="yaml.parser.html#YamlParser">YamlParser</a> object is needed. This object stores some state while parsing that may be useful for error reporting to the user. The <a class="reference external" href="yaml.parser.html#parse,YamlParser,Stream">parse</a> proc implements the YAML processing step of the same name. All syntax errors in the input character stream are processed by <tt class="docutils literal"><span class="pre">parse</span></tt>, which will raise a <tt class="docutils literal"><span class="pre">YamlParserError</span></tt> if it encounters a syntax error.</p>
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<p>Transforming a <tt class="docutils literal"><span class="pre">YamlStream</span></tt> to a native YAML object is done via <tt class="docutils literal"><span class="pre">construct</span></tt>. It skips the <tt class="docutils literal"><span class="pre">compose</span></tt> step for efficiency reasons. As Nim is statically typed, you have to know the target type when you write your loading code. This is different from YAML APIs of dynamically typed languages. If you cannot know the type of your YAML input at compile time, you have to manually process the <tt class="docutils literal"><span class="pre">YamlStream</span></tt> to serve your needs.</p>
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<h1 id="dumping-yaml">Dumping YAML</h1><p>Dumping is preferredly done with <a class="reference external" href="yaml.serialization.html#dump,K,Stream,TagStyle,AnchorStyle,PresentationOptions">dump</a>, which serializes a native Nim variable to a character stream. As with <tt class="docutils literal"><span class="pre">load</span></tt>, the following paragraph describes how <tt class="docutils literal"><span class="pre">dump</span></tt> is implemented using the low-level API.</p>
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<p>A Nim value is transformed into a <tt class="docutils literal"><span class="pre">YamlStream</span></tt> with <a class="reference external" href="yaml.serialization.html#represent,T,TagStyle,AnchorStyle">represent</a>. Depending on the <tt class="docutils literal"><span class="pre">AnchorStyle</span></tt> you specify, this will transform <tt class="docutils literal"><span class="pre">ref</span></tt> variables with multiple instances into anchored elements and aliases (for <tt class="docutils literal"><span class="pre">asTidy</span></tt> and <tt class="docutils literal"><span class="pre">asAlways</span></tt>) or write the same element into all places it occurs (for <tt class="docutils literal"><span class="pre">asNone</span></tt>). Be aware that if you use <tt class="docutils literal"><span class="pre">asNone</span></tt>, the value you serialize might not round-trip.</p>
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<p>Transforming a <tt class="docutils literal"><span class="pre">YamlStream</span></tt> into YAML character data is done with <a class="reference external" href="yaml.presenter.html#present,YamlStream,Stream,TagLibrary,PresentationOptions">present</a>. You can choose from multiple presentation styles. <tt class="docutils literal"><span class="pre">psJson</span></tt> is not able to process some features of <tt class="docutils literal"><span class="pre">YamlStream</span></tt> s, the other styles support all features and are guaranteed to round-trip to the same <tt class="docutils literal"><span class="pre">YamlStream</span></tt> if you parse the generated YAML character stream again.</p>
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<h1 id="the-document-object-model">The Document Object Model</h1><p>Much like XML, YAML also defines a <em>document object model</em>. If you cannot or do not want to load a YAML character stream to native Nim types, you can instead load it into a <a class="reference external" href="yaml.dom.html#YamlDocument">YamlDocument</a>. This <tt class="docutils literal"><span class="pre">YamlDocument</span></tt> can also be serialized into a YAML character stream. All tags will be preserved exactly as they are when transforming from and to a <tt class="docutils literal"><span class="pre">YamlDocument</span></tt>. The only important thing to remember is that when a value has no tag, it will get the non-specific tag <tt class="docutils literal"><span class="pre">"!"</span></tt> for quoted scalars and <tt class="docutils literal"><span class="pre">"?"</span></tt> for all other nodes.</p>
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<p>While tags are preserved, anchors will be resolved during loading and re-added during serialization. It is allowed for a <tt class="docutils literal"><span class="pre">YamlNode</span></tt> to occur multiple times within a <tt class="docutils literal"><span class="pre">YamlDocument</span></tt>, in which case it will be serialized once and referred to afterwards via aliases.</p>
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<p>The document object model is provided for completeness, but you are encouraged to use native Nim types as start- or endpoint instead. That may be significantly faster, as every <tt class="docutils literal"><span class="pre">YamlNode</span></tt> is allocated on the heap and subject to garbage collection. </p>
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<small>Made with Nim. Generated: 2017-09-20 19:08:33 UTC</small>
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