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<h1 class="title">std/wrapnils</h1>
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<li>
<a class="reference reference-toplevel" href="#6" id="56">Imports</a>
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<summary><a class="reference reference-toplevel" href="#12" id="62">Procs</a></summary>
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<ul class="simple nested-toc-section">[]
<li><a class="reference" href="#%5B%5D%2COption%5BT%5D%2CI" title="`[]`[T, I](a: Option[T]; i: I): auto">`[]`[T, I](a: Option[T]; i: I): auto</a></li>
<li><a class="reference" href="#%5B%5D%2COption%5BU%5D" title="`[]`[U](a: Option[U]): auto">`[]`[U](a: Option[U]): auto</a></li>
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<summary><a class="reference reference-toplevel" href="#17" id="67">Macros</a></summary>
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<ul class="simple nested-toc-section">?.
<li><a class="reference" href="#%3F..m%2Ctyped" title="`?.`(a: typed): auto">`?.`(a: typed): auto</a></li>
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<ul class="simple nested-toc-section">??.
<li><a class="reference" href="#%3F%3F..m%2Ctyped" title="`??.`(a: typed): Option">`??.`(a: typed): Option</a></li>
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<a href="https://github.com/nim-lang/Nim/tree/devel/lib/std/wrapnils.nim#L1" class="link-seesrc" target="_blank">Source</a>&nbsp;&nbsp;
<a href="https://github.com/nim-lang/Nim/edit/devel/lib/std/wrapnils.nim#L1" class="link-seesrc" target="_blank" >Edit</a>&nbsp;&nbsp;
<div id="tocRoot"></div>
<p class="module-desc">This module allows evaluating expressions safely against the following conditions:<ul class="simple"><li>nil dereferences</li>
<li>field accesses with incorrect discriminant in case objects</li>
</ul>
<p><tt class="docutils literal"><span class="pre"><span class="Identifier">default</span><span class="Punctuation">(</span><span class="Identifier">T</span><span class="Punctuation">)</span></span></tt> is returned in those cases when evaluating an expression of type <tt class="docutils literal"><span class="pre"><span class="Identifier">T</span></span></tt>. This simplifies code by reducing need for if-else branches.</p>
<p>Note: experimental module, unstable API.</p>
<p><strong class="examples_text">Example:</strong></p>
<pre class="listing"><span class="Keyword">import</span> <span class="Identifier">std</span><span class="Operator">/</span><span class="Identifier">wrapnils</span>
<span class="Keyword">type</span> <span class="Identifier">Foo</span> <span class="Operator">=</span> <span class="Keyword">ref</span> <span class="Keyword">object</span>
<span class="Identifier">x1</span><span class="Punctuation">:</span> <span class="Identifier">string</span>
<span class="Identifier">x2</span><span class="Punctuation">:</span> <span class="Identifier">Foo</span>
<span class="Identifier">x3</span><span class="Punctuation">:</span> <span class="Keyword">ref</span> <span class="Identifier">int</span>
<span class="Keyword">var</span> <span class="Identifier">f</span><span class="Punctuation">:</span> <span class="Identifier">Foo</span>
<span class="Identifier">assert</span> <span class="Operator">?.</span><span class="Identifier">f</span><span class="Operator">.</span><span class="Identifier">x2</span><span class="Operator">.</span><span class="Identifier">x1</span> <span class="Operator">==</span> <span class="StringLit">&quot;&quot;</span> <span class="Comment"># returns default value since `f` is nil</span>
<span class="Keyword">var</span> <span class="Identifier">f2</span> <span class="Operator">=</span> <span class="Identifier">Foo</span><span class="Punctuation">(</span><span class="Identifier">x1</span><span class="Punctuation">:</span> <span class="StringLit">&quot;a&quot;</span><span class="Punctuation">)</span>
<span class="Identifier">f2</span><span class="Operator">.</span><span class="Identifier">x2</span> <span class="Operator">=</span> <span class="Identifier">f2</span>
<span class="Identifier">assert</span> <span class="Operator">?.</span><span class="Identifier">f2</span><span class="Operator">.</span><span class="Identifier">x1</span> <span class="Operator">==</span> <span class="StringLit">&quot;a&quot;</span> <span class="Comment"># same as f2.x1 (no nil LHS in this chain)</span>
<span class="Identifier">assert</span> <span class="Operator">?.</span><span class="Identifier">Foo</span><span class="Punctuation">(</span><span class="Identifier">x1</span><span class="Punctuation">:</span> <span class="StringLit">&quot;a&quot;</span><span class="Punctuation">)</span><span class="Operator">.</span><span class="Identifier">x1</span> <span class="Operator">==</span> <span class="StringLit">&quot;a&quot;</span> <span class="Comment"># can use constructor inside</span>
<span class="Comment"># when you know a sub-expression doesn't involve a `nil` (e.g. `f2.x2.x2`),</span>
<span class="Comment"># you can scope it as follows:</span>
<span class="Identifier">assert</span> <span class="Operator">?.</span><span class="Punctuation">(</span><span class="Identifier">f2</span><span class="Operator">.</span><span class="Identifier">x2</span><span class="Operator">.</span><span class="Identifier">x2</span><span class="Punctuation">)</span><span class="Operator">.</span><span class="Identifier">x3</span><span class="Punctuation">[</span><span class="Punctuation">]</span> <span class="Operator">==</span> <span class="DecNumber">0</span>
<span class="Identifier">assert</span> <span class="Punctuation">(</span><span class="Operator">?.</span><span class="Identifier">f2</span><span class="Operator">.</span><span class="Identifier">x2</span><span class="Operator">.</span><span class="Identifier">x2</span><span class="Punctuation">)</span><span class="Operator">.</span><span class="Identifier">x3</span> <span class="Operator">==</span> <span class="Keyword">nil</span> <span class="Comment"># this terminates ?. early</span></pre>
<p><strong class="examples_text">Example:</strong></p>
<pre class="listing"><span class="Keyword">import</span> <span class="Identifier">std</span><span class="Operator">/</span><span class="Identifier">wrapnils</span>
<span class="Comment"># ?. also allows case object</span>
<span class="Keyword">type</span> <span class="Identifier">B</span> <span class="Operator">=</span> <span class="Keyword">object</span>
<span class="Identifier">b0</span><span class="Punctuation">:</span> <span class="Identifier">int</span>
<span class="Keyword">case</span> <span class="Identifier">cond</span><span class="Punctuation">:</span> <span class="Identifier">bool</span>
<span class="Keyword">of</span> <span class="Identifier">false</span><span class="Punctuation">:</span> <span class="Keyword">discard</span>
<span class="Keyword">of</span> <span class="Identifier">true</span><span class="Punctuation">:</span>
<span class="Identifier">b1</span><span class="Punctuation">:</span> <span class="Identifier">float</span>
<span class="Keyword">var</span> <span class="Identifier">b</span> <span class="Operator">=</span> <span class="Identifier">B</span><span class="Punctuation">(</span><span class="Identifier">cond</span><span class="Punctuation">:</span> <span class="Identifier">false</span><span class="Punctuation">,</span> <span class="Identifier">b0</span><span class="Punctuation">:</span> <span class="DecNumber">3</span><span class="Punctuation">)</span>
<span class="Identifier">doAssertRaises</span><span class="Punctuation">(</span><span class="Identifier">FieldDefect</span><span class="Punctuation">)</span><span class="Punctuation">:</span> <span class="Keyword">discard</span> <span class="Identifier">b</span><span class="Operator">.</span><span class="Identifier">b1</span> <span class="Comment"># wrong discriminant</span>
<span class="Identifier">doAssert</span> <span class="Operator">?.</span><span class="Identifier">b</span><span class="Operator">.</span><span class="Identifier">b1</span> <span class="Operator">==</span> <span class="FloatNumber">0.0</span> <span class="Comment"># safe</span>
<span class="Identifier">b</span> <span class="Operator">=</span> <span class="Identifier">B</span><span class="Punctuation">(</span><span class="Identifier">cond</span><span class="Punctuation">:</span> <span class="Identifier">true</span><span class="Punctuation">,</span> <span class="Identifier">b1</span><span class="Punctuation">:</span> <span class="FloatNumber">4.5</span><span class="Punctuation">)</span>
<span class="Identifier">doAssert</span> <span class="Operator">?.</span><span class="Identifier">b</span><span class="Operator">.</span><span class="Identifier">b1</span> <span class="Operator">==</span> <span class="FloatNumber">4.5</span>
<span class="Comment"># lvalue semantics are preserved:</span>
<span class="Keyword">if</span> <span class="Punctuation">(</span><span class="Keyword">let</span> <span class="Identifier">p</span> <span class="Operator">=</span> <span class="Operator">?.</span><span class="Identifier">b</span><span class="Operator">.</span><span class="Identifier">b1</span><span class="Operator">.</span><span class="Keyword">addr</span><span class="Punctuation">;</span> <span class="Identifier">p</span> <span class="Operator">!=</span> <span class="Keyword">nil</span><span class="Punctuation">)</span><span class="Punctuation">:</span> <span class="Identifier">p</span><span class="Punctuation">[</span><span class="Punctuation">]</span> <span class="Operator">=</span> <span class="FloatNumber">4.7</span>
<span class="Identifier">doAssert</span> <span class="Identifier">b</span><span class="Operator">.</span><span class="Identifier">b1</span> <span class="Operator">==</span> <span class="FloatNumber">4.7</span></pre></p>
<div class="section" id="6">
<h1><a class="toc-backref" href="#6">Imports</a></h1>
<dl class="item">
<a class="reference external" href="macros.html">macros</a>, <a class="reference external" href="options.html">options</a>
</dl>
</div>
<div class="section" id="12">
<h1><a class="toc-backref" href="#12">Procs</a></h1>
<dl class="item">
<div id="[]-procs-all">
<div id="[],Option[T],I">
<dt><pre><span class="Keyword">func</span> <a href="#%5B%5D%2COption%5BT%5D%2CI"><span class="Identifier">`[]`</span></a><span class="Other">[</span><span class="Identifier">T</span><span class="Other">,</span> <span class="Identifier">I</span><span class="Other">]</span><span class="Other">(</span><span class="Identifier">a</span><span class="Other">:</span> <a href="options.html#Option"><span class="Identifier">Option</span></a><span class="Other">[</span><span class="Identifier">T</span><span class="Other">]</span><span class="Other">;</span> <span class="Identifier">i</span><span class="Other">:</span> <span class="Identifier">I</span><span class="Other">)</span><span class="Other">:</span> <a href="system.html#auto"><span class="Identifier">auto</span></a> {.<span class="Identifier">inline</span>.}</pre></dt>
<dd>
See top-level example.
<a href="https://github.com/nim-lang/Nim/tree/devel/lib/std/wrapnils.nim#L178" class="link-seesrc" target="_blank">Source</a>&nbsp;&nbsp;
<a href="https://github.com/nim-lang/Nim/edit/devel/lib/std/wrapnils.nim#L178" class="link-seesrc" target="_blank" >Edit</a>&nbsp;&nbsp;
</dd>
</div>
<div id="[],Option[U]">
<dt><pre><span class="Keyword">func</span> <a href="#%5B%5D%2COption%5BU%5D"><span class="Identifier">`[]`</span></a><span class="Other">[</span><span class="Identifier">U</span><span class="Other">]</span><span class="Other">(</span><span class="Identifier">a</span><span class="Other">:</span> <a href="options.html#Option"><span class="Identifier">Option</span></a><span class="Other">[</span><span class="Identifier">U</span><span class="Other">]</span><span class="Other">)</span><span class="Other">:</span> <a href="system.html#auto"><span class="Identifier">auto</span></a> {.<span class="Identifier">inline</span>.}</pre></dt>
<dd>
See top-level example.
<a href="https://github.com/nim-lang/Nim/tree/devel/lib/std/wrapnils.nim#L184" class="link-seesrc" target="_blank">Source</a>&nbsp;&nbsp;
<a href="https://github.com/nim-lang/Nim/edit/devel/lib/std/wrapnils.nim#L184" class="link-seesrc" target="_blank" >Edit</a>&nbsp;&nbsp;
</dd>
</div>
</div>
</dl>
</div>
<div class="section" id="17">
<h1><a class="toc-backref" href="#17">Macros</a></h1>
<dl class="item">
<div id="?.-macros-all">
<div id="?..m,typed">
<dt><pre><span class="Keyword">macro</span> <a href="#%3F..m%2Ctyped"><span class="Identifier">`?.`</span></a><span class="Other">(</span><span class="Identifier">a</span><span class="Other">:</span> <a href="system.html#typed"><span class="Identifier">typed</span></a><span class="Other">)</span><span class="Other">:</span> <a href="system.html#auto"><span class="Identifier">auto</span></a></pre></dt>
<dd>
Transforms <tt class="docutils literal"><span class="pre"><span class="Identifier">a</span></span></tt> into an expression that can be safely evaluated even in presence of intermediate nil pointers/references, in which case a default value is produced.
<a href="https://github.com/nim-lang/Nim/tree/devel/lib/std/wrapnils.nim#L112" class="link-seesrc" target="_blank">Source</a>&nbsp;&nbsp;
<a href="https://github.com/nim-lang/Nim/edit/devel/lib/std/wrapnils.nim#L112" class="link-seesrc" target="_blank" >Edit</a>&nbsp;&nbsp;
</dd>
</div>
</div>
<div id="??.-macros-all">
<div id="??..m,typed">
<dt><pre><span class="Keyword">macro</span> <a href="#%3F%3F..m%2Ctyped"><span class="Identifier">`??.`</span></a><span class="Other">(</span><span class="Identifier">a</span><span class="Other">:</span> <a href="system.html#typed"><span class="Identifier">typed</span></a><span class="Other">)</span><span class="Other">:</span> <a href="options.html#Option"><span class="Identifier">Option</span></a></pre></dt>
<dd>
Same as <tt class="docutils literal"><span class="pre"><span class="Operator">?.</span></span></tt> but returns an <tt class="docutils literal"><span class="pre"><span class="Identifier">Option</span></span></tt>.
<p><strong class="examples_text">Example:</strong></p>
<pre class="listing"><span class="Keyword">import</span> <span class="Identifier">std</span><span class="Operator">/</span><span class="Identifier">options</span>
<span class="Keyword">type</span> <span class="Identifier">Foo</span> <span class="Operator">=</span> <span class="Keyword">ref</span> <span class="Keyword">object</span>
<span class="Identifier">x1</span><span class="Punctuation">:</span> <span class="Keyword">ref</span> <span class="Identifier">int</span>
<span class="Identifier">x2</span><span class="Punctuation">:</span> <span class="Identifier">int</span>
<span class="Comment"># `?.` can't distinguish between a valid vs invalid default value, but `??.` can:</span>
<span class="Keyword">var</span> <span class="Identifier">f1</span> <span class="Operator">=</span> <span class="Identifier">Foo</span><span class="Punctuation">(</span><span class="Identifier">x1</span><span class="Punctuation">:</span> <span class="Identifier">int</span><span class="Operator">.</span><span class="Identifier">new</span><span class="Punctuation">,</span> <span class="Identifier">x2</span><span class="Punctuation">:</span> <span class="DecNumber">2</span><span class="Punctuation">)</span>
<span class="Identifier">doAssert</span> <span class="Punctuation">(</span><span class="Operator">??.</span><span class="Identifier">f1</span><span class="Operator">.</span><span class="Identifier">x1</span><span class="Punctuation">[</span><span class="Punctuation">]</span><span class="Punctuation">)</span><span class="Operator">.</span><span class="Identifier">get</span> <span class="Operator">==</span> <span class="DecNumber">0</span> <span class="Comment"># not enough to tell when the chain was valid.</span>
<span class="Identifier">doAssert</span> <span class="Punctuation">(</span><span class="Operator">??.</span><span class="Identifier">f1</span><span class="Operator">.</span><span class="Identifier">x1</span><span class="Punctuation">[</span><span class="Punctuation">]</span><span class="Punctuation">)</span><span class="Operator">.</span><span class="Identifier">isSome</span> <span class="Comment"># a nil didn't occur in the chain</span>
<span class="Identifier">doAssert</span> <span class="Punctuation">(</span><span class="Operator">??.</span><span class="Identifier">f1</span><span class="Operator">.</span><span class="Identifier">x2</span><span class="Punctuation">)</span><span class="Operator">.</span><span class="Identifier">get</span> <span class="Operator">==</span> <span class="DecNumber">2</span>
<span class="Keyword">var</span> <span class="Identifier">f2</span><span class="Punctuation">:</span> <span class="Identifier">Foo</span>
<span class="Identifier">doAssert</span> <span class="Keyword">not</span> <span class="Punctuation">(</span><span class="Operator">??.</span><span class="Identifier">f2</span><span class="Operator">.</span><span class="Identifier">x1</span><span class="Punctuation">[</span><span class="Punctuation">]</span><span class="Punctuation">)</span><span class="Operator">.</span><span class="Identifier">isSome</span> <span class="Comment"># f2 was nil</span>
<span class="Identifier">doAssertRaises</span><span class="Punctuation">(</span><span class="Identifier">UnpackDefect</span><span class="Punctuation">)</span><span class="Punctuation">:</span> <span class="Keyword">discard</span> <span class="Punctuation">(</span><span class="Operator">??.</span><span class="Identifier">f2</span><span class="Operator">.</span><span class="Identifier">x1</span><span class="Punctuation">[</span><span class="Punctuation">]</span><span class="Punctuation">)</span><span class="Operator">.</span><span class="Identifier">get</span>
<span class="Identifier">doAssert</span> <span class="Operator">?.</span><span class="Identifier">f2</span><span class="Operator">.</span><span class="Identifier">x1</span><span class="Punctuation">[</span><span class="Punctuation">]</span> <span class="Operator">==</span> <span class="DecNumber">0</span> <span class="Comment"># in contrast, this returns default(int)</span></pre>
<a href="https://github.com/nim-lang/Nim/tree/devel/lib/std/wrapnils.nim#L128" class="link-seesrc" target="_blank">Source</a>&nbsp;&nbsp;
<a href="https://github.com/nim-lang/Nim/edit/devel/lib/std/wrapnils.nim#L128" class="link-seesrc" target="_blank" >Edit</a>&nbsp;&nbsp;
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<h1><a class="toc-backref" href="#18">Templates</a></h1>
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<div id="fakeDot-templates-all">
<div id="fakeDot.t,Option,">
<dt><pre><span class="Keyword">template</span> <a href="#fakeDot.t%2COption%2C"><span class="Identifier">fakeDot</span></a><span class="Other">(</span><span class="Identifier">a</span><span class="Other">:</span> <a href="options.html#Option"><span class="Identifier">Option</span></a><span class="Other">;</span> <span class="Identifier">b</span><span class="Other">)</span><span class="Other">:</span> <a href="system.html#untyped"><span class="Identifier">untyped</span></a></pre></dt>
<dd>
See top-level example.
<a href="https://github.com/nim-lang/Nim/tree/devel/lib/std/wrapnils.nim#L159" class="link-seesrc" target="_blank">Source</a>&nbsp;&nbsp;
<a href="https://github.com/nim-lang/Nim/edit/devel/lib/std/wrapnils.nim#L159" class="link-seesrc" target="_blank" >Edit</a>&nbsp;&nbsp;
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