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<h1>
<small class="visible-xs">Elixir v1.7.2</small>
Collectable <small>protocol</small>
<a href="https://github.com/elixir-lang/elixir/blob/v1.7.2/lib/elixir/lib/collectable.ex#L1" title="View Source" class="view-source" rel="help">
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<section id="moduledoc">
<p>A protocol to traverse data structures.</p>
<p>The <a href="Enum.html#into/2"><code class="inline">Enum.into/2</code></a> function uses this protocol to insert an
enumerable into a collection:</p>
<pre><code class="nohighlight makeup elixir"><span class="gp unselectable">iex> </span><span class="nc">Enum</span><span class="o">.</span><span class="n">into</span><span class="p" data-group-id="1914926035-1">(</span><span class="p" data-group-id="1914926035-2">[</span><span class="ss">a</span><span class="p">:</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="ss">b</span><span class="p">:</span><span class="w"> </span><span class="mi">2</span><span class="p" data-group-id="1914926035-2">]</span><span class="p">,</span><span class="w"> </span><span class="p" data-group-id="1914926035-3">%{</span><span class="p" data-group-id="1914926035-3">}</span><span class="p" data-group-id="1914926035-1">)</span><span class="w">
</span><span class="p" data-group-id="1914926035-4">%{</span><span class="ss">a</span><span class="p">:</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="ss">b</span><span class="p">:</span><span class="w"> </span><span class="mi">2</span><span class="p" data-group-id="1914926035-4">}</span></code></pre>
<h2 id="module-why-collectable" class="section-heading">
<a href="#module-why-collectable" class="hover-link"><span class="icon-link" aria-hidden="true"></span></a>
Why Collectable?
</h2>
<p>The <a href="Enumerable.html"><code class="inline">Enumerable</code></a> protocol is useful to take values out of a collection.
In order to support a wide range of values, the functions provided by
the <a href="Enumerable.html"><code class="inline">Enumerable</code></a> protocol do not keep shape. For example, passing a
map to <a href="Enum.html#map/2"><code class="inline">Enum.map/2</code></a> always returns a list.</p>
<p>This design is intentional. <a href="Enumerable.html"><code class="inline">Enumerable</code></a> was designed to support infinite
collections, resources and other structures with fixed shape. For example,
it doesn’t make sense to insert values into a range, as it has a fixed
shape where just the range limits are stored.</p>
<p>The <a href="Collectable.html#content"><code class="inline">Collectable</code></a> module was designed to fill the gap left by the
<a href="Enumerable.html"><code class="inline">Enumerable</code></a> protocol. <a href="#into/1"><code class="inline">into/1</code></a> can be seen as the opposite of
<a href="Enumerable.html#reduce/3"><code class="inline">Enumerable.reduce/3</code></a>. If <a href="Enumerable.html"><code class="inline">Enumerable</code></a> is about taking values out,
<a href="Collectable.html#into/1"><code class="inline">Collectable.into/1</code></a> is about collecting those values into a structure.</p>
<h2 id="module-examples" class="section-heading">
<a href="#module-examples" class="hover-link"><span class="icon-link" aria-hidden="true"></span></a>
Examples
</h2>
<p>To show how to manually use the <a href="Collectable.html#content"><code class="inline">Collectable</code></a> protocol, let’s play with its
implementation for <a href="MapSet.html"><code class="inline">MapSet</code></a>.</p>
<pre><code class="nohighlight makeup elixir"><span class="gp unselectable">iex> </span><span class="p" data-group-id="1753321125-1">{</span><span class="n">initial_acc</span><span class="p">,</span><span class="w"> </span><span class="n">collector_fun</span><span class="p" data-group-id="1753321125-1">}</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nc">Collectable</span><span class="o">.</span><span class="n">into</span><span class="p" data-group-id="1753321125-2">(</span><span class="nc">MapSet</span><span class="o">.</span><span class="n">new</span><span class="p" data-group-id="1753321125-3">(</span><span class="p" data-group-id="1753321125-3">)</span><span class="p" data-group-id="1753321125-2">)</span><span class="w">
</span><span class="gp unselectable">iex> </span><span class="n">updated_acc</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nc">Enum</span><span class="o">.</span><span class="n">reduce</span><span class="p" data-group-id="1753321125-4">(</span><span class="p" data-group-id="1753321125-5">[</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p" data-group-id="1753321125-5">]</span><span class="p">,</span><span class="w"> </span><span class="n">initial_acc</span><span class="p">,</span><span class="w"> </span><span class="k" data-group-id="1753321125-6">fn</span><span class="w"> </span><span class="n">elem</span><span class="p">,</span><span class="w"> </span><span class="n">acc</span><span class="w"> </span><span class="o">-></span><span class="w">
</span><span class="gp unselectable">...> </span><span class="w"> </span><span class="n">collector_fun</span><span class="o">.</span><span class="p" data-group-id="1753321125-7">(</span><span class="n">acc</span><span class="p">,</span><span class="w"> </span><span class="p" data-group-id="1753321125-8">{</span><span class="ss">:cont</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p" data-group-id="1753321125-8">}</span><span class="p" data-group-id="1753321125-7">)</span><span class="w">
</span><span class="gp unselectable">...> </span><span class="k" data-group-id="1753321125-6">end</span><span class="p" data-group-id="1753321125-4">)</span><span class="w">
</span><span class="gp unselectable">iex> </span><span class="n">collector_fun</span><span class="o">.</span><span class="p" data-group-id="1753321125-9">(</span><span class="n">updated_acc</span><span class="p">,</span><span class="w"> </span><span class="ss">:done</span><span class="p" data-group-id="1753321125-9">)</span><span class="w">
</span><span class="p" data-group-id="1753321125-10">#</span><span class="nc" data-group-id="1753321125-10">MapSet</span><span class="p" data-group-id="1753321125-10"><</span><span class="p" data-group-id="1753321125-11">[</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p" data-group-id="1753321125-11">]</span><span class="p" data-group-id="1753321125-10">></span></code></pre>
<p>To show how the protocol can be implemented, we can take again a look at the
implementation for <a href="MapSet.html"><code class="inline">MapSet</code></a>. In this implementation “collecting” elements
simply means inserting them in the set through <a href="MapSet.html#put/2"><code class="inline">MapSet.put/2</code></a>.</p>
<pre><code class="nohighlight makeup elixir"><span class="kd">defimpl</span><span class="w"> </span><span class="nc">Collectable</span><span class="w"> </span><span class="k" data-group-id="8080708269-1">do</span><span class="w">
</span><span class="kd">def</span><span class="w"> </span><span class="nf">into</span><span class="p" data-group-id="8080708269-2">(</span><span class="n">original</span><span class="p" data-group-id="8080708269-2">)</span><span class="w"> </span><span class="k" data-group-id="8080708269-3">do</span><span class="w">
</span><span class="n">collector_fun</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k" data-group-id="8080708269-4">fn</span><span class="w">
</span><span class="n">set</span><span class="p">,</span><span class="w"> </span><span class="p" data-group-id="8080708269-5">{</span><span class="ss">:cont</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p" data-group-id="8080708269-5">}</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="nc">MapSet</span><span class="o">.</span><span class="n">put</span><span class="p" data-group-id="8080708269-6">(</span><span class="n">set</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p" data-group-id="8080708269-6">)</span><span class="w">
</span><span class="n">set</span><span class="p">,</span><span class="w"> </span><span class="ss">:done</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">set</span><span class="w">
</span><span class="c">_set</span><span class="p">,</span><span class="w"> </span><span class="ss">:halt</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="ss">:ok</span><span class="w">
</span><span class="k" data-group-id="8080708269-4">end</span><span class="w">
</span><span class="p" data-group-id="8080708269-7">{</span><span class="n">original</span><span class="p">,</span><span class="w"> </span><span class="n">collector_fun</span><span class="p" data-group-id="8080708269-7">}</span><span class="w">
</span><span class="k" data-group-id="8080708269-3">end</span><span class="w">
</span><span class="k" data-group-id="8080708269-1">end</span></code></pre>
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Summary
</h1>
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<h2>
<a href="#types">Types</a>
</h2>
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<a href="#t:command/0">command()</a>
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<a href="#t:t/0">t()</a>
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<h2>
<a href="#functions">Functions</a>
</h2>
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<a href="#into/1">into(collectable)</a>
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<div class="summary-synopsis"><p>Returns an initial accumulator and a “collector” function</p>
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Types
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<span class="signature">command()</span>
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<pre>command() :: {:cont, <a href="typespecs.html#built-in-types">term</a>()} | :done | :halt</pre>
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<pre>t() :: <a href="typespecs.html#built-in-types">term</a>()</pre>
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Functions
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<span class="signature">into(collectable)</span>
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<section class="docstring">
<p>Returns an initial accumulator and a “collector” function.</p>
<p>The returned function receives a term and a command and injects the term into
the collectable on every <code class="inline">{:cont, term}</code> command.</p>
<p><code class="inline">:done</code> is passed as a command when no further values will be injected. This
is useful when there’s a need to close resources or normalizing values. A
collectable must be returned when the command is <code class="inline">:done</code>.</p>
<p>If injection is suddenly interrupted, <code class="inline">:halt</code> is passed and the function
can return any value as it won’t be used.</p>
<p>For examples on how to use the <a href="Collectable.html#content"><code class="inline">Collectable</code></a> protocol and <a href="#into/1"><code class="inline">into/1</code></a> see the
module documentation.</p>
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