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  <div class="section" id="design-of-scope-in-paddle">
<span id="design-of-scope-in-paddle"></span><h1>Design of Scope in Paddle<a class="headerlink" href="#design-of-scope-in-paddle" title="Permalink to this headline"></a></h1>
<div class="section" id="overview">
<span id="overview"></span><h2>Overview<a class="headerlink" href="#overview" title="Permalink to this headline"></a></h2>
<p>Scope is an important concept in programming languages, which defines a program region that a set of bindings between names and entities applies. In a specific scope, a valid name is uniquely associated with an entity, such as a variable. And in another scope, this name may refer to other entity or nothing at all. It clearly restricts the visibility and validity of names in a program. Hence <strong>Scope</strong> is introduced to PaddlePaddle to manage variables in context. But different from the original abstract concept, Scope now becomes an object with two important attributes:</p>
<ul class="simple">
<li>Scope is an association of a name to variable.</li>
<li>Variables in a parent scope can be retrieved from local scope.</li>
</ul>
<p>A detailed explanation of these two attributes goes as following.</p>
</div>
<div class="section" id="scope-is-an-association-of-a-name-to-variable">
<span id="scope-is-an-association-of-a-name-to-variable"></span><h2>Scope is an association of a name to variable.<a class="headerlink" href="#scope-is-an-association-of-a-name-to-variable" title="Permalink to this headline"></a></h2>
<p>Scope is an association of a name to variable. All variables belong to <code class="docutils literal"><span class="pre">Scope</span></code>. You need to specify a scope to run a Net, i.e., <code class="docutils literal"><span class="pre">net.Run(&amp;scope)</span></code>. One net can run in different scopes and update different variable in the scope.</p>
<ol>
<li><p class="first">Scope only contains a map of a name to variable.</p>
<p>All parameters, data, states in a Net should be variables and stored inside a scope. Each op should get inputs and outputs to do computation from a scope, such as data buffer, state(momentum) etc.</p>
</li>
<li><p class="first">Variable can only be created by Scope and a variable can only be got from Scope. User cannot create or get a variable outside a scope. This is a constraints of our framework, and will keep our framework simple and clear.</p>
</li>
<li><p class="first">Scope only contains methods that are used to Create and Get Variables. Scope do not contain Operators and have no information to run them.
<code class="docutils literal"><span class="pre">Net</span></code> is designed to drive the computation and Scope only contains a map of variables. There is no computation logic inside a <code class="docutils literal"><span class="pre">Scope</span></code>. Scope just handles the lifetime management of variables.</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">Create</span></code> is used to create a Variable by its name and add the mapping relation.</li>
<li><code class="docutils literal"><span class="pre">Get</span></code> is used to find a Variable by name.</li>
</ul>
</li>
<li><p class="first">Every variable only belongs to one certain Scope.</p>
<p>Variable can not belong to many scopes. If you want to use variables from parent scope, you can use <code class="docutils literal"><span class="pre">parent</span> <span class="pre">scope</span></code>.</p>
</li>
<li><p class="first">Scope should destruct all Variables inside it when itself is destructed. User can never store <code class="docutils literal"><span class="pre">Variable</span></code> pointer somewhere else.</p>
<p>Because Variable can only be got from Scope. When destroying Scope, we also need to destroy all the Variables in it. If user store <code class="docutils literal"><span class="pre">Variable</span></code> pointer to private data member or some global variable, the pointer will be a invalid pointer when associated <code class="docutils literal"><span class="pre">Scope</span></code> is destroyed.</p>
</li>
</ol>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Scope</span> <span class="p">{</span>
 <span class="k">public</span><span class="o">:</span>
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  <span class="n">Variable</span><span class="o">*</span> <span class="n">NewVar</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span> <span class="n">name</span><span class="p">);</span>
  <span class="k">const</span> <span class="n">Variable</span><span class="o">*</span> <span class="nf">FindVar</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span> <span class="n">name</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
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 <span class="k">private</span><span class="o">:</span>
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    <span class="n">std</span><span class="o">::</span><span class="n">unordered_map</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">unique_ptr</span><span class="o">&lt;</span><span class="n">Variable</span><span class="o">&gt;&gt;</span> <span class="n">vars_</span><span class="p">;</span>
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<span class="p">};</span>
</pre></div>
</div>
</div>
<div class="section" id="parent-scope-and-local-scope">
<span id="parent-scope-and-local-scope"></span><h2>Parent scope and local scope<a class="headerlink" href="#parent-scope-and-local-scope" title="Permalink to this headline"></a></h2>
<p>Just like <a class="reference external" href="https://en.wikipedia.org/wiki/Scope_(computer_science)">scope</a> in programming languages, <code class="docutils literal"><span class="pre">Scope</span></code> in the neural network can also be a local scope. There are two attributes about local scope.</p>
<ol class="simple">
<li>We can create local variables in a local scope. When that local scope are destroyed, all local variables should also be destroyed.</li>
<li>Variables in a parent scope can be retrieved from local scopes of that parent scope, i.e., when user get a variable from a scope, it will try to search this variable in current scope. If there is no such variable in the local scope, <code class="docutils literal"><span class="pre">scope</span></code> will keep searching from its parent, until the variable is found or there is no parent.</li>
</ol>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Scope</span> <span class="p">{</span>
 <span class="k">public</span><span class="o">:</span>
  <span class="n">Scope</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">Scope</span><span class="o">&gt;&amp;</span> <span class="n">scope</span><span class="p">)</span><span class="o">:</span> <span class="n">parent_</span><span class="p">(</span><span class="n">scope</span><span class="p">)</span> <span class="p">{}</span>
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  <span class="n">Variable</span><span class="o">*</span> <span class="n">FindVar</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span> <span class="n">name</span><span class="p">)</span> <span class="k">const</span> <span class="p">{</span>
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    <span class="k">auto</span> <span class="n">it</span> <span class="o">=</span> <span class="n">vars_</span><span class="p">.</span><span class="n">find</span><span class="p">(</span><span class="n">name</span><span class="p">);</span>
    <span class="k">if</span> <span class="p">(</span><span class="n">it</span> <span class="o">!=</span> <span class="n">vars_</span><span class="p">.</span><span class="n">end</span><span class="p">())</span> <span class="p">{</span>
      <span class="k">return</span> <span class="n">it</span><span class="o">-&gt;</span><span class="n">second</span><span class="p">.</span><span class="n">get</span><span class="p">();</span>
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    <span class="p">}</span> <span class="k">else</span> <span class="k">if</span> <span class="p">(</span><span class="n">parent_</span> <span class="o">!=</span> <span class="k">nullptr</span><span class="p">)</span> <span class="p">{</span>
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      <span class="k">return</span> <span class="n">parent_</span><span class="o">-&gt;</span><span class="n">FindVar</span><span class="p">(</span><span class="n">name</span><span class="p">);</span>
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    <span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
      <span class="k">return</span> <span class="k">nullptr</span><span class="p">;</span>
    <span class="p">}</span>
  <span class="p">}</span>

 <span class="k">private</span><span class="o">:</span>
  <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">Scope</span><span class="o">&gt;</span> <span class="n">parent_</span> <span class="p">{</span><span class="k">nullptr</span><span class="p">};</span>
<span class="p">};</span>
</pre></div>
</div>
<p>In <code class="docutils literal"><span class="pre">Scope</span></code> class, there is a private data member called <code class="docutils literal"><span class="pre">parent_</span></code>. <code class="docutils literal"><span class="pre">parent_</span></code> is a smart pointer to its parent scope. When user <code class="docutils literal"><span class="pre">Get</span></code> a variable by its <code class="docutils literal"><span class="pre">name</span></code>, the <code class="docutils literal"><span class="pre">name</span></code> will be searched inside the current scope. If the variable cannot be found locally and parent scope is not a <code class="docutils literal"><span class="pre">nullptr</span></code>, the variable will be searched inside that parent scope. <code class="docutils literal"><span class="pre">parent_</span></code> pointer&#8217;s default value is <code class="docutils literal"><span class="pre">nullptr</span></code>. It means that the scope is a global scope when <code class="docutils literal"><span class="pre">parent_</span></code> is nullptr.</p>
<p>A local scope is very useful when we implement Recurrent Neural Network. Each timestep of an RNN should be a <code class="docutils literal"><span class="pre">Net</span></code>. Each <code class="docutils literal"><span class="pre">Net</span></code> of timestep (<code class="docutils literal"><span class="pre">StepNet</span></code> for short) should use an independent local scope. Just like variables in a while loop is inside a local scope in programming languages. By using a single <code class="docutils literal"><span class="pre">StepNet</span></code> and changing local scope, we can implement an RNN easily.</p>
</div>
</div>
<div class="section" id="interface-design">
<span id="interface-design"></span><h1>Interface Design<a class="headerlink" href="#interface-design" title="Permalink to this headline"></a></h1>
<div class="highlight-cpp"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Variable</span> <span class="p">{</span>
 <span class="k">private</span><span class="o">:</span>
  <span class="n">Variable</span><span class="p">()</span> <span class="o">=</span> <span class="k">default</span><span class="p">;</span>
  <span class="k">friend</span> <span class="k">class</span> <span class="nc">Scope</span><span class="p">;</span>
<span class="p">};</span>

<span class="k">class</span> <span class="nc">Scope</span> <span class="p">{</span>
 <span class="k">private</span><span class="o">:</span>
  <span class="n">Scope</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">Scope</span><span class="o">&gt;&amp;</span> <span class="n">parent</span> <span class="o">=</span> <span class="k">nullptr</span><span class="p">);</span>

 <span class="k">public</span><span class="o">:</span>
  <span class="k">static</span> <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">Scope</span><span class="o">&gt;</span> <span class="n">Create</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">Scope</span><span class="o">&gt;&amp;</span> <span class="n">parent</span> <span class="o">=</span> <span class="k">nullptr</span><span class="p">);</span>

  <span class="c1">// return nullptr if not found.</span>
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  <span class="n">Variable</span><span class="o">*</span> <span class="nf">FindVar</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span> <span class="n">name</span><span class="p">)</span> <span class="k">const</span><span class="p">;</span>
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  <span class="c1">// return if already contains same name variable.</span>
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  <span class="n">Variable</span><span class="o">*</span> <span class="nf">NewVar</span><span class="p">(</span><span class="k">const</span> <span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span> <span class="n">name</span><span class="p">);</span>
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 <span class="k">private</span><span class="o">:</span>
  <span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">Scope</span><span class="o">&gt;</span> <span class="n">parent_</span><span class="p">;</span>
  <span class="n">std</span><span class="o">::</span><span class="n">unordered_map</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="p">,</span> <span class="n">std</span><span class="o">::</span><span class="n">unique_ptr</span><span class="o">&lt;</span><span class="n">Variable</span><span class="o">&gt;&gt;</span> <span class="n">vars_</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<div class="section" id="only-scope-can-create-a-variable">
<span id="only-scope-can-create-a-variable"></span><h2>Only scope can create a variable<a class="headerlink" href="#only-scope-can-create-a-variable" title="Permalink to this headline"></a></h2>
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<p>To ensure <code class="docutils literal"><span class="pre">only</span> <span class="pre">scope</span> <span class="pre">can</span> <span class="pre">create</span> <span class="pre">a</span> <span class="pre">variable</span></code>, we should mark <code class="docutils literal"><span class="pre">Variable</span></code>&#8216;s constructor as a private member function, and Scope is a friend class of Variable. And then only <code class="docutils literal"><span class="pre">NewVar</span></code> can construct <code class="docutils literal"><span class="pre">Variable</span></code>.</p>
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</div>
<div class="section" id="when-scope-destroyed-all-variables-inside-this-scope-should-be-destroyed-together">
<span id="when-scope-destroyed-all-variables-inside-this-scope-should-be-destroyed-together"></span><h2>When scope destroyed, all variables inside this scope should be destroyed together<a class="headerlink" href="#when-scope-destroyed-all-variables-inside-this-scope-should-be-destroyed-together" title="Permalink to this headline"></a></h2>
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<p>The scope hold unique pointers for all variables. User can <code class="docutils literal"><span class="pre">FindVar</span></code> from scope, but he should not hold this pointer as a member variable. Because when scope is destroyed, all variables inside this scope will be destroyed together.</p>
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</div>
<div class="section" id="sharing-a-parent-scope">
<span id="sharing-a-parent-scope"></span><h2>Sharing a parent scope<a class="headerlink" href="#sharing-a-parent-scope" title="Permalink to this headline"></a></h2>
<p>Local scope contains a <code class="docutils literal"><span class="pre">parent_</span></code> pointer. It is a linked-list for scopes. Using a <code class="docutils literal"><span class="pre">shared_ptr</span></code> because when a local scope is using, its parents cannot be destroyed.</p>
<p>Also, as the parent scope is a <code class="docutils literal"><span class="pre">shared_ptr</span></code>, we can only <code class="docutils literal"><span class="pre">Create()</span></code> a scope shared pointer. We cannot construct a scope variable, because it cannot be passed to other scope as <code class="docutils literal"><span class="pre">parent</span></code> pointer.</p>
</div>
<div class="section" id="orthogonal-interface">
<span id="orthogonal-interface"></span><h2>Orthogonal interface<a class="headerlink" href="#orthogonal-interface" title="Permalink to this headline"></a></h2>
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<p><code class="docutils literal"><span class="pre">FindVar</span></code> will return <code class="docutils literal"><span class="pre">nullptr</span></code> when <code class="docutils literal"><span class="pre">name</span></code> is not found. It can be used as <code class="docutils literal"><span class="pre">Contains</span></code> method. <code class="docutils literal"><span class="pre">NewVar</span></code> will return a <code class="docutils literal"><span class="pre">Error</span></code> when there is a name conflict locally. Combine <code class="docutils literal"><span class="pre">FindVar</span></code> and <code class="docutils literal"><span class="pre">NewVar</span></code>, we can implement <code class="docutils literal"><span class="pre">NewVar</span></code> easily.</p>
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