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<article>
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<h2><a href="lsbasi-part19/index.html">Let’s Build A Simple Interpreter. Part 19: Nested Procedure Calls</a></h2>
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<div class="well well-sm">
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<span class="label label-default">Date</span>
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<span class="published">
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<i class="fa fa-calendar"></i><time datetime="2020-03-19T10:00:00-04:00"> Thu, March 19, 2020</time>
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</span>
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</footer><!-- /.post-info --> </div>
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<div class="summary">
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<blockquote>
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<p><em><span class="dquo">“</span>What I cannot create, I do not understand.” — Richard Feynman</em></p>
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</blockquote>
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<p>As I promised you last time, today we’re going to expand on the material covered in the previous article and talk about executing nested procedure calls. Just like last time, we will limit our focus today to procedures that can access their parameters and local variables only. We will cover accessing non-local variables in the next article.</p>
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<p>Here is the sample program for today:</p>
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<div class="highlight"><pre><span></span><span class="k">program</span> <span class="n">Main</span><span class="o">;</span>
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<span class="k">procedure</span> <span class="nf">Alpha</span><span class="p">(</span><span class="n">a</span> <span class="o">:</span> <span class="kt">integer</span><span class="o">;</span> <span class="n">b</span> <span class="o">:</span> <span class="kt">integer</span><span class="p">)</span><span class="o">;</span>
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<span class="k">var</span> <span class="n">x</span> <span class="o">:</span> <span class="kt">integer</span><span class="o">;</span>
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<span class="k">procedure</span> <span class="nf">Beta</span><span class="p">(</span><span class="n">a</span> <span class="o">:</span> <span class="kt">integer</span><span class="o">;</span> <span class="n">b</span> <span class="o">:</span> <span class="kt">integer</span><span class="p">)</span><span class="o">;</span>
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<span class="k">var</span> <span class="n">x</span> <span class="o">:</span> <span class="kt">integer</span><span class="o">;</span>
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<span class="k">begin</span>
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<span class="n">x</span> <span class="o">:=</span> <span class="n">a</span> <span class="o">*</span> <span class="mi">10</span> <span class="o">+</span> <span class="n">b</span> <span class="o">*</span> <span class="mi">2</span><span class="o">;</span>
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<span class="k">end</span><span class="o">;</span>
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<span class="k">begin</span>
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<span class="n">x</span> <span class="o">:=</span> <span class="p">(</span><span class="n">a</span> <span class="o">+</span> <span class="n">b</span> <span class="p">)</span> <span class="o">*</span> <span class="mi">2</span><span class="o">;</span>
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<span class="n">Beta</span><span class="p">(</span><span class="mi">5</span><span class="o">,</span> <span class="mi">10</span><span class="p">)</span><span class="o">;</span> <span class="cm">{ procedure call }</span>
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<span class="k">end</span><span class="o">;</span>
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<span class="k">begin</span> <span class="cm">{ Main }</span>
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<span class="n">Alpha</span><span class="p">(</span><span class="mi">3</span> <span class="o">+</span> <span class="mi">5</span><span class="o">,</span> <span class="mi">7</span><span class="p">)</span><span class="o">;</span> <span class="cm">{ procedure call }</span>
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<span class="k">end</span><span class="o">.</span> <span class="cm">{ Main }</span>
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</pre></div>
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<p>The nesting relationships diagram for the program looks like this:</p>
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<p><img alt="" src="lsbasi-part19/lsbasi_part19_nestingrel.png" width="240"></p>
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<p>Some things to note about the above program:</p>
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<ul>
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<li>
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<p>it has two procedure declarations, <em>Alpha</em> and <em>Beta</em></p>
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</li>
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<li>
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<p><em>Alpha</em> is declared inside the main program (the <em>global</em> scope)</p>
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</li>
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<li>
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<p>the <em>Beta</em> procedure is declared inside the <em>Alpha</em> procedure</p>
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</li>
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<li>
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<p>both <em>Alpha</em> and <em>Beta</em> have the same names for their formal parameters: integers <em>a</em> and <em>b</em></p>
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</li>
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<li>
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<p>and both <em>Alpha</em> and <em>Beta</em> have the same local variable <em>x</em></p>
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</li>
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<li>
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<p>the program has nested calls: the <em>Beta</em> procedure is called from the <em>Alpha</em> procedure, which, in turn, is called from the main program</p>
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</li>
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</ul>
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<p><br>
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Now, let’s do an experiment. Download the <a href="https://github.com/rspivak/lsbasi/blob/master/part19/part19.pas">part19.pas</a> file from <a href="https://github.com/rspivak/lsbasi/blob/master/part19">GitHub</a> and run the <a href="https://github.com/rspivak/lsbasi/blob/master/part18/spi.py">interpreter from the previous article</a> with the <a href="https://github.com/rspivak/lsbasi/blob/master/part19/part19.pas">part19.pas</a> file as its input to see what happens when the interpreter executes nested procedure calls (the main program calling <em>Alpha</em> calling <em>Beta</em>):</p>
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<div class="highlight"><pre><span></span>$ python spi.py part19.pas --stack
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ENTER: PROGRAM Main
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CALL STACK
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<span class="m">1</span>: PROGRAM Main
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...
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ENTER: PROCEDURE Beta
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CALL STACK
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<span class="m">2</span>: PROCEDURE Beta
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a : <span class="m">5</span>
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b : <span class="m">10</span>
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<span class="m">2</span>: PROCEDURE Alpha
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a : <span class="m">8</span>
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b : <span class="m">7</span>
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x : <span class="m">30</span>
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<span class="m">1</span>: PROGRAM Main
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LEAVE: PROCEDURE Beta
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CALL STACK
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<span class="m">2</span>: PROCEDURE Beta
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a : <span class="m">5</span>
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b : <span class="m">10</span>
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x : <span class="m">70</span>
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<span class="m">2</span>: PROCEDURE Alpha
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a : <span class="m">8</span>
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b : <span class="m">7</span>
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x : <span class="m">30</span>
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<span class="m">1</span>: PROGRAM Main
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...
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LEAVE: PROGRAM Main
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CALL STACK
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<span class="m">1</span>: PROGRAM Main
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</pre></div>
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<p><br>
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It just works! There are no errors. And if you study the contents of the ARs(activation records), you can see that the values stored in the activation records for the <em>Alpha</em> and <em>Beta</em> procedure calls are correct. So, what’s the catch then? There is one small issue. If you take a look at the output where it says ‘<span class="caps">ENTER</span>: <span class="caps">PROCEDURE</span> Beta’, you can see that the nesting level for the <em>Beta</em> and <em>Alpha</em> procedure call is the same, it’s 2 (two). The nesting level for <em>Alpha</em> should be 2 and the nesting level for <em>Beta</em> should be 3. That’s the issue that we need to fix. Right now the <em>nesting_level</em> value in the <em>visit_ProcedureCall</em> method is hardcoded to be 2 (two):</p>
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<div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">visit_ProcedureCall</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">node</span><span class="p">):</span>
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<span class="n">proc_name</span> <span class="o">=</span> <span class="n">node</span><span class="o">.</span><span class="n">proc_name</span>
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<span class="n">ar</span> <span class="o">=</span> <span class="n">ActivationRecord</span><span class="p">(</span>
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<span class="n">name</span><span class="o">=</span><span class="n">proc_name</span><span class="p">,</span>
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<span class="nb">type</span><span class="o">=</span><span class="n">ARType</span><span class="o">.</span><span class="n">PROCEDURE</span><span class="p">,</span>
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<span class="n">nesting_level</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span>
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<span class="p">)</span>
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<span class="n">proc_symbol</span> <span class="o">=</span> <span class="n">node</span><span class="o">.</span><span class="n">proc_symbol</span>
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<span class="o">...</span>
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</pre></div>
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<p><br>
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Let’s get rid of the hardcoded value. How do we determine a nesting level for a procedure call? In the method above we have a procedure symbol and it is stored in a scoped symbol table that has the right scope level that we can use as the value of the <em>nesting_level</em> parameter (see <a href="lsbasi-part14/index.html">Part 14</a> for more details about scopes and scope levels).</p>
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<p>How do we get to the scoped symbol table’s scope level through the procedure symbol?</p>
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<p>Let’s look at the following parts of the <em>ScopedSymbolTable</em> class:</p>
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<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ScopedSymbolTable</span><span class="p">:</span>
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<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">scope_name</span><span class="p">,</span> <span class="n">scope_level</span><span class="p">,</span> <span class="n">enclosing_scope</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
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<span class="o">...</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">scope_level</span> <span class="o">=</span> <span class="n">scope_level</span>
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<span class="o">...</span>
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<span class="k">def</span> <span class="nf">insert</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">symbol</span><span class="p">):</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">f</span><span class="s1">'Insert: {symbol.name}'</span><span class="p">)</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">_symbols</span><span class="p">[</span><span class="n">symbol</span><span class="o">.</span><span class="n">name</span><span class="p">]</span> <span class="o">=</span> <span class="n">symbol</span>
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</pre></div>
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<p>Looking at the code above, we can see that we could assign the scope level of a scoped symbol table to a symbol when we store the symbol in the scoped symbol table (scope) inside the <em>insert</em> method. This way we will have access to the procedure symbol’s scope level in the <em>visit_Procedure</em> method during the interpretation phase. And that’s exactly what we need.</p>
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<p>Let’s make the necessary changes:</p>
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<ul>
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<li>
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<p>First, let’s add a <em>scope_level</em> member to the <em>Symbol</em> class and give it a default value of zero:</p>
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<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Symbol</span><span class="p">:</span>
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<span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
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<span class="o">...</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">scope_level</span> <span class="o">=</span> <span class="mi">0</span>
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</pre></div>
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</li>
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<li>
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<p>Next, let’s assign the corresponding scope level to a symbol when storing the symbol in a scoped symbol table:</p>
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<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ScopedSymbolTable</span><span class="p">:</span>
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<span class="o">...</span>
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<span class="k">def</span> <span class="nf">insert</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">symbol</span><span class="p">):</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">log</span><span class="p">(</span><span class="n">f</span><span class="s1">'Insert: {symbol.name}'</span><span class="p">)</span>
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<span class="n">symbol</span><span class="o">.</span><span class="n">scope_level</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">scope_level</span>
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<span class="bp">self</span><span class="o">.</span><span class="n">_symbols</span><span class="p">[</span><span class="n">symbol</span><span class="o">.</span><span class="n">name</span><span class="p">]</span> <span class="o">=</span> <span class="n">symbol</span>
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</pre></div>
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</li>
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</ul>
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<p><br>
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Now, when creating an <span class="caps">AR</span> for a procedure call in the <em>visit_ProcedureCall</em> method, we have access to the scope level of the procedure symbol. All that’s left to do is use the scope level of the procedure symbol as the value of the <em>nesting_level</em> parameter:</p>
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<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Interpreter</span><span class="p">(</span><span class="n">NodeVisitor</span><span class="p">):</span>
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<span class="o">...</span>
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<span class="k">def</span> <span class="nf">visit_ProcedureCall</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">node</span><span class="p">):</span>
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<span class="n">proc_name</span> <span class="o">=</span> <span class="n">node</span><span class="o">.</span><span class="n">proc_name</span>
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<span class="n">proc_symbol</span> <span class="o">=</span> <span class="n">node</span><span class="o">.</span><span class="n">proc_symbol</span>
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<span class="n">ar</span> <span class="o">=</span> <span class="n">ActivationRecord</span><span class="p">(</span>
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<span class="n">name</span><span class="o">=</span><span class="n">proc_name</span><span class="p">,</span>
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<span class="nb">type</span><span class="o">=</span><span class="n">ARType</span><span class="o">.</span><span class="n">PROCEDURE</span><span class="p">,</span>
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<span class="n">nesting_level</span><span class="o">=</span><span class="n">proc_symbol</span><span class="o">.</span><span class="n">scope_level</span> <span class="o">+</span> <span class="mi">1</span><span class="p">,</span>
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<span class="p">)</span>
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</pre></div>
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<p>That’s great, no more hardcoded nesting levels. One thing worth mentioning is why we put <em>proc_symbol.scope_level + 1</em> as the value of the <em>nesting_level</em> parameter and not just <em>proc_symbol.scope_level.</em> In <a href="lsbasi-part17.html">Part 17</a>, I mentioned that the nesting level of an <span class="caps">AR</span> corresponds to the scope level of the respective procedure or function declaration plus one. Let’s see why.</p>
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<p>In our sample program for today, the <em>Alpha</em> procedure symbol - the symbol that contains information about the <em>Alpha</em> procedure declaration - is stored in the <em>global</em> scope at level 1 (one). So 1 is the value of the <em>Alpha</em> procedure symbol’s <em>scope_level</em>. But as we know from <a href="lsbasi-part14/index.html">Part14</a>, the scope level of the procedure declaration <em>Alpha</em> is one less than the level of the variables declared inside the procedure <em>Alpha</em>. So, to get the scope level of the scope where the <em>Alpha</em> procedure’s parameters and local variables are stored, we need to increment the procedure symbol’s scope level by 1. That’s the reason we use <em>proc_symbol.scope_level + 1</em> as the value of the <em>nesting_level</em> parameter when creating an <span class="caps">AR</span> for a procedure call and not simply <em>proc_symbol.scope_level</em>.</p>
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<p>Let’s see the changes we’ve made so far in action. Download <a href="https://github.com/rspivak/lsbasi/blob/master/part19">the updated interpreter</a> and test it again with the <a href="https://github.com/rspivak/lsbasi/blob/master/part19">part19.pas</a> file as its input:</p>
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<div class="highlight"><pre><span></span>$ python spi.py part19.pas --stack
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ENTER: PROGRAM Main
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CALL STACK
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<span class="m">1</span>: PROGRAM Main
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ENTER: PROCEDURE Alpha
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CALL STACK
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<span class="m">2</span>: PROCEDURE Alpha
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a : <span class="m">8</span>
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b : <span class="m">7</span>
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<span class="m">1</span>: PROGRAM Main
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ENTER: PROCEDURE Beta
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CALL STACK
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<span class="m">3</span>: PROCEDURE Beta
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a : <span class="m">5</span>
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b : <span class="m">10</span>
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<span class="m">2</span>: PROCEDURE Alpha
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a : <span class="m">8</span>
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b : <span class="m">7</span>
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x : <span class="m">30</span>
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<span class="m">1</span>: PROGRAM Main
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LEAVE: PROCEDURE Beta
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|
CALL STACK
|
|
<span class="m">3</span>: PROCEDURE Beta
|
|
a : <span class="m">5</span>
|
|
b : <span class="m">10</span>
|
|
x : <span class="m">70</span>
|
|
<span class="m">2</span>: PROCEDURE Alpha
|
|
a : <span class="m">8</span>
|
|
b : <span class="m">7</span>
|
|
x : <span class="m">30</span>
|
|
<span class="m">1</span>: PROGRAM Main
|
|
|
|
|
|
LEAVE: PROCEDURE Alpha
|
|
CALL STACK
|
|
<span class="m">2</span>: PROCEDURE Alpha
|
|
a : <span class="m">8</span>
|
|
b : <span class="m">7</span>
|
|
x : <span class="m">30</span>
|
|
<span class="m">1</span>: PROGRAM Main
|
|
|
|
|
|
LEAVE: PROGRAM Main
|
|
CALL STACK
|
|
<span class="m">1</span>: PROGRAM Main
|
|
</pre></div>
|
|
|
|
|
|
<p>As you can see from the output above, the nesting levels of the activation records (<span class="caps">AR</span>) now have the correct values:</p>
|
|
<ul>
|
|
<li>
|
|
<p>The <em>Main</em> program <span class="caps">AR</span>: nesting level 1</p>
|
|
</li>
|
|
<li>
|
|
<p>The <em>Alpha</em> procedure <span class="caps">AR</span>: nesting level 2</p>
|
|
</li>
|
|
<li>
|
|
<p>The <em>Beta</em> procedure <span class="caps">AR</span>: nesting level 3</p>
|
|
</li>
|
|
</ul>
|
|
<p><br>
|
|
Let’s take a look at how the scope tree (scoped symbol tables) and the call stack look visually during the execution of the program. Here is how the call stack looks right after the message “<span class="caps">LEAVE</span>: <span class="caps">PROCEDURE</span> Beta” and before the <span class="caps">AR</span> for the <em>Beta</em> procedure call is popped off the stack:</p>
|
|
<p><img alt="" src="lsbasi-part19/lsbasi_part19_callstack.png" width="640"></p>
|
|
<p>And if we flip the call stack (so that the top of the stack is at the “bottom”), you can see how the call stack with activation records relates to the scope tree with scopes (scoped symbol tables). In fact, we can say that activation records are run-time equivalents of scopes. Scopes are created during semantic analysis of a source program (the source program is read, parsed, and analyzed at this stage, but not executed) and the call stack with activation records is created at run-time when the interpreter executes the source program:</p>
|
|
<p><img alt="" src="lsbasi-part19/lsbasi_part19_scopes_callstack.png" width="640"></p>
|
|
<p>As you’ve seen in this article, we haven’t made a lot of changes to support the execution of nested procedure calls. The only real change was to make sure the nesting level in ARs was correct. The rest of the codebase stayed the same. The main reason why our code continues to work pretty much unchanged with nested procedure calls is because the <em>Alpha</em> and <em>Beta</em> procedures in the sample program access the values of local variables only (including their own parameters). And because those values are stored in the <span class="caps">AR</span> at the top of the stack, this allows us to continue to use the methods <em>visit_Assignment</em> and <em>visit_Var</em> without any change, when executing the body of the procedures. Here is the source code of the methods again:</p>
|
|
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">visit_Assign</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">node</span><span class="p">):</span>
|
|
<span class="n">var_name</span> <span class="o">=</span> <span class="n">node</span><span class="o">.</span><span class="n">left</span><span class="o">.</span><span class="n">value</span>
|
|
<span class="n">var_value</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">visit</span><span class="p">(</span><span class="n">node</span><span class="o">.</span><span class="n">right</span><span class="p">)</span>
|
|
|
|
<span class="n">ar</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">call_stack</span><span class="o">.</span><span class="n">peek</span><span class="p">()</span>
|
|
<span class="n">ar</span><span class="p">[</span><span class="n">var_name</span><span class="p">]</span> <span class="o">=</span> <span class="n">var_value</span>
|
|
|
|
<span class="k">def</span> <span class="nf">visit_Var</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">node</span><span class="p">):</span>
|
|
<span class="n">var_name</span> <span class="o">=</span> <span class="n">node</span><span class="o">.</span><span class="n">value</span>
|
|
|
|
<span class="n">ar</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">call_stack</span><span class="o">.</span><span class="n">peek</span><span class="p">()</span>
|
|
<span class="n">var_value</span> <span class="o">=</span> <span class="n">ar</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="n">var_name</span><span class="p">)</span>
|
|
|
|
<span class="k">return</span> <span class="n">var_value</span>
|
|
</pre></div>
|
|
|
|
|
|
<p><br>
|
|
Okay, today we’ve been able to successfully execute nested procedure calls with our interpreter with very few changes. And now we’re one step closer to properly executing recursive procedure calls.</p>
|
|
<p>That’s it for today. In the next article, we’ll talk about how procedures can access non-local variables during run-time.</p>
|
|
<p><br/>
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<strong>Stay safe, stay healthy, and take care of each other! See you next time.</strong></p>
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<p><br/>
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<p><br/>
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<strong>All articles in this series:</strong>
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<ul>
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<li>
|
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<a href="lsbasi-part1/index.html">Let's Build A Simple Interpreter. Part 1.</a>
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</li>
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<li>
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<a href="lsbasi-part2/index.html">Let's Build A Simple Interpreter. Part 2.</a>
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</li>
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<li>
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<a href="lsbasi-part3/index.html">Let's Build A Simple Interpreter. Part 3.</a>
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</li>
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<li>
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<a href="lsbasi-part4/index.html">Let's Build A Simple Interpreter. Part 4.</a>
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</li>
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<li>
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<a href="lsbasi-part5/index.html">Let's Build A Simple Interpreter. Part 5.</a>
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</li>
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<li>
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<a href="lsbasi-part6/index.html">Let's Build A Simple Interpreter. Part 6.</a>
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</li>
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<li>
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<a href="lsbasi-part7/index.html">Let's Build A Simple Interpreter. Part 7: Abstract Syntax Trees</a>
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</li>
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<li>
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|
<a href="lsbasi-part8/index.html">Let's Build A Simple Interpreter. Part 8.</a>
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</li>
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<li>
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<a href="lsbasi-part9/index.html">Let's Build A Simple Interpreter. Part 9.</a>
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</li>
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<li>
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<a href="lsbasi-part10/index.html">Let's Build A Simple Interpreter. Part 10.</a>
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</li>
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<li>
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<a href="lsbasi-part11/index.html">Let's Build A Simple Interpreter. Part 11.</a>
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</li>
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<li>
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<a href="lsbasi-part12/index.html">Let's Build A Simple Interpreter. Part 12.</a>
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</li>
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<a href="lsbasi-part13.html">Let's Build A Simple Interpreter. Part 13: Semantic Analysis</a>
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</li>
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<li>
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<a href="lsbasi-part14/index.html">Let's Build A Simple Interpreter. Part 14: Nested Scopes and a Source-to-Source Compiler</a>
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</li>
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<li>
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<a href="lsbasi-part15/index.html">Let's Build A Simple Interpreter. Part 15.</a>
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<li>
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<a href="lsbasi-part16/index.html">Let's Build A Simple Interpreter. Part 16: Recognizing Procedure Calls</a>
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<a href="lsbasi-part17.html">Let's Build A Simple Interpreter. Part 17: Call Stack and Activation Records</a>
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<a href="lsbasi-part18/index.html">Let's Build A Simple Interpreter. Part 18: Executing Procedure Calls</a>
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<li>
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<a href="lsbasi-part19/index.html">Let's Build A Simple Interpreter. Part 19: Nested Procedure Calls</a>
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