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<title>Let’s Build A Simple Interpreter. Part 12. - Ruslan's Blog</title>
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Let&#8217;s Build A Simple Interpreter. Part&nbsp;12.
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<blockquote>
<p><em><span class="dquo">&#8220;</span>Be not afraid of going slowly; be afraid only of standing still.&#8221; - Chinese&nbsp;proverb.</em></p>
</blockquote>
<p>Hello, and welcome&nbsp;back!</p>
<p>Today we are going to take a few more baby steps and learn how to parse Pascal procedure&nbsp;declarations.</p>
<p><img alt="" src="lsbasi_part12_babysteps.png" width="700"></p>
<p>What is a <em><strong>procedure declaration</strong></em>? A <em><strong>procedure declaration</strong></em> is a language construct that defines an identifier (a procedure name) and associates it with a block of Pascal&nbsp;code.</p>
<p>Before we dive in, a few words about Pascal procedures and their&nbsp;declarations:</p>
<ul>
<li>Pascal procedures don’t have return statements. They exit when they reach the end of their corresponding&nbsp;block.</li>
<li>Pascal procedures can be nested within each&nbsp;other.</li>
<li>For simplicity reasons, procedure declarations in this article won&#8217;t have any formal parameters. But, don&#8217;t worry, we&#8217;ll cover that later in the&nbsp;series.</li>
</ul>
<p>This is our test program for&nbsp;today:</p>
<div class="highlight"><pre><span></span><span class="k">PROGRAM</span> <span class="n">Part12</span><span class="o">;</span>
<span class="k">VAR</span>
<span class="n">a</span> <span class="o">:</span> <span class="kt">INTEGER</span><span class="o">;</span>
<span class="k">PROCEDURE</span> <span class="nf">P1</span><span class="o">;</span>
<span class="k">VAR</span>
<span class="n">a</span> <span class="o">:</span> <span class="kt">REAL</span><span class="o">;</span>
<span class="n">k</span> <span class="o">:</span> <span class="kt">INTEGER</span><span class="o">;</span>
<span class="k">PROCEDURE</span> <span class="nf">P2</span><span class="o">;</span>
<span class="k">VAR</span>
<span class="n">a</span><span class="o">,</span> <span class="n">z</span> <span class="o">:</span> <span class="kt">INTEGER</span><span class="o">;</span>
<span class="k">BEGIN</span> <span class="cm">{P2}</span>
<span class="n">z</span> <span class="o">:=</span> <span class="mi">777</span><span class="o">;</span>
<span class="k">END</span><span class="o">;</span> <span class="cm">{P2}</span>
<span class="k">BEGIN</span> <span class="cm">{P1}</span>
<span class="k">END</span><span class="o">;</span> <span class="cm">{P1}</span>
<span class="k">BEGIN</span> <span class="cm">{Part12}</span>
<span class="n">a</span> <span class="o">:=</span> <span class="mi">10</span><span class="o">;</span>
<span class="k">END</span><span class="o">.</span> <span class="cm">{Part12}</span>
</pre></div>
<p>As you can see above, we have defined two procedures (<em>P1</em> and <em>P2</em>) and <em>P2</em> is nested within <em>P1</em>.
In the code above, I used comments with a procedure’s name to clearly indicate where the body of every procedure begins and where it&nbsp;ends.</p>
<p>Our objective for today is pretty clear: learn how to parse a code like&nbsp;that.</p>
<p><br/></p>
<p>First, we need to make some changes to our grammar to add procedure declarations. Well, let’s just do&nbsp;that!</p>
<p>Here is the updated <em>declarations</em> grammar&nbsp;rule:</p>
<p><img alt="" src="lsbasi_part12_grammar.png" width="640"></p>
<p>The procedure declaration sub-rule consists of the reserved keyword <strong><span class="caps">PROCEDURE</span></strong> followed by an identifier (a procedure name),
followed by a semicolon, which in turn is followed by a <em>block</em> rule, which is terminated by a semicolon.
Whoa! This is a case where I think the picture is actually worth however many words I just put in the previous sentence!&nbsp;:)</p>
<p>Here is the updated syntax diagram for the <em>declarations</em>&nbsp;rule:</p>
<p><img alt="" src="lsbasi_part12_syntaxdiagram.png" width="640"></p>
<p>From the grammar and the diagram above you can see that you can have as many procedure declarations on the same level as you want.
For example, in the code snippet below we define two procedure declarations, <em>P1</em> and <em><span class="caps">P1A</span></em>, on the same&nbsp;level:</p>
<div class="highlight"><pre><span></span><span class="k">PROGRAM</span> <span class="n">Test</span><span class="o">;</span>
<span class="k">VAR</span>
<span class="n">a</span> <span class="o">:</span> <span class="kt">INTEGER</span><span class="o">;</span>
<span class="k">PROCEDURE</span> <span class="nf">P1</span><span class="o">;</span>
<span class="k">BEGIN</span> <span class="cm">{P1}</span>
<span class="k">END</span><span class="o">;</span> <span class="cm">{P1}</span>
<span class="k">PROCEDURE</span> <span class="nf">P1A</span><span class="o">;</span>
<span class="k">BEGIN</span> <span class="cm">{P1A}</span>
<span class="k">END</span><span class="o">;</span> <span class="cm">{P1A}</span>
<span class="k">BEGIN</span> <span class="cm">{Test}</span>
<span class="n">a</span> <span class="o">:=</span> <span class="mi">10</span><span class="o">;</span>
<span class="k">END</span><span class="o">.</span> <span class="cm">{Test}</span>
</pre></div>
<p>The diagram and the grammar rule above also indicate that procedure declarations can be nested because the
<em>procedure declaration</em> sub-rule references the <em>block</em> rule which contains the <em>declarations</em> rule, which in turn
contains the <em>procedure declaration</em> sub-rule. As a reminder, here is the syntax diagram and the grammar
for the block rule from <a href="../lsbasi-part10/index.html">Part10</a>:</p>
<p><img alt="" src="lsbasi_part12_block_rule_from_part10.png" width="800"></p>
<p><br/>
Okay, now let&#8217;s focus on the interpreter components that need to be updated to support procedure&nbsp;declarations:</p>
<p><em><strong>Updating the&nbsp;Lexer</strong></em></p>
<p>All we need to do is add a new token named <strong><span class="caps">PROCEDURE</span></strong>:</p>
<div class="highlight"><pre><span></span><span class="n">PROCEDURE</span> <span class="o">=</span> <span class="s1">&#39;PROCEDURE&#39;</span>
</pre></div>
<p>And add <em>&#8216;<span class="caps">PROCEDURE</span>&#8217;</em> to the reserved keywords. Here is the complete mapping of reserved keywords to&nbsp;tokens:</p>
<div class="highlight"><pre><span></span><span class="n">RESERVED_KEYWORDS</span> <span class="o">=</span> <span class="p">{</span>
<span class="s1">&#39;PROGRAM&#39;</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">&#39;PROGRAM&#39;</span><span class="p">,</span> <span class="s1">&#39;PROGRAM&#39;</span><span class="p">),</span>
<span class="s1">&#39;VAR&#39;</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">&#39;VAR&#39;</span><span class="p">,</span> <span class="s1">&#39;VAR&#39;</span><span class="p">),</span>
<span class="s1">&#39;DIV&#39;</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">&#39;INTEGER_DIV&#39;</span><span class="p">,</span> <span class="s1">&#39;DIV&#39;</span><span class="p">),</span>
<span class="s1">&#39;INTEGER&#39;</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">&#39;INTEGER&#39;</span><span class="p">,</span> <span class="s1">&#39;INTEGER&#39;</span><span class="p">),</span>
<span class="s1">&#39;REAL&#39;</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">&#39;REAL&#39;</span><span class="p">,</span> <span class="s1">&#39;REAL&#39;</span><span class="p">),</span>
<span class="s1">&#39;BEGIN&#39;</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">&#39;BEGIN&#39;</span><span class="p">,</span> <span class="s1">&#39;BEGIN&#39;</span><span class="p">),</span>
<span class="s1">&#39;END&#39;</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">&#39;END&#39;</span><span class="p">,</span> <span class="s1">&#39;END&#39;</span><span class="p">),</span>
<span class="s1">&#39;PROCEDURE&#39;</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">&#39;PROCEDURE&#39;</span><span class="p">,</span> <span class="s1">&#39;PROCEDURE&#39;</span><span class="p">),</span>
<span class="p">}</span>
</pre></div>
<p><br/>
<em><strong>Updating the&nbsp;Parser</strong></em></p>
<p>Here is a summary of the parser&nbsp;changes:</p>
<ol>
<li>New <em>ProcedureDecl</em> <span class="caps">AST</span>&nbsp;node</li>
<li>Update to the parser’s <em>declarations</em> method to support procedure&nbsp;declarations</li>
</ol>
<p>Let’s go over the&nbsp;changes.</p>
<ol>
<li>
<p>The <em>ProcedureDecl</em> <span class="caps">AST</span> node represents a <em>procedure declaration</em>. The class constructor takes as parameters the name of the procedure and the <span class="caps">AST</span> node of the block of code that the procedure’s name refers&nbsp;to.</p>
<div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">ProcedureDecl</span><span class="p">(</span><span class="n">AST</span><span class="p">):</span>
<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">proc_name</span><span class="p">,</span> <span class="n">block_node</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">proc_name</span> <span class="o">=</span> <span class="n">proc_name</span>
<span class="bp">self</span><span class="o">.</span><span class="n">block_node</span> <span class="o">=</span> <span class="n">block_node</span>
</pre></div>
</li>
<li>
<p>Here is the updated <em>declarations</em> method of the <em>Parser</em>&nbsp;class</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">declarations</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">&quot;&quot;&quot;declarations : VAR (variable_declaration SEMI)+</span>
<span class="sd"> | (PROCEDURE ID SEMI block SEMI)*</span>
<span class="sd"> | empty</span>
<span class="sd"> &quot;&quot;&quot;</span>
<span class="n">declarations</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_token</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">VAR</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">VAR</span><span class="p">)</span>
<span class="k">while</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_token</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">ID</span><span class="p">:</span>
<span class="n">var_decl</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">variable_declaration</span><span class="p">()</span>
<span class="n">declarations</span><span class="o">.</span><span class="n">extend</span><span class="p">(</span><span class="n">var_decl</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">SEMI</span><span class="p">)</span>
<span class="k">while</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_token</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">PROCEDURE</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">PROCEDURE</span><span class="p">)</span>
<span class="n">proc_name</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_token</span><span class="o">.</span><span class="n">value</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">ID</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">SEMI</span><span class="p">)</span>
<span class="n">block_node</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">block</span><span class="p">()</span>
<span class="n">proc_decl</span> <span class="o">=</span> <span class="n">ProcedureDecl</span><span class="p">(</span><span class="n">proc_name</span><span class="p">,</span> <span class="n">block_node</span><span class="p">)</span>
<span class="n">declarations</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">proc_decl</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">SEMI</span><span class="p">)</span>
<span class="k">return</span> <span class="n">declarations</span>
</pre></div>
<p>Hopefully, the code above is pretty self-explanatory. It follows the grammar/syntax diagram for procedure declarations that you’ve seen earlier in the&nbsp;article.</p>
</li>
</ol>
<p><br/>
<em><strong>Updating the SymbolTable&nbsp;builder</strong></em></p>
<p>Because we’re not ready yet to handle nested procedure scopes, we’ll simply add an empty <em>visit_ProcedureDecl</em> method to the <em>SymbolTreeBuilder</em> <span class="caps">AST</span> visitor class. We&#8217;ll fill it out in the next&nbsp;article.</p>
<div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">visit_ProcedureDecl</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="k">pass</span>
</pre></div>
<p></br>
<em><strong>Updating the&nbsp;Interpreter</strong></em></p>
<p>We also need to add an empty <em>visit_ProcedureDecl</em> method to the <em>Interpreter</em> class, which will cause our interpreter to silently ignore all our procedure&nbsp;declarations.</p>
<p>So far, so&nbsp;good.</p>
<p></br>
Now that we&#8217;ve made all the necessary changes, let&#8217;s see what the <em>Abstract Syntax Tree</em> looks like with the new <em>ProcedureDecl</em>&nbsp;nodes.</p>
<p>Here is our Pascal program again (you can download it directly from <a href="https://github.com/rspivak/lsbasi/blob/master/part12/python/part12.pas">GitHub</a>):</p>
<div class="highlight"><pre><span></span><span class="k">PROGRAM</span> <span class="n">Part12</span><span class="o">;</span>
<span class="k">VAR</span>
<span class="n">a</span> <span class="o">:</span> <span class="kt">INTEGER</span><span class="o">;</span>
<span class="k">PROCEDURE</span> <span class="nf">P1</span><span class="o">;</span>
<span class="k">VAR</span>
<span class="n">a</span> <span class="o">:</span> <span class="kt">REAL</span><span class="o">;</span>
<span class="n">k</span> <span class="o">:</span> <span class="kt">INTEGER</span><span class="o">;</span>
<span class="k">PROCEDURE</span> <span class="nf">P2</span><span class="o">;</span>
<span class="k">VAR</span>
<span class="n">a</span><span class="o">,</span> <span class="n">z</span> <span class="o">:</span> <span class="kt">INTEGER</span><span class="o">;</span>
<span class="k">BEGIN</span> <span class="cm">{P2}</span>
<span class="n">z</span> <span class="o">:=</span> <span class="mi">777</span><span class="o">;</span>
<span class="k">END</span><span class="o">;</span> <span class="cm">{P2}</span>
<span class="k">BEGIN</span> <span class="cm">{P1}</span>
<span class="k">END</span><span class="o">;</span> <span class="cm">{P1}</span>
<span class="k">BEGIN</span> <span class="cm">{Part12}</span>
<span class="n">a</span> <span class="o">:=</span> <span class="mi">10</span><span class="o">;</span>
<span class="k">END</span><span class="o">.</span> <span class="cm">{Part12}</span>
</pre></div>
<p><br/>
Let&#8217;s generate an <span class="caps">AST</span> and visualize it with the <a href="https://github.com/rspivak/lsbasi/blob/master/part12/python/genastdot.py">genastdot.py</a>&nbsp;utility:</p>
<div class="highlight"><pre><span></span>$ python genastdot.py part12.pas &gt; ast.dot <span class="o">&amp;&amp;</span> dot -Tpng -o ast.png ast.dot
</pre></div>
<p><img alt="" src="lsbasi_part12_procdecl_ast.png"></p>
<p>In the picture above you can see two <em>ProcedureDecl</em> nodes: <em>ProcDecl:P1</em> and <em>ProcDecl:P2</em> that
correspond to procedures <em>P1</em> and <em>P2</em>. Mission accomplished.&nbsp;:)</p>
<p>As a last item for today, let&#8217;s quickly check that our updated interpreter works as before when
a Pascal program has procedure declarations in it. Download <a href="https://github.com/rspivak/lsbasi/blob/master/part12/python/spi.py">the interpreter</a>
and <a href="https://github.com/rspivak/lsbasi/blob/master/part12/python/part12.pas">the test program</a> if you haven&#8217;t done so yet, and run it on the command line.
Your output should look similar to&nbsp;this:</p>
<div class="highlight"><pre><span></span>$ python spi.py part12.pas
Define: INTEGER
Define: REAL
Lookup: INTEGER
Define: &lt;a:INTEGER&gt;
Lookup: a
Symbol Table contents:
Symbols: <span class="o">[</span>INTEGER, REAL, &lt;a:INTEGER&gt;<span class="o">]</span>
Run-time GLOBAL_MEMORY contents:
<span class="nv">a</span> <span class="o">=</span> <span class="m">10</span>
</pre></div>
<p></br>
Okay, with all that knowledge and experience under our belt, we’re ready to tackle the topic of nested scopes
that we need to understand in order to be able to analyze nested procedures and prepare ourselves to
handle procedure and function calls. And that&#8217;s exactly what we are going to do in the next article:
dive deep into nested scopes. So don&#8217;t forget to bring your swimming gear next time! Stay tuned and see you&nbsp;soon!</p>
<p><br/>
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<p><br/>
<strong>All articles in this series:</strong>
<ul>
<li>
<a href="../lsbasi-part1/index.html">Let's Build A Simple Interpreter. Part 1.</a>
</li>
<li>
<a href="../lsbasi-part2/index.html">Let's Build A Simple Interpreter. Part 2.</a>
</li>
<li>
<a href="../lsbasi-part3/index.html">Let's Build A Simple Interpreter. Part 3.</a>
</li>
<li>
<a href="../lsbasi-part4/index.html">Let's Build A Simple Interpreter. Part 4.</a>
</li>
<li>
<a href="../lsbasi-part5/index.html">Let's Build A Simple Interpreter. Part 5.</a>
</li>
<li>
<a href="../lsbasi-part6/index.html">Let's Build A Simple Interpreter. Part 6.</a>
</li>
<li>
<a href="../lsbasi-part7/index.html">Let's Build A Simple Interpreter. Part 7: Abstract Syntax Trees</a>
</li>
<li>
<a href="../lsbasi-part8/index.html">Let's Build A Simple Interpreter. Part 8.</a>
</li>
<li>
<a href="../lsbasi-part9/index.html">Let's Build A Simple Interpreter. Part 9.</a>
</li>
<li>
<a href="../lsbasi-part10/index.html">Let's Build A Simple Interpreter. Part 10.</a>
</li>
<li>
<a href="../lsbasi-part11/index.html">Let's Build A Simple Interpreter. Part 11.</a>
</li>
<li>
<a href="index.html">Let's Build A Simple Interpreter. Part 12.</a>
</li>
<li>
<a href="../lsbasi-part13.html">Let's Build A Simple Interpreter. Part 13: Semantic Analysis</a>
</li>
<li>
<a href="../lsbasi-part14/index.html">Let's Build A Simple Interpreter. Part 14: Nested Scopes and a Source-to-Source Compiler</a>
</li>
<li>
<a href="../lsbasi-part15/index.html">Let's Build A Simple Interpreter. Part 15.</a>
</li>
<li>
<a href="../lsbasi-part16/index.html">Let's Build A Simple Interpreter. Part 16: Recognizing Procedure Calls</a>
</li>
<li>
<a href="../lsbasi-part17.html">Let's Build A Simple Interpreter. Part 17: Call Stack and Activation Records</a>
</li>
<li>
<a href="../lsbasi-part18/index.html">Let's Build A Simple Interpreter. Part 18: Executing Procedure Calls</a>
</li>
<li>
<a href="../lsbasi-part19/index.html">Let's Build A Simple Interpreter. Part 19: Nested Procedure Calls</a>
</li>
</ul>
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