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<title>Let’s Build A Simple Interpreter. Part 12. - Ruslan's Blog</title>
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<meta name="description" content="“Be not afraid of going slowly; be afraid only of standing still.” - Chinese proverb. Hello, and welcome back! Today we are going to take a few more baby steps and learn how to parse Pascal procedure declarations. What is a procedure declaration? A procedure declaration is a language construct that …" />
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<meta property="og:description" content="“Be not afraid of going slowly; be afraid only of standing still.” - Chinese proverb. Hello, and welcome back! Today we are going to take a few more baby steps and learn how to parse Pascal procedure declarations. What is a procedure declaration? A procedure declaration is a language construct that …"/>
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<i class="fa fa-calendar"></i><time datetime="2016-12-01T16:20:00-05:00"> Thu, December 01, 2016</time>
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<p><em><span class="dquo">“</span>Be not afraid of going slowly; be afraid only of standing still.” - Chinese proverb.</em></p>
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</blockquote>
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<p>Hello, and welcome back!</p>
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<p>Today we are going to take a few more baby steps and learn how to parse Pascal procedure declarations.</p>
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<p><img alt="" src="lsbasi_part12_babysteps.png" width="700"></p>
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<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 code.</p>
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<p>Before we dive in, a few words about Pascal procedures and their declarations:</p>
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<ul>
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<li>Pascal procedures don’t have return statements. They exit when they reach the end of their corresponding block.</li>
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<li>Pascal procedures can be nested within each other.</li>
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<li>For simplicity reasons, procedure declarations in this article won’t have any formal parameters. But, don’t worry, we’ll cover that later in the series.</li>
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</ul>
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<p>This is our test program for today:</p>
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<div class="highlight"><pre><span></span><span class="k">PROGRAM</span> <span class="n">Part12</span><span class="o">;</span>
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<span class="k">VAR</span>
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<span class="n">a</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">P1</span><span class="o">;</span>
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<span class="k">VAR</span>
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<span class="n">a</span> <span class="o">:</span> <span class="kt">REAL</span><span class="o">;</span>
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<span class="n">k</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">P2</span><span class="o">;</span>
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<span class="k">VAR</span>
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<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>
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<span class="k">BEGIN</span> <span class="cm">{P2}</span>
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<span class="n">z</span> <span class="o">:=</span> <span class="mi">777</span><span class="o">;</span>
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<span class="k">END</span><span class="o">;</span> <span class="cm">{P2}</span>
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<span class="k">BEGIN</span> <span class="cm">{P1}</span>
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<span class="k">END</span><span class="o">;</span> <span class="cm">{P1}</span>
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<span class="k">BEGIN</span> <span class="cm">{Part12}</span>
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<span class="n">a</span> <span class="o">:=</span> <span class="mi">10</span><span class="o">;</span>
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<span class="k">END</span><span class="o">.</span> <span class="cm">{Part12}</span>
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</pre></div>
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<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>.
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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 ends.</p>
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<p>Our objective for today is pretty clear: learn how to parse a code like that.</p>
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<p><br/></p>
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<p>First, we need to make some changes to our grammar to add procedure declarations. Well, let’s just do that!</p>
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<p>Here is the updated <em>declarations</em> grammar rule:</p>
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<p><img alt="" src="lsbasi_part12_grammar.png" width="640"></p>
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<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),
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followed by a semicolon, which in turn is followed by a <em>block</em> rule, which is terminated by a semicolon.
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Whoa! This is a case where I think the picture is actually worth however many words I just put in the previous sentence! :)</p>
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<p>Here is the updated syntax diagram for the <em>declarations</em> rule:</p>
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<p><img alt="" src="lsbasi_part12_syntaxdiagram.png" width="640"></p>
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<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.
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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 level:</p>
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<div class="highlight"><pre><span></span><span class="k">PROGRAM</span> <span class="n">Test</span><span class="o">;</span>
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<span class="k">VAR</span>
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<span class="n">a</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">P1</span><span class="o">;</span>
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<span class="k">BEGIN</span> <span class="cm">{P1}</span>
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<span class="k">END</span><span class="o">;</span> <span class="cm">{P1}</span>
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<span class="k">PROCEDURE</span> <span class="nf">P1A</span><span class="o">;</span>
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<span class="k">BEGIN</span> <span class="cm">{P1A}</span>
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<span class="k">END</span><span class="o">;</span> <span class="cm">{P1A}</span>
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<span class="k">BEGIN</span> <span class="cm">{Test}</span>
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<span class="n">a</span> <span class="o">:=</span> <span class="mi">10</span><span class="o">;</span>
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<span class="k">END</span><span class="o">.</span> <span class="cm">{Test}</span>
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</pre></div>
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<p>The diagram and the grammar rule above also indicate that procedure declarations can be nested because the
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<em>procedure declaration</em> sub-rule references the <em>block</em> rule which contains the <em>declarations</em> rule, which in turn
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contains the <em>procedure declaration</em> sub-rule. As a reminder, here is the syntax diagram and the grammar
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for the block rule from <a href="../lsbasi-part10/index.html">Part10</a>:</p>
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<p><img alt="" src="lsbasi_part12_block_rule_from_part10.png" width="800"></p>
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<p><br/>
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Okay, now let’s focus on the interpreter components that need to be updated to support procedure declarations:</p>
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<p><em><strong>Updating the Lexer</strong></em></p>
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<p>All we need to do is add a new token named <strong><span class="caps">PROCEDURE</span></strong>:</p>
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<div class="highlight"><pre><span></span><span class="n">PROCEDURE</span> <span class="o">=</span> <span class="s1">'PROCEDURE'</span>
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</pre></div>
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<p>And add <em>‘<span class="caps">PROCEDURE</span>’</em> to the reserved keywords. Here is the complete mapping of reserved keywords to tokens:</p>
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<div class="highlight"><pre><span></span><span class="n">RESERVED_KEYWORDS</span> <span class="o">=</span> <span class="p">{</span>
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<span class="s1">'PROGRAM'</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">'PROGRAM'</span><span class="p">,</span> <span class="s1">'PROGRAM'</span><span class="p">),</span>
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<span class="s1">'VAR'</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">'VAR'</span><span class="p">,</span> <span class="s1">'VAR'</span><span class="p">),</span>
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<span class="s1">'DIV'</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">'INTEGER_DIV'</span><span class="p">,</span> <span class="s1">'DIV'</span><span class="p">),</span>
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<span class="s1">'INTEGER'</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">'INTEGER'</span><span class="p">,</span> <span class="s1">'INTEGER'</span><span class="p">),</span>
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<span class="s1">'REAL'</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">'REAL'</span><span class="p">,</span> <span class="s1">'REAL'</span><span class="p">),</span>
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<span class="s1">'BEGIN'</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">'BEGIN'</span><span class="p">,</span> <span class="s1">'BEGIN'</span><span class="p">),</span>
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<span class="s1">'END'</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">'END'</span><span class="p">,</span> <span class="s1">'END'</span><span class="p">),</span>
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<span class="s1">'PROCEDURE'</span><span class="p">:</span> <span class="n">Token</span><span class="p">(</span><span class="s1">'PROCEDURE'</span><span class="p">,</span> <span class="s1">'PROCEDURE'</span><span class="p">),</span>
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<span class="p">}</span>
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</pre></div>
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<p><br/>
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<em><strong>Updating the Parser</strong></em></p>
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<p>Here is a summary of the parser changes:</p>
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<ol>
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<li>New <em>ProcedureDecl</em> <span class="caps">AST</span> node</li>
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<li>Update to the parser’s <em>declarations</em> method to support procedure declarations</li>
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</ol>
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<p>Let’s go over the changes.</p>
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<ol>
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<li>
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<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 to.</p>
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<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>
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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">proc_name</span><span class="p">,</span> <span class="n">block_node</span><span class="p">):</span>
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<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>
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<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>
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</pre></div>
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</li>
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<li>
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<p>Here is the updated <em>declarations</em> method of the <em>Parser</em> class</p>
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<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>
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<span class="sd">"""declarations : VAR (variable_declaration SEMI)+</span>
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<span class="sd"> | (PROCEDURE ID SEMI block SEMI)*</span>
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<span class="sd"> | empty</span>
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<span class="sd"> """</span>
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<span class="n">declarations</span> <span class="o">=</span> <span class="p">[]</span>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<span class="k">return</span> <span class="n">declarations</span>
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</pre></div>
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<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 article.</p>
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</li>
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</ol>
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<p><br/>
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<em><strong>Updating the SymbolTable builder</strong></em></p>
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<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’ll fill it out in the next article.</p>
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<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>
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<span class="k">pass</span>
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</pre></div>
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<p></br>
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<em><strong>Updating the Interpreter</strong></em></p>
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<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 declarations.</p>
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<p>So far, so good.</p>
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<p></br>
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Now that we’ve made all the necessary changes, let’s see what the <em>Abstract Syntax Tree</em> looks like with the new <em>ProcedureDecl</em> nodes.</p>
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<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>
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<div class="highlight"><pre><span></span><span class="k">PROGRAM</span> <span class="n">Part12</span><span class="o">;</span>
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<span class="k">VAR</span>
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<span class="n">a</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">P1</span><span class="o">;</span>
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<span class="k">VAR</span>
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<span class="n">a</span> <span class="o">:</span> <span class="kt">REAL</span><span class="o">;</span>
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||
<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’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> utility:</p>
|
||
<div class="highlight"><pre><span></span>$ python genastdot.py part12.pas > ast.dot <span class="o">&&</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. :)</p>
|
||
<p>As a last item for today, let’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’t done so yet, and run it on the command line.
|
||
Your output should look similar to this:</p>
|
||
<div class="highlight"><pre><span></span>$ python spi.py part12.pas
|
||
Define: INTEGER
|
||
Define: REAL
|
||
Lookup: INTEGER
|
||
Define: <a:INTEGER>
|
||
Lookup: a
|
||
|
||
Symbol Table contents:
|
||
Symbols: <span class="o">[</span>INTEGER, REAL, <a:INTEGER><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’s exactly what we are going to do in the next article:
|
||
dive deep into nested scopes. So don’t forget to bring your swimming gear next time! Stay tuned and see you soon!</p>
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<p><br/>
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<strong>All articles in this series:</strong>
|
||
|
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<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>
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||
</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>
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||
</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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