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<!DOCTYPE html>
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<title>kizzy的个人博客 - Let’s Build A Slmple Interpreter. Part 3.</title>
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<h1>Let’s Build A Slmple Interpreter. Part 3.</h1>
Fri 19 June 2020
by <a class="url fn" href="https://kuikui.tech/author/kizzy.html">kizzy</a>
</div>
<div><blockquote>
<p><strong><em>I'm scared of myself. I've always run away from myself. I have always been pursuing Brahman and God. I hope to destroy myself and leave myself in order to discover the core of this Dharma, God-I, life, divinity and absolute in this unknown deepest place. However, I lost myself in the road because of doing so. —— 流浪者之歌</em></strong></p>
</blockquote>
<p> 我們是如何學習一件新的技能呢？例如游泳，我們需要花費很長的時間在水裏進行實踐。因此學習一件事情最爲重要的就實踐，前端程序員需要花費大量的實踐去練習框架組件的demo，會計財務需要進行十分特殊的訓練才能對業務十分熟練，我們編寫編譯器/解釋器同樣如此。</p>
<p> 上一節我們實現了簡單的語法分析和詞法分析，當然語法僅僅實現的是加和減這樣十分簡單的語法。本節將要實現一個可以進行持續運算的解釋器，語法圖如下：</p>
<p><img src="https://ruslanspivak.com/lsbasi-part3/lsbasi_part3_syntax_diagram.png" alt="img" style="zoom:80%;" /></p>
<p>可以看到，exper表達式，在獲取到一個term之後可以進行選擇，儅term之後還存在加或者減的運算符的時候，那麽需要在獲取運算符后還應該再獲取一個term，如果term之後不存在運算符那麽語法匹配就應該進入下一個流程。</p>
<p>語法圖有以下兩個主要的功能：</p>
<ol>
<li>以圖形方式表達編程語言的規範。</li>
<li>可以幫助我們來寫自己的parser。</li>
</ol>
<p>根据上面的语法图，以下所有算术表达式均有效：</p>
<div class="highlight"><pre><span></span><code><span class="o">>>></span> <span class="mi">3</span>
<span class="mi">3</span>
<span class="o">>>></span> <span class="mi">3</span> <span class="o">+</span> <span class="mi">4</span>
<span class="mi">7</span>
<span class="o">>>></span> <span class="mi">7</span> <span class="o">-</span> <span class="mi">3</span> <span class="o">+</span> <span class="mi">2</span> <span class="o">-</span> <span class="mi">1</span>
<span class="mi">5</span>
</code></pre></div>
<p>具體代碼如下：</p>
<div class="highlight"><pre><span></span><code><span class="c1"># Token types</span>
<span class="c1">#</span>
<span class="c1"># EOF (end-of-file) token is used to indicate that</span>
<span class="c1"># there is no more input left for lexical analysis</span>
<span class="n">INTEGER</span><span class="p">,</span> <span class="n">PLUS</span><span class="p">,</span> <span class="n">MINUS</span><span class="p">,</span> <span class="n">EOF</span> <span class="o">=</span> <span class="s1">'INTEGER'</span><span class="p">,</span> <span class="s1">'PLUS'</span><span class="p">,</span> <span class="s1">'MINUS'</span><span class="p">,</span> <span class="s1">'EOF'</span>
<span class="k">class</span> <span class="nc">Token</span><span class="p">(</span><span class="nb">object</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="nb">type</span><span class="p">,</span> <span class="n">value</span><span class="p">):</span>
<span class="c1"># token type: INTEGER, PLUS, MINUS, or EOF</span>
<span class="bp">self</span><span class="o">.</span><span class="n">type</span> <span class="o">=</span> <span class="nb">type</span>
<span class="c1"># token value: non-negative integer value, '+', '-', or None</span>
<span class="bp">self</span><span class="o">.</span><span class="n">value</span> <span class="o">=</span> <span class="n">value</span>
<span class="k">def</span> <span class="fm">__str__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">"""String representation of the class instance.</span>
<span class="sd"> Examples:</span>
<span class="sd"> Token(INTEGER, 3)</span>
<span class="sd"> Token(PLUS, '+')</span>
<span class="sd"> """</span>
<span class="k">return</span> <span class="s1">'Token(</span><span class="si">{type}</span><span class="s1">, </span><span class="si">{value}</span><span class="s1">)'</span><span class="o">.</span><span class="n">format</span><span class="p">(</span>
<span class="nb">type</span><span class="o">=</span><span class="bp">self</span><span class="o">.</span><span class="n">type</span><span class="p">,</span>
<span class="n">value</span><span class="o">=</span><span class="nb">repr</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">value</span><span class="p">)</span>
<span class="p">)</span>
<span class="k">def</span> <span class="fm">__repr__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="fm">__str__</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">Interpreter</span><span class="p">(</span><span class="nb">object</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">text</span><span class="p">):</span>
<span class="c1"># client string input, e.g. "3 + 5", "12 - 5 + 3", etc</span>
<span class="bp">self</span><span class="o">.</span><span class="n">text</span> <span class="o">=</span> <span class="n">text</span>
<span class="c1"># self.pos is an index into self.text</span>
<span class="bp">self</span><span class="o">.</span><span class="n">pos</span> <span class="o">=</span> <span class="mi">0</span>
<span class="c1"># current token instance</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_token</span> <span class="o">=</span> <span class="kc">None</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">text</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">pos</span><span class="p">]</span>
<span class="c1">##########################################################</span>
<span class="c1"># Lexer code #</span>
<span class="c1">##########################################################</span>
<span class="k">def</span> <span class="nf">error</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s1">'Invalid syntax'</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">advance</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">"""Advance the `pos` pointer and set the `current_char` variable."""</span>
<span class="bp">self</span><span class="o">.</span><span class="n">pos</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">pos</span> <span class="o">></span> <span class="nb">len</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">text</span><span class="p">)</span> <span class="o">-</span> <span class="mi">1</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="o">=</span> <span class="kc">None</span> <span class="c1"># Indicates end of input</span>
<span class="k">else</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">text</span><span class="p">[</span><span class="bp">self</span><span class="o">.</span><span class="n">pos</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">skip_whitespace</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="k">while</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span><span class="o">.</span><span class="n">isspace</span><span class="p">():</span>
<span class="bp">self</span><span class="o">.</span><span class="n">advance</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">integer</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">"""Return a (multidigit) integer consumed from the input."""</span>
<span class="n">result</span> <span class="o">=</span> <span class="s1">''</span>
<span class="k">while</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span> <span class="ow">and</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span><span class="o">.</span><span class="n">isdigit</span><span class="p">():</span>
<span class="n">result</span> <span class="o">+=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span>
<span class="bp">self</span><span class="o">.</span><span class="n">advance</span><span class="p">()</span>
<span class="k">return</span> <span class="nb">int</span><span class="p">(</span><span class="n">result</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">get_next_token</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">"""Lexical analyzer (also known as scanner or tokenizer)</span>
<span class="sd"> This method is responsible for breaking a sentence</span>
<span class="sd"> apart into tokens. One token at a time.</span>
<span class="sd"> """</span>
<span class="k">while</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span><span class="o">.</span><span class="n">isspace</span><span class="p">():</span>
<span class="bp">self</span><span class="o">.</span><span class="n">skip_whitespace</span><span class="p">()</span>
<span class="k">continue</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span><span class="o">.</span><span class="n">isdigit</span><span class="p">():</span>
<span class="k">return</span> <span class="n">Token</span><span class="p">(</span><span class="n">INTEGER</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">integer</span><span class="p">())</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="o">==</span> <span class="s1">'+'</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">advance</span><span class="p">()</span>
<span class="k">return</span> <span class="n">Token</span><span class="p">(</span><span class="n">PLUS</span><span class="p">,</span> <span class="s1">'+'</span><span class="p">)</span>
<span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_char</span> <span class="o">==</span> <span class="s1">'-'</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">advance</span><span class="p">()</span>
<span class="k">return</span> <span class="n">Token</span><span class="p">(</span><span class="n">MINUS</span><span class="p">,</span> <span class="s1">'-'</span><span class="p">)</span>
<span class="bp">self</span><span class="o">.</span><span class="n">error</span><span class="p">()</span>
<span class="k">return</span> <span class="n">Token</span><span class="p">(</span><span class="n">EOF</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
<span class="c1">##########################################################</span>
<span class="c1"># Parser / Interpreter code #</span>
<span class="c1">##########################################################</span>
<span class="k">def</span> <span class="nf">eat</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">token_type</span><span class="p">):</span>
<span class="c1"># compare the current token type with the passed token</span>
<span class="c1"># type and if they match then "eat" the current token</span>
<span class="c1"># and assign the next token to the self.current_token,</span>
<span class="c1"># otherwise raise an exception.</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">token_type</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_token</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get_next_token</span><span class="p">()</span>
<span class="k">else</span><span class="p">:</span>
<span class="bp">self</span><span class="o">.</span><span class="n">error</span><span class="p">()</span>
<span class="k">def</span> <span class="nf">term</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">"""Return an INTEGER token value."""</span>
<span class="n">token</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_token</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">INTEGER</span><span class="p">)</span>
<span class="k">return</span> <span class="n">token</span><span class="o">.</span><span class="n">value</span>
<span class="k">def</span> <span class="nf">expr</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="sd">"""Arithmetic expression parser / interpreter."""</span>
<span class="c1"># set current token to the first token taken from the input</span>
<span class="bp">self</span><span class="o">.</span><span class="n">current_token</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">get_next_token</span><span class="p">()</span>
<span class="n">result</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">term</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="ow">in</span> <span class="p">(</span><span class="n">PLUS</span><span class="p">,</span> <span class="n">MINUS</span><span class="p">):</span>
<span class="n">token</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">current_token</span>
<span class="k">if</span> <span class="n">token</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">PLUS</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">PLUS</span><span class="p">)</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">result</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">term</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">token</span><span class="o">.</span><span class="n">type</span> <span class="o">==</span> <span class="n">MINUS</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">MINUS</span><span class="p">)</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">result</span> <span class="o">-</span> <span class="bp">self</span><span class="o">.</span><span class="n">term</span><span class="p">()</span>
<span class="k">return</span> <span class="n">result</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="k">while</span> <span class="kc">True</span><span class="p">:</span>
<span class="k">try</span><span class="p">:</span>
<span class="c1"># To run under Python3 replace 'raw_input' call</span>
<span class="c1"># with 'input'</span>
<span class="n">text</span> <span class="o">=</span> <span class="n">raw_input</span><span class="p">(</span><span class="s1">'calc> '</span><span class="p">)</span>
<span class="k">except</span> <span class="ne">EOFError</span><span class="p">:</span>
<span class="k">break</span>
<span class="k">if</span> <span class="ow">not</span> <span class="n">text</span><span class="p">:</span>
<span class="k">continue</span>
<span class="n">interpreter</span> <span class="o">=</span> <span class="n">Interpreter</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">interpreter</span><span class="o">.</span><span class="n">expr</span><span class="p">()</span>
<span class="nb">print</span><span class="p">(</span><span class="n">result</span><span class="p">)</span>
<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</code></pre></div>
<p>保存cal.py并且運行：</p>
<div class="highlight"><pre><span></span><code><span class="err">$</span> <span class="n">python</span> <span class="n">calc3</span><span class="o">.</span><span class="n">py</span>
<span class="n">calc</span><span class="o">></span> <span class="mi">3</span>
<span class="mi">3</span>
<span class="n">calc</span><span class="o">></span> <span class="mi">7</span> <span class="o">-</span> <span class="mi">4</span>
<span class="mi">3</span>
<span class="n">calc</span><span class="o">></span> <span class="mi">10</span> <span class="o">+</span> <span class="mi">5</span>
<span class="mi">15</span>
<span class="n">calc</span><span class="o">></span> <span class="mi">7</span> <span class="o">-</span> <span class="mi">3</span> <span class="o">+</span> <span class="mi">2</span> <span class="o">-</span> <span class="mi">1</span>
<span class="mi">5</span>
<span class="n">calc</span><span class="o">></span> <span class="mi">10</span> <span class="o">+</span> <span class="mi">1</span> <span class="o">+</span> <span class="mi">2</span> <span class="o">-</span> <span class="mi">3</span> <span class="o">+</span> <span class="mi">4</span> <span class="o">+</span> <span class="mi">6</span> <span class="o">-</span> <span class="mi">15</span>
<span class="mi">5</span>
<span class="n">calc</span><span class="o">></span> <span class="mi">3</span> <span class="o">+</span>
<span class="n">Traceback</span> <span class="p">(</span><span class="n">most</span> <span class="n">recent</span> <span class="n">call</span> <span class="n">last</span><span class="p">):</span>
<span class="n">File</span> <span class="s2">"calc3.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">147</span><span class="p">,</span> <span class="ow">in</span> <span class="o"><</span><span class="n">module</span><span class="o">></span>
<span class="n">main</span><span class="p">()</span>
<span class="n">File</span> <span class="s2">"calc3.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">142</span><span class="p">,</span> <span class="ow">in</span> <span class="n">main</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">interpreter</span><span class="o">.</span><span class="n">expr</span><span class="p">()</span>
<span class="n">File</span> <span class="s2">"calc3.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">123</span><span class="p">,</span> <span class="ow">in</span> <span class="n">expr</span>
<span class="n">result</span> <span class="o">=</span> <span class="n">result</span> <span class="o">+</span> <span class="bp">self</span><span class="o">.</span><span class="n">term</span><span class="p">()</span>
<span class="n">File</span> <span class="s2">"calc3.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">110</span><span class="p">,</span> <span class="ow">in</span> <span class="n">term</span>
<span class="bp">self</span><span class="o">.</span><span class="n">eat</span><span class="p">(</span><span class="n">INTEGER</span><span class="p">)</span>
<span class="n">File</span> <span class="s2">"calc3.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">105</span><span class="p">,</span> <span class="ow">in</span> <span class="n">eat</span>
<span class="bp">self</span><span class="o">.</span><span class="n">error</span><span class="p">()</span>
<span class="n">File</span> <span class="s2">"calc3.py"</span><span class="p">,</span> <span class="n">line</span> <span class="mi">45</span><span class="p">,</span> <span class="ow">in</span> <span class="n">error</span>
<span class="k">raise</span> <span class="ne">Exception</span><span class="p">(</span><span class="s1">'Invalid syntax'</span><span class="p">)</span>
<span class="ne">Exception</span><span class="p">:</span> <span class="n">Invalid</span> <span class="n">syntax</span>
</code></pre></div>
<p>Over~~~~</p></div>
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