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<!DOCTYPE html>
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<title>kizzy的个人博客 - Let’s Build A Slmple Interpreter. Part 4.</title>
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<h1>Let’s Build A Slmple Interpreter. Part 4.</h1>
Sun 21 June 2020
by <a class="url fn" href="https://kuikui.tech/author/kizzy.html">kizzy</a>
</div>
<div><p> 在之前的文章裏，我們學會了如何解析和翻譯加減的數學表達式，例如“7 - 3 + 2 -1”。并且瞭解了使用語法圖來定義編程語言的語法。</p>
<p><img alt="image" src="https://ruslanspivak.com/lsbasi-part4/LSBAWS_confucius_do.png"></p>
<p> 今天來嘗試構建乘法和除法的語法，例如“7 * 4/2 * 3”，本次構建的結果是整除結果，所以有9/2 = 4 這類結果的產生。</p>
<p> 此外還會介紹一些其他定義編程語言語法的表示法，例如上下文無關文法或者BNF，本文使用的是EBNF表示法簡稱範式，下面是使用範式定義的原因：</p>
<ol>
<li>
<p>範式以一種十分簡潔的方式指定變成語言的語法，相比於語法圖它更爲緊凑。</p>
</li>
<li>
<p>語法範式可以作爲十分好的文檔進行閲讀。</p>
</li>
<li>
<p>一組簡單的規則可以將語法轉化爲代碼。</p>
</li>
<li>
<p>有一套稱爲解析器生成器的工具，可以接受語法作爲輸入，并根據該語法自動為您生成解析器。我將在本系列的稍後部分討論這些工具。</p>
</li>
</ol>
<p>例如簡單計算器的計算範式：</p>
<p><img alt="img" src="https://ruslanspivak.com/lsbasi-part4/lsbasi_part4_bnf1.png"></p>
<p>映射到與發圖和代碼：</p>
<p><img alt="img" src="https://ruslanspivak.com/lsbasi-part4/lsbasi_part4_rules.png"></p>
<p>完整代碼如下：</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">MUL</span><span class="p">,</span> <span class="n">DIV</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">'MUL'</span><span class="p">,</span> <span class="s1">'DIV'</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, MUL, DIV, 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(MUL, '*')</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">Lexer</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 / 3 * 4", 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="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">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 character'</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">MUL</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">DIV</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="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">lexer</span><span class="p">):</span>
<span class="bp">self</span><span class="o">.</span><span class="n">lexer</span> <span class="o">=</span> <span class="n">lexer</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">lexer</span><span class="o">.</span><span class="n">get_next_token</span><span class="p">()</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">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">lexer</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">factor</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="sd"> factor : INTEGER</span>
<span class="sd"> """</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="sd"> expr : factor ((MUL | DIV) factor)*</span>
<span class="sd"> factor : INTEGER</span>
<span class="sd"> """</span>
<span class="n">result</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">factor</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">MUL</span><span class="p">,</span> <span class="n">DIV</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">MUL</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">MUL</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">factor</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">DIV</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">DIV</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">factor</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">lexer</span> <span class="o">=</span> <span class="n">Lexer</span><span class="p">(</span><span class="n">text</span><span class="p">)</span>
<span class="n">interpreter</span> <span class="o">=</span> <span class="n">Interpreter</span><span class="p">(</span><span class="n">lexer</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>
<blockquote>
<p>”这个城市没有草长莺飞的传说，它永远活在现实里面，快速的鼓点，匆忙的身影，麻木的眼神，虚假的笑容，而我正在被同化。—— <strong>kizzy</strong> “</p>
</blockquote>
<p>持續更新！！！！</p></div>
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