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POCrockfords Top Down Operator Precedence
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copied!<p>Out of interest, i want to learn how to write a parser for a simple language, to ultimately write an interpreter for my own little code-golfing language, as soon as i understood how such things work in general.</p> <p>So I started reading Douglas Crockfords article <a href="http://javascript.crockford.com/tdop/tdop.html">Top Down Operator Precedence</a>.</p> <p><em>Note: You should probably read the article if you want a deeper understanding of the context of the code snippets below</em></p> <p>I have trouble understanding how the <code>var</code> statement and the assignment operator <code>=</code> should work together.</p> <p>D.C. defines an assignment operator like</p> <pre><code>var assignment = function (id) { return infixr(id, 10, function (left) { if (left.id !== "." && left.id !== "[" && left.arity !== "name") { left.error("Bad lvalue."); } this.first = left; this.second = expression(9); this.assignment = true; this.arity = "binary"; return this; }); }; assignment("="); </code></pre> <p><em>Note: [[value]] refers to a token, simplified to its value</em> </p> <p>Now if the expression function reaches e.g. <code>[[t]],[[=]],[[2]]</code>,the result of <code>[[=]].led</code> is something like this. </p> <pre><code>{ "arity": "binary", "value": "=", "assignment": true, //<- "first": { "arity": "name", "value": "t" }, "second": { "arity": "literal", "value": "2" } } </code></pre> <p>D.C. makes the <code>assignment</code> function because </p> <blockquote> <p>we want it to do two extra bits of business: examine the left operand to make sure that it is a proper lvalue, and <strong>set an assignment member</strong> so that we can <strong>later quickly identify</strong> assignment statements.</p> </blockquote> <p>Which makes sense to me up to the point where he introduces the <code>var</code> statement, which is defined as follows.</p> <blockquote> <p>The var statement defines one or more variables in the current block. Each name can optionally be followed by = and an initializing expression. </p> </blockquote> <pre><code>stmt("var", function () { var a = [], n, t; while (true) { n = token; if (n.arity !== "name") { n.error("Expected a new variable name."); } scope.define(n); advance(); if (token.id === "=") { t = token; advance("="); t.first = n; t.second = expression(0); t.arity = "binary"; a.push(t); } if (token.id !== ",") { break; } advance(","); } advance(";"); return a.length === 0 ? null : a.length === 1 ? a[0] : a; }); </code></pre> <p>Now if the parser reaches a set of tokens like <code>[[var]],[[t]],[[=]],[[1]]</code> the generated tree would look something like.</p> <pre><code>{ "arity": "binary", "value": "=", "first": { "arity": "name", "value": "t" }, "second": { "arity": "literal", "value": "1" } } </code></pre> <p><strong>The keypart of my question is the <code>if (token.id === "=") {...}</code> part.</strong></p> <p>I don't understand why we call </p> <pre><code> t = token; advance("="); t.first = n; t.second = expression(0); t.arity = "binary"; a.push(t); </code></pre> <p>rather than </p> <pre><code> t = token; advance("="); t.led (n); a.push(t); </code></pre> <p>in the <code>...</code> part.</p> <p>which would call our <code>[[=]]</code> operators <code>led</code> function <em>(the assignment function)</em>, which does</p> <blockquote> <p>make sure that it is a proper lvalue, and <strong>set an assignment member</strong> so that we can <strong>later quickly identify</strong> assignment statements. <em>e.g</em> </p> </blockquote> <pre><code>{ "arity": "binary", "value": "=", "assignment": true, "first": { "arity": "name", "value": "t" }, "second": { "arity": "literal", "value": "1" } } </code></pre> <p><em>since there is no operator with a <code>lbp</code> between 0 and 10, calling <code>expression(0) vs. expression (9)</code> makes no difference. (<code>!(0<0) && !(9<0) && 0<10 && 9<10)</code>)</em> </p> <p><em>And the <code>token.id === "="</code> condition prevents assignments to an object member as <code>token.id</code> would either be <code>'['</code> or <code>'.'</code> and <code>t.led</code> wouldn't be called.</em> </p> <p>My question in short is: </p> <p><strong>Why do we not call the, optionally after a variable declaration followable, assignment operators' available <code>led</code> function. But instead manually set the <code>first</code> and <code>second</code> members of the statement but not the <code>assignment</code> member ?</strong></p> <p>Here are two fiddles parsing a simple string. Using the <a href="http://jsfiddle.net/jYLYr/">original</a> code and one using the <a href="http://jsfiddle.net/hQXFT/">assignment</a> operators <code>led</code>.</p>

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