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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
                      "http://www.w3.org/TR/html4/strict.dtd">

<html>
<head>
  <title>Kaleidoscope: Implementing a Parser and AST</title>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <meta name="author" content="Chris Lattner">
  <link rel="stylesheet" href="../llvm.css" type="text/css">
</head>

<body>

<div class="doc_title">Kaleidoscope: Implementing a Parser and AST</div>

<ul>
<li><a href="index.html">Up to Tutorial Index</a></li>
<li>Chapter 2
  <ol>
    <li><a href="#intro">Chapter 2 Introduction</a></li>
    <li><a href="#ast">The Abstract Syntax Tree (AST)</a></li>
    <li><a href="#parserbasics">Parser Basics</a></li>
    <li><a href="#parserprimexprs">Basic Expression Parsing</a></li>
    <li><a href="#parserbinops">Binary Expression Parsing</a></li>
    <li><a href="#parsertop">Parsing the Rest</a></li>
    <li><a href="#driver">The Driver</a></li>
    <li><a href="#conclusions">Conclusions</a></li>
    <li><a href="#code">Full Code Listing</a></li>
  </ol>
</li>
<li><a href="LangImpl3.html">Chapter 3</a>: Code generation to LLVM IR</li>
</ul>

<div class="doc_author">
  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
</div>

<!-- *********************************************************************** -->
<div class="doc_section"><a name="intro">Chapter 2 Introduction</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>Welcome to Chapter 2 of the "<a href="index.html">Implementing a language
with LLVM</a>" tutorial.  This chapter shows you how to use the lexer, built in 
<a href="LangImpl1.html">Chapter 1</a>, to build a full <a
href="http://en.wikipedia.org/wiki/Parsing">parser</a> for
our Kaleidoscope language.  Once we have a parser, we'll define and build an <a 
href="http://en.wikipedia.org/wiki/Abstract_syntax_tree">Abstract Syntax 
Tree</a> (AST).</p>

<p>The parser we will build uses a combination of <a 
href="http://en.wikipedia.org/wiki/Recursive_descent_parser">Recursive Descent
Parsing</a> and <a href=
"http://en.wikipedia.org/wiki/Operator-precedence_parser">Operator-Precedence 
Parsing</a> to parse the Kaleidoscope language (the latter for 
binary expressions and the former for everything else).  Before we get to
parsing though, lets talk about the output of the parser: the Abstract Syntax
Tree.</p>

</div>

<!-- *********************************************************************** -->
<div class="doc_section"><a name="ast">The Abstract Syntax Tree (AST)</a></div>
<!-- *********************************************************************** -->

<div class="doc_text">

<p>The AST for a program captures its behavior in such a way that it is easy for
later stages of the compiler (e.g. code generation) to interpret.  We basically
want one object for each construct in the language, and the AST should closely
model the language.  In Kaleidoscope, we have expressions, a prototype, and a
function object.  We'll start with expressions first:</p>

<div class="doc_code">
<pre>
/// ExprAST - Base class for all