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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<html>
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<head>
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<title>Kaleidoscope: Tutorial Introduction and the Lexer</title>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="author" content="Chris Lattner">
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<meta name="author" content="Erick Tryzelaar">
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<link rel="stylesheet" href="../llvm.css" type="text/css">
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</head>
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<body>
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<h1>Kaleidoscope: Tutorial Introduction and the Lexer</h1>
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<ul>
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<li><a href="index.html">Up to Tutorial Index</a></li>
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<li>Chapter 1
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<ol>
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<li><a href="#intro">Tutorial Introduction</a></li>
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<li><a href="#language">The Basic Language</a></li>
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<li><a href="#lexer">The Lexer</a></li>
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</ol>
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</li>
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<li><a href="OCamlLangImpl2.html">Chapter 2</a>: Implementing a Parser and
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AST</li>
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</ul>
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<div class="doc_author">
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<p>
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Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
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and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
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</p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="intro">Tutorial Introduction</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>Welcome to the "Implementing a language with LLVM" tutorial. This tutorial
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runs through the implementation of a simple language, showing how fun and
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easy it can be. This tutorial will get you up and started as well as help to
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build a framework you can extend to other languages. The code in this tutorial
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can also be used as a playground to hack on other LLVM specific things.
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</p>
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<p>
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The goal of this tutorial is to progressively unveil our language, describing
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how it is built up over time. This will let us cover a fairly broad range of
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language design and LLVM-specific usage issues, showing and explaining the code
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for it all along the way, without overwhelming you with tons of details up
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front.</p>
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<p>It is useful to point out ahead of time that this tutorial is really about
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teaching compiler techniques and LLVM specifically, <em>not</em> about teaching
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modern and sane software engineering principles. In practice, this means that
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we'll take a number of shortcuts to simplify the exposition. For example, the
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code leaks memory, uses global variables all over the place, doesn't use nice
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design patterns like <a
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href="http://en.wikipedia.org/wiki/Visitor_pattern">visitors</a>, etc... but it
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is very simple. If you dig in and use the code as a basis for future projects,
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fixing these deficiencies shouldn't be hard.</p>
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<p>I've tried to put this tutorial together in a way that makes chapters easy to
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skip over if you are already familiar with or are uninterested in the various
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pieces. The structure of the tutorial is:
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</p>
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<ul>
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<li><b><a href="#language">Chapter #1</a>: Introduction to the Kaleidoscope
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language, and the definition of its Lexer</b> - This shows where we are going
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and the basic functionality that we want it to do. In order to make this
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tutorial maximally understandable and hackable, we choose to implement
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everything in Objective Caml instead of using lexer and parser generators.
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LLVM obviously works just fine with such tools, feel free to use one if you
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prefer.</li>
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<li><b><a href="OCamlLangImpl2.html">Chapter #2</a>: Implementing a Parser and
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AST</b> - With the lexer in place, we can talk about parsing techniques and
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basic AST construction. This tutorial describes recursive descent parsing and
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operator precedence parsing. Nothing in Chapters 1 or 2 is LLVM-specific,
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the code doesn't even link in LLVM at this point. :)</li>
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<li><b><a href="OCamlLangImpl3.html">Chapter #3</a>: Code generation to LLVM
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IR</b> - With the AST ready, we can show off how easy generation of LLVM IR
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really is.</li>
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<li><b><a href="OCamlLangImpl4.html">Chapter #4</a>: Adding JIT and Optimizer
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Support</b> - Because a lot of people are interested in using LLVM as a JIT,
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we'll dive right into it and show you the 3 lines it takes to add JIT support.
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LLVM is also useful in many other ways, but this is one simple and "sexy" way
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to shows off its power. :)</li>
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<li><b><a href="OCamlLangImpl5.html">Chapter #5</a>: Extending the Language:
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Control Flow</b> - With the language up and running, we show how to extend it
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with control flow operations (if/then/else and a 'for' loop). This gives us a
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chance to talk about simple SSA construction and control flow.</li>
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<li><b><a href="OCamlLangImpl6.html">Chapter #6</a>: Extending the Language:
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User-defined Operators</b> - This is a silly but fun chapter that talks about
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extending the language to let the user program define their own arbitrary
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unary and binary operators (with assignable precedence!). This lets us build a
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significant piece of the "language" as library routines.</li>
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<li><b><a href="OCamlLangImpl7.html">Chapter #7</a>: Extending the Language:
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Mutable Variables</b> - This chapter talks about adding user-defined local
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variables along with an assignment operator. The interesting part about this
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is how easy and trivial it is to construct SSA form in LLVM: no, LLVM does
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<em>not</em> require your front-end to construct SSA form!</li>
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<li><b><a href="OCamlLangImpl8.html">Chapter #8</a>: Conclusion and other
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useful LLVM tidbits</b> - This chapter wraps up the series by talking about
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potential ways to extend the language, but also includes a bunch of pointers to
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info about "special topics" like adding garbage collection support, exceptions,
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debugging, support for "spaghetti stacks", and a bunch of other tips and
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tricks.</li>
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</ul>
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<p>By the end of the tutorial, we'll have written a bit less than 700 lines of
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non-comment, non-blank, lines of code. With this small amount of code, we'll
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have built up a very reasonable compiler for a non-trivial language including
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a hand-written lexer, parser, AST, as well as code generation support with a JIT
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compiler. While other systems may have interesting "hello world" tutorials,
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I think the breadth of this tutorial is a great testament to the strengths of
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LLVM and why you should consider it if you're interested in language or compiler
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design.</p>
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<p>A note about this tutorial: we expect you to extend the language and play
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with it on your own. Take the code and go crazy hacking away at it, compilers
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don't need to be scary creatures - it can be a lot of fun to play with
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languages!</p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="language">The Basic Language</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>This tutorial will be illustrated with a toy language that we'll call
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"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>" (derived
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from "meaning beautiful, form, and view").
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Kaleidoscope is a procedural language that allows you to define functions, use
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conditionals, math, etc. Over the course of the tutorial, we'll extend
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Kaleidoscope to support the if/then/else construct, a for loop, user defined
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operators, JIT compilation with a simple command line interface, etc.</p>
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<p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
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64-bit floating point type (aka 'float' in O'Caml parlance). As such, all
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values are implicitly double precision and the language doesn't require type
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declarations. This gives the language a very nice and simple syntax. For
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example, the following simple example computes <a
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href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>
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<div class="doc_code">
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<pre>
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# Compute the x'th fibonacci number.
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def fib(x)
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if x < 3 then
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1
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else
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fib(x-1)+fib(x-2)
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# This expression will compute the 40th number.
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fib(40)
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</pre>
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</div>
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<p>We also allow Kaleidoscope to call into standard library functions (the LLVM
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JIT makes this completely trivial). This means that you can use the 'extern'
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keyword to define a function before you use it (this is also useful for mutually
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recursive functions). For example:</p>
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<div class="doc_code">
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<pre>
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extern sin(arg);
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extern cos(arg);
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extern atan2(arg1 arg2);
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atan2(sin(.4), cos(42))
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</pre>
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</div>
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<p>A more interesting example is included in Chapter 6 where we write a little
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Kaleidoscope application that <a href="OCamlLangImpl6.html#example">displays
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a Mandelbrot Set</a> at various levels of magnification.</p>
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<p>Lets dive into the implementation of this language!</p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="lexer">The Lexer</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>When it comes to implementing a language, the first thing needed is
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the ability to process a text file and recognize what it says. The traditional
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way to do this is to use a "<a
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href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
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to break the input up into "tokens". Each token returned by the lexer includes
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a token code and potentially some metadata (e.g. the numeric value of a number).
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First, we define the possibilities:
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</p>
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<div class="doc_code">
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<pre>
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(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
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* these others for known things. *)
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type token =
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(* commands *)
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| Def | Extern
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(* primary *)
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| Ident of string | Number of float
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(* unknown *)
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| Kwd of char
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</pre>
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</div>
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<p>Each token returned by our lexer will be one of the token variant values.
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An unknown character like '+' will be returned as <tt>Token.Kwd '+'</tt>. If
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the curr token is an identifier, the value will be <tt>Token.Ident s</tt>. If
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the current token is a numeric literal (like 1.0), the value will be
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<tt>Token.Number 1.0</tt>.
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</p>
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<p>The actual implementation of the lexer is a collection of functions driven
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by a function named <tt>Lexer.lex</tt>. The <tt>Lexer.lex</tt> function is
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called to return the next token from standard input. We will use
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<a href="http://caml.inria.fr/pub/docs/manual-camlp4/index.html">Camlp4</a>
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to simplify the tokenization of the standard input. Its definition starts
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as:</p>
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<div class="doc_code">
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<pre>
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(*===----------------------------------------------------------------------===
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* Lexer
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*===----------------------------------------------------------------------===*)
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let rec lex = parser
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(* Skip any whitespace. *)
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| [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
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</pre>
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</div>
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<p>
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<tt>Lexer.lex</tt> works by recursing over a <tt>char Stream.t</tt> to read
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characters one at a time from the standard input. It eats them as it recognizes
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them and stores them in in a <tt>Token.token</tt> variant. The first thing that
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it has to do is ignore whitespace between tokens. This is accomplished with the
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recursive call above.</p>
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<p>The next thing <tt>Lexer.lex</tt> needs to do is recognize identifiers and
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specific keywords like "def". Kaleidoscope does this with a pattern match
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and a helper function.<p>
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<div class="doc_code">
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<pre>
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(* identifier: [a-zA-Z][a-zA-Z0-9] *)
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| [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
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let buffer = Buffer.create 1 in
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Buffer.add_char buffer c;
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lex_ident buffer stream
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...
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and lex_ident buffer = parser
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| [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
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Buffer.add_char buffer c;
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lex_ident buffer stream
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| [< stream=lex >] ->
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match Buffer.contents buffer with
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| "def" -> [< 'Token.Def; stream >]
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| "extern" -> [< 'Token.Extern; stream >]
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| id -> [< 'Token.Ident id; stream >]
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</pre>
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</div>
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<p>Numeric values are similar:</p>
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<div class="doc_code">
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<pre>
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(* number: [0-9.]+ *)
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| [< ' ('0' .. '9' as c); stream >] ->
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let buffer = Buffer.create 1 in
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Buffer.add_char buffer c;
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lex_number buffer stream
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...
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and lex_number buffer = parser
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| [< ' ('0' .. '9' | '.' as c); stream >] ->
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Buffer.add_char buffer c;
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lex_number buffer stream
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| [< stream=lex >] ->
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[< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
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</pre>
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</div>
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<p>This is all pretty straight-forward code for processing input. When reading
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a numeric value from input, we use the ocaml <tt>float_of_string</tt> function
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to convert it to a numeric value that we store in <tt>Token.Number</tt>. Note
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that this isn't doing sufficient error checking: it will raise <tt>Failure</tt>
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if the string "1.23.45.67". Feel free to extend it :). Next we handle
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comments:
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</p>
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<div class="doc_code">
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<pre>
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(* Comment until end of line. *)
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| [< ' ('#'); stream >] ->
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lex_comment stream
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...
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and lex_comment = parser
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| [< ' ('\n'); stream=lex >] -> stream
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| [< 'c; e=lex_comment >] -> e
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| [< >] -> [< >]
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</pre>
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</div>
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<p>We handle comments by skipping to the end of the line and then return the
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next token. Finally, if the input doesn't match one of the above cases, it is
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either an operator character like '+' or the end of the file. These are handled
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with this code:</p>
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<div class="doc_code">
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<pre>
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(* Otherwise, just return the character as its ascii value. *)
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| [< 'c; stream >] ->
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[< 'Token.Kwd c; lex stream >]
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(* end of stream. *)
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| [< >] -> [< >]
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</pre>
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</div>
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<p>With this, we have the complete lexer for the basic Kaleidoscope language
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(the <a href="OCamlLangImpl2.html#code">full code listing</a> for the Lexer is
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available in the <a href="OCamlLangImpl2.html">next chapter</a> of the
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tutorial). Next we'll <a href="OCamlLangImpl2.html">build a simple parser that
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uses this to build an Abstract Syntax Tree</a>. When we have that, we'll
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include a driver so that you can use the lexer and parser together.
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</p>
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<a href="OCamlLangImpl2.html">Next: Implementing a Parser and AST</a>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
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<a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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</address>
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</body>
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</html>
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