docs/tutorial/LangImpl1.html - fp2-dev/platform/external/llvm - Gitiles

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 <div class="doc_title">Kaleidoscope: The basic language, with its lexer</div>

 <ul>
 <li><a href="index.html">Up to Tutorial Index</a></li>
 <li>Chapter 1
   <ol>
     <li><a href="#intro">Tutorial Introduction</a></li>
     <li><a href="#language">The Basic Language</a></li>
     <li><a href="#lexer">The Lexer</a></li>
   </ol>
 </li>
 <li><a href="LangImpl2.html">Chapter 2</a>: Implementing a Parser and AST</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">Tutorial Introduction</a></div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>Welcome to the "Implementing a language with LLVM" tutorial.  This tutorial
 will run the through implementation of a simple language, showing how fun and
 easy it can be.  This tutorial will get you up and started as well as help to
 build a framework you can extend to other languages.  You can also use this
 tutorial to help you start playing with other LLVM specific things.
 </p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section"><a name="language">The Basic Language</a></div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>This tutorial will be illustrated with a toy language that we'll call
 "<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>".
 Kaleidoscope is a procedural language that allows you to define functions, use
 conditionals, math, etc.  Over the course of the tutorial, we'll extend
 Kaleidoscope to support the if/then/else construct, a for loop, user defined
 operators, JIT compilation with a simple command line interface, etc.</p>

 <p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
 64-bit floating point type (aka 'double' in C parlance).  As such, all values
 are implicitly double precision and the language doesn't require type
 declarations.  This gives the language a very nice and simple syntax.  For
 example, the following simple example computes <a
 href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>

 <div class="doc_code">
 <pre>
 # Compute the x'th fibonacci number.
 def fib(x)
   if x &lt; 3 then
     1
   else
     fib(x-1)+fib(x-2)

 # This expression will compute the 40th number.
 fib(40)
 </pre>
 </div>

 <p>We also allow Kaleidoscope to call into standard library functions (the LLVM
 JIT makes this completely trivial).  This means that you can use the 'extern'
 keyword to define a function before you use it (this is also useful for mutually
 recursive functions).  For example:</p>

 <div class="doc_code">
 <pre>
 extern sin(arg);
 extern cos(arg);
 extern atan2(arg1 arg2);

 atan2(sin(.4), cos(42))
 </pre>
 </div>

 <p>In the first incarnation of the language, we will only support basic
 arithmetic: if/then/else will be added in a future installment.  Another
 interesting aspect of the first implementation is that it is a completely
 functional language, which does not allow you to have side-effects, etc.  We
 will eventually add side effects for those who prefer them.</p>

 <p>In order to make this tutorial
 maximally understandable and hackable, we choose to implement everything in C++
 instead of using lexer and parser generators.  LLVM obviously works just fine
 with such tools, and making use of them doesn't impact the overall design.</p>

 <p>A note about this tutorial: we expect you to extend the language and play
 with it on your own.  Take the code and go crazy hacking away at it.  It can be
 a lot of fun to play with languages!</p>

 </div>

 <!-- *********************************************************************** -->
 <div class="doc_section"><a name="lexer">The Lexer</a></div>
 <!-- *********************************************************************** -->

 <div class="doc_text">

 <p>When it comes to implementing a language, the first thing needed is
 the ability to process a text file and recognize what it says.  The traditional
 way to do this is to use a "<a
 href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
 to break the input up into "tokens".  Each token returned by the lexer includes
 a token code and potentially some metadata (e.g. the numeric value of a number).
 First, we define the possibilities:
 </p>

 <div class="doc_code">
 <pre>
 // The lexer returns tokens [0-255] if it is an unknown character, otherwise one
 // of these for known things.
 enum Token {
   tok_eof = -1,

   // commands
   tok_def = -2, tok_extern = -3,

   // primary
   tok_identifier = -4, tok_number = -5,
 };

 static std::string IdentifierStr;  // Filled in if tok_identifier
 static double NumVal;              // Filled in if tok_number
 </pre>
 </div>

 <p>Each token returned by our lexer will either be one of the Token enum values
 or it will be an 'unknown' character like '+', which is returned as its ascii
 value.  If the current token is an identifier, the <tt>IdentifierStr</tt>
 global variable holds the name of the identifier.  If the current token is a
 numeric literal (like 1.0), <tt>NumVal</tt> holds its value.  Note that we use
 global variables for simplicity, this is not the best choice for a real language
 implementation :).
 </p>

 <p>The actual implementation of the lexer is a single function named
 <tt>gettok</tt>. The <tt>gettok</tt> function is called to return the next token
 from standard input.  Its definition starts as:</p>

 <div class="doc_code">
 <pre>
 /// gettok - Return the next token from standard input.
 static int gettok() {
   static int LastChar = ' ';

   // Skip any whitespace.
   while (isspace(LastChar))
     LastChar = getchar();
 </pre>
 </div>

 <p>
 <tt>gettok</tt> works by calling the C <tt>getchar()</tt> function to read
 characters one at a time from standard input.  It eats them as it recognizes
 them and stores the last character read, but not processed, in LastChar.  The
 first thing that it has to do is ignore whitespace between tokens.  This is
 accomplished with the loop above.</p>

 <p>The next thing <tt>gettok</tt> needs to do is recognize identifiers and
 specific keywords like "def".  Kaleidoscope does this with this simple loop:</p>

 <div class="doc_code">
 <pre>
   if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
     IdentifierStr = LastChar;
     while (isalnum((LastChar = getchar())))
       IdentifierStr += LastChar;

     if (IdentifierStr == "def") return tok_def;
     if (IdentifierStr == "extern") return tok_extern;
     return tok_identifier;
   }
 </pre>
 </div>

 <p>Note that this code sets the '<tt>IdentifierStr</tt>' global whenever it
 lexes an identifier.  Also, since language keywords are matched by the same
 loop, we handle them here inline.  Numeric values are similar:</p>

 <div class="doc_code">
 <pre>
   if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
     std::string NumStr;
     do {
       NumStr += LastChar;
       LastChar = getchar();
     } while (isdigit(LastChar) || LastChar == '.');

     NumVal = strtod(NumStr.c_str(), 0);
     return tok_number;
   }
 </pre>
 </div>

 <p>This is all pretty straight-forward code for processing input.  When reading
 a numeric value from input, we use the C <tt>strtod</tt> function to convert it
 to a numeric value that we store in <tt>NumVal</tt>.  Note that this isn't doing
 sufficient error checking: it will incorrectly read "1.23.45.67" and handle it as
 if you typed in "1.23".  Feel free to extend it :).  Next we handle comments:
 </p>

 <div class="doc_code">
 <pre>
   if (LastChar == '#') {
     // Comment until end of line.
     do LastChar = getchar();
     while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp; LastChar != '\r');

     if (LastChar != EOF)
       return gettok();
   }
 </pre>
 </div>

 <p>We handle comments by skipping to the end of the line and then returning the
 next comment.  Finally, if the input doesn't match one of the above cases, it is
 either an operator character like '+' or the end of the file.  These are handled with
 this code:</p>

 <div class="doc_code">
 <pre>
   // Check for end of file.  Don't eat the EOF.
   if (LastChar == EOF)
     return tok_eof;

   // Otherwise, just return the character as its ascii value.
   int ThisChar = LastChar;
   LastChar = getchar();
   return ThisChar;
 }
 </pre>
 </div>

 <p>With this, we have the complete lexer for the basic Kaleidoscope language
 (the <a href="LangImpl2.html#code">full code listing</a> for the Lexer is
 available in the <a href="LangImpl2.html">next chapter</a> of the tutorial).
 Next we'll <a href="LangImpl2.html">build a simple parser that uses this to
 build an Abstract Syntax Tree</a>.  When we have that, we'll include a driver
 so that you can use the lexer and parser together.
 </p>

 </div>

 <!-- *********************************************************************** -->
 <hr>
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   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
   <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
   Last modified: $Date: 2007-10-17 11:05:13 -0700 (Wed, 17 Oct 2007) $
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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: The basic language, with its lexer</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: The basic language, with its lexer</div>

	<ul>
	<li><a href="index.html">Up to Tutorial Index</a></li>
	<li>Chapter 1
	<ol>
	<li><a href="#intro">Tutorial Introduction</a></li>
	<li><a href="#language">The Basic Language</a></li>
	<li><a href="#lexer">The Lexer</a></li>
	</ol>
	</li>
	<li><a href="LangImpl2.html">Chapter 2</a>: Implementing a Parser and AST</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">Tutorial Introduction</a></div>
	<!-- *********************************************************************** -->

	<div class="doc_text">

	<p>Welcome to the "Implementing a language with LLVM" tutorial. This tutorial
	will run the through implementation of a simple language, showing how fun and
	easy it can be. This tutorial will get you up and started as well as help to
	build a framework you can extend to other languages. You can also use this
	tutorial to help you start playing with other LLVM specific things.
	</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section"><a name="language">The Basic Language</a></div>
	<!-- *********************************************************************** -->

	<div class="doc_text">

	<p>This tutorial will be illustrated with a toy language that we'll call
	"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>".
	Kaleidoscope is a procedural language that allows you to define functions, use
	conditionals, math, etc. Over the course of the tutorial, we'll extend
	Kaleidoscope to support the if/then/else construct, a for loop, user defined
	operators, JIT compilation with a simple command line interface, etc.</p>

	<p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
	64-bit floating point type (aka 'double' in C parlance). As such, all values
	are implicitly double precision and the language doesn't require type
	declarations. This gives the language a very nice and simple syntax. For
	example, the following simple example computes <a
	href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>

	<div class="doc_code">
	<pre>
	# Compute the x'th fibonacci number.
	def fib(x)
	if x < 3 then
	1
	else
	fib(x-1)+fib(x-2)

	# This expression will compute the 40th number.
	fib(40)
	</pre>
	</div>

	<p>We also allow Kaleidoscope to call into standard library functions (the LLVM
	JIT makes this completely trivial). This means that you can use the 'extern'
	keyword to define a function before you use it (this is also useful for mutually
	recursive functions). For example:</p>

	<div class="doc_code">
	<pre>
	extern sin(arg);
	extern cos(arg);
	extern atan2(arg1 arg2);

	atan2(sin(.4), cos(42))
	</pre>
	</div>

	<p>In the first incarnation of the language, we will only support basic
	arithmetic: if/then/else will be added in a future installment. Another
	interesting aspect of the first implementation is that it is a completely
	functional language, which does not allow you to have side-effects, etc. We
	will eventually add side effects for those who prefer them.</p>

	<p>In order to make this tutorial
	maximally understandable and hackable, we choose to implement everything in C++
	instead of using lexer and parser generators. LLVM obviously works just fine
	with such tools, and making use of them doesn't impact the overall design.</p>

	<p>A note about this tutorial: we expect you to extend the language and play
	with it on your own. Take the code and go crazy hacking away at it. It can be
	a lot of fun to play with languages!</p>

	</div>

	<!-- *********************************************************************** -->
	<div class="doc_section"><a name="lexer">The Lexer</a></div>
	<!-- *********************************************************************** -->

	<div class="doc_text">

	<p>When it comes to implementing a language, the first thing needed is
	the ability to process a text file and recognize what it says. The traditional
	way to do this is to use a "<a
	href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
	to break the input up into "tokens". Each token returned by the lexer includes
	a token code and potentially some metadata (e.g. the numeric value of a number).
	First, we define the possibilities:
	</p>

	<div class="doc_code">
	<pre>
	// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
	// of these for known things.
	enum Token {
	tok_eof = -1,

	// commands
	tok_def = -2, tok_extern = -3,

	// primary
	tok_identifier = -4, tok_number = -5,
	};

	static std::string IdentifierStr; // Filled in if tok_identifier
	static double NumVal; // Filled in if tok_number
	</pre>
	</div>

	<p>Each token returned by our lexer will either be one of the Token enum values
	or it will be an 'unknown' character like '+', which is returned as its ascii
	value. If the current token is an identifier, the <tt>IdentifierStr</tt>
	global variable holds the name of the identifier. If the current token is a
	numeric literal (like 1.0), <tt>NumVal</tt> holds its value. Note that we use
	global variables for simplicity, this is not the best choice for a real language
	implementation :).
	</p>

	<p>The actual implementation of the lexer is a single function named
	<tt>gettok</tt>. The <tt>gettok</tt> function is called to return the next token
	from standard input. Its definition starts as:</p>

	<div class="doc_code">
	<pre>
	/// gettok - Return the next token from standard input.
	static int gettok() {
	static int LastChar = ' ';

	// Skip any whitespace.
	while (isspace(LastChar))
	LastChar = getchar();
	</pre>
	</div>

	<p>
	<tt>gettok</tt> works by calling the C <tt>getchar()</tt> function to read
	characters one at a time from standard input. It eats them as it recognizes
	them and stores the last character read, but not processed, in LastChar. The
	first thing that it has to do is ignore whitespace between tokens. This is
	accomplished with the loop above.</p>

	<p>The next thing <tt>gettok</tt> needs to do is recognize identifiers and
	specific keywords like "def". Kaleidoscope does this with this simple loop:</p>

	<div class="doc_code">
	<pre>
	if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
	IdentifierStr = LastChar;
	while (isalnum((LastChar = getchar())))
	IdentifierStr += LastChar;

	if (IdentifierStr == "def") return tok_def;
	if (IdentifierStr == "extern") return tok_extern;
	return tok_identifier;
	}
	</pre>
	</div>

	<p>Note that this code sets the '<tt>IdentifierStr</tt>' global whenever it
	lexes an identifier. Also, since language keywords are matched by the same
	loop, we handle them here inline. Numeric values are similar:</p>

	<div class="doc_code">
	<pre>
	if (isdigit(LastChar) \|\| LastChar == '.') { // Number: [0-9.]+
	std::string NumStr;
	do {
	NumStr += LastChar;
	LastChar = getchar();
	} while (isdigit(LastChar) \|\| LastChar == '.');

	NumVal = strtod(NumStr.c_str(), 0);
	return tok_number;
	}
	</pre>
	</div>

	<p>This is all pretty straight-forward code for processing input. When reading
	a numeric value from input, we use the C <tt>strtod</tt> function to convert it
	to a numeric value that we store in <tt>NumVal</tt>. Note that this isn't doing
	sufficient error checking: it will incorrectly read "1.23.45.67" and handle it as
	if you typed in "1.23". Feel free to extend it :). Next we handle comments:
	</p>

	<div class="doc_code">
	<pre>
	if (LastChar == '#') {
	// Comment until end of line.
	do LastChar = getchar();
	while (LastChar != EOF && LastChar != '\n' & LastChar != '\r');

	if (LastChar != EOF)
	return gettok();
	}
	</pre>
	</div>

	<p>We handle comments by skipping to the end of the line and then returning the
	next comment. Finally, if the input doesn't match one of the above cases, it is
	either an operator character like '+' or the end of the file. These are handled with
	this code:</p>

	<div class="doc_code">
	<pre>
	// Check for end of file. Don't eat the EOF.
	if (LastChar == EOF)
	return tok_eof;

	// Otherwise, just return the character as its ascii value.
	int ThisChar = LastChar;
	LastChar = getchar();
	return ThisChar;
	}
	</pre>
	</div>

	<p>With this, we have the complete lexer for the basic Kaleidoscope language
	(the <a href="LangImpl2.html#code">full code listing</a> for the Lexer is
	available in the <a href="LangImpl2.html">next chapter</a> of the tutorial).
	Next we'll <a href="LangImpl2.html">build a simple parser that uses this to
	build an Abstract Syntax Tree</a>. When we have that, we'll include a driver
	so that you can use the lexer and parser together.
	</p>

	</div>

	<!-- *********************************************************************** -->
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	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
	<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
	Last modified: $Date: 2007-10-17 11:05:13 -0700 (Wed, 17 Oct 2007) $
	</address>
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