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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<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">
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<body>
<div class="doc_title">Kaleidoscope: The basic language, with its lexer</div>
<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 through implementation of a simple language, showing how fun and easy
it can be. This tutorial will get you up and started and build a framework you
can extend to other languages and to play with other 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 if/then/else, operator overloading, JIT compilation with
a simple command line interface, etc.</p>
<p>Because we want to keep things simple, in Kaleidoscope the only datatype 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, A simple example computes <a
href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers</a>,
which looks like this:</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 (this 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 these tools, and choice of these tools doesn't impact 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="language">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 <tt>gettok</tt>.
<tt>gettok</tt> 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 it 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 it 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 incorrect 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 returnning the
next comment. Finally, if the input doesn't match one of the above cases, it is
either an operator character like '+', the end of 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.
Next we'll <a href="LangImpl2.html">build a simple parser that uses this to
build an Abstract Syntax Tree</a>. If you prefer, you can jump to the <a
href="index.html">main tutorial index page</a>.
</p>
</div>
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