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				6	<title>Kaleidoscope: Tutorial Introduction and the Lexer</title>
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				8	<meta name="author" content="Chris Lattner">
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				15	<div class="doc_title">Kaleidoscope: Tutorial Introduction and the Lexer</div>
				16
				17	<ul>
				18	<li><a href="index.html">Up to Tutorial Index</a></li>
				19	<li>Chapter 1
				20	<ol>
				21	<li><a href="#intro">Tutorial Introduction</a></li>
				22	<li><a href="#language">The Basic Language</a></li>
				23	<li><a href="#lexer">The Lexer</a></li>
				24	</ol>
				25	</li>
				26	<li><a href="OCamlLangImpl2.html">Chapter 2</a>: Implementing a Parser and
				27	AST</li>
				28	</ul>
				29
				30	<div class="doc_author">
				31	<p>
				32	Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
				33	and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
				34	</p>
				35	</div>
				36
				37	<!-- *********************************************************************** -->
				38	<div class="doc_section"><a name="intro">Tutorial Introduction</a></div>
				39	<!-- *********************************************************************** -->
				40
				41	<div class="doc_text">
				42
				43	<p>Welcome to the "Implementing a language with LLVM" tutorial. This tutorial
				44	runs through the implementation of a simple language, showing how fun and
				45	easy it can be. This tutorial will get you up and started as well as help to
				46	build a framework you can extend to other languages. The code in this tutorial
				47	can also be used as a playground to hack on other LLVM specific things.
				48	</p>
				49
				50	<p>
				51	The goal of this tutorial is to progressively unveil our language, describing
				52	how it is built up over time. This will let us cover a fairly broad range of
				53	language design and LLVM-specific usage issues, showing and explaining the code
				54	for it all along the way, without overwhelming you with tons of details up
				55	front.</p>
				56
				57	<p>It is useful to point out ahead of time that this tutorial is really about
				58	teaching compiler techniques and LLVM specifically, <em>not</em> about teaching
				59	modern and sane software engineering principles. In practice, this means that
				60	we'll take a number of shortcuts to simplify the exposition. For example, the
				61	code leaks memory, uses global variables all over the place, doesn't use nice
				62	design patterns like <a
				63	href="http://en.wikipedia.org/wiki/Visitor_pattern">visitors</a>, etc... but it
				64	is very simple. If you dig in and use the code as a basis for future projects,
				65	fixing these deficiencies shouldn't be hard.</p>
				66
				67	<p>I've tried to put this tutorial together in a way that makes chapters easy to
				68	skip over if you are already familiar with or are uninterested in the various
				69	pieces. The structure of the tutorial is:
				70	</p>
				71
				72	<ul>
				73	<li><b><a href="#language">Chapter #1</a>: Introduction to the Kaleidoscope
				74	language, and the definition of its Lexer</b> - This shows where we are going
				75	and the basic functionality that we want it to do. In order to make this
				76	tutorial maximally understandable and hackable, we choose to implement
				77	everything in Objective Caml instead of using lexer and parser generators.
				78	LLVM obviously works just fine with such tools, feel free to use one if you
				79	prefer.</li>
				80	<li><b><a href="OCamlLangImpl2.html">Chapter #2</a>: Implementing a Parser and
				81	AST</b> - With the lexer in place, we can talk about parsing techniques and
				82	basic AST construction. This tutorial describes recursive descent parsing and
				83	operator precedence parsing. Nothing in Chapters 1 or 2 is LLVM-specific,
				84	the code doesn't even link in LLVM at this point. :)</li>
				85	<li><b><a href="OCamlLangImpl3.html">Chapter #3</a>: Code generation to LLVM
				86	IR</b> - With the AST ready, we can show off how easy generation of LLVM IR
				87	really is.</li>
				88	<li><b><a href="OCamlLangImpl4.html">Chapter #4</a>: Adding JIT and Optimizer
				89	Support</b> - Because a lot of people are interested in using LLVM as a JIT,
				90	we'll dive right into it and show you the 3 lines it takes to add JIT support.
				91	LLVM is also useful in many other ways, but this is one simple and "sexy" way
				92	to shows off its power. :)</li>
				93	<li><b><a href="OCamlLangImpl5.html">Chapter #5</a>: Extending the Language:
				94	Control Flow</b> - With the language up and running, we show how to extend it
				95	with control flow operations (if/then/else and a 'for' loop). This gives us a
				96	chance to talk about simple SSA construction and control flow.</li>
				97	<li><b><a href="OCamlLangImpl6.html">Chapter #6</a>: Extending the Language:
				98	User-defined Operators</b> - This is a silly but fun chapter that talks about
				99	extending the language to let the user program define their own arbitrary
				100	unary and binary operators (with assignable precedence!). This lets us build a
				101	significant piece of the "language" as library routines.</li>
				102	<li><b><a href="OCamlLangImpl7.html">Chapter #7</a>: Extending the Language:
				103	Mutable Variables</b> - This chapter talks about adding user-defined local
				104	variables along with an assignment operator. The interesting part about this
				105	is how easy and trivial it is to construct SSA form in LLVM: no, LLVM does
				106	<em>not</em> require your front-end to construct SSA form!</li>
				107	<li><b><a href="OCamlLangImpl8.html">Chapter #8</a>: Conclusion and other
				108	useful LLVM tidbits</b> - This chapter wraps up the series by talking about
				109	potential ways to extend the language, but also includes a bunch of pointers to
				110	info about "special topics" like adding garbage collection support, exceptions,
				111	debugging, support for "spaghetti stacks", and a bunch of other tips and
				112	tricks.</li>
				113
				114	</ul>
				115
				116	<p>By the end of the tutorial, we'll have written a bit less than 700 lines of
				117	non-comment, non-blank, lines of code. With this small amount of code, we'll
				118	have built up a very reasonable compiler for a non-trivial language including
				119	a hand-written lexer, parser, AST, as well as code generation support with a JIT
				120	compiler. While other systems may have interesting "hello world" tutorials,
				121	I think the breadth of this tutorial is a great testament to the strengths of
				122	LLVM and why you should consider it if you're interested in language or compiler
				123	design.</p>
				124
				125	<p>A note about this tutorial: we expect you to extend the language and play
				126	with it on your own. Take the code and go crazy hacking away at it, compilers
				127	don't need to be scary creatures - it can be a lot of fun to play with
				128	languages!</p>
				129
				130	</div>
				131
				132	<!-- *********************************************************************** -->
				133	<div class="doc_section"><a name="language">The Basic Language</a></div>
				134	<!-- *********************************************************************** -->
				135
				136	<div class="doc_text">
				137
				138	<p>This tutorial will be illustrated with a toy language that we'll call
				139	"<a href="http://en.wikipedia.org/wiki/Kaleidoscope">Kaleidoscope</a>" (derived
				140	from "meaning beautiful, form, and view").
				141	Kaleidoscope is a procedural language that allows you to define functions, use
				142	conditionals, math, etc. Over the course of the tutorial, we'll extend
				143	Kaleidoscope to support the if/then/else construct, a for loop, user defined
				144	operators, JIT compilation with a simple command line interface, etc.</p>
				145
				146	<p>Because we want to keep things simple, the only datatype in Kaleidoscope is a
				147	64-bit floating point type (aka 'float' in O'Caml parlance). As such, all
				148	values are implicitly double precision and the language doesn't require type
				149	declarations. This gives the language a very nice and simple syntax. For
				150	example, the following simple example computes <a
				151	href="http://en.wikipedia.org/wiki/Fibonacci_number">Fibonacci numbers:</a></p>
				152
				153	<div class="doc_code">
				154	<pre>
				155	# Compute the x'th fibonacci number.
				156	def fib(x)
				157	if x < 3 then
				158	1
				159	else
				160	fib(x-1)+fib(x-2)
				161
				162	# This expression will compute the 40th number.
				163	fib(40)
				164	</pre>
				165	</div>
				166
				167	<p>We also allow Kaleidoscope to call into standard library functions (the LLVM
				168	JIT makes this completely trivial). This means that you can use the 'extern'
				169	keyword to define a function before you use it (this is also useful for mutually
				170	recursive functions). For example:</p>
				171
				172	<div class="doc_code">
				173	<pre>
				174	extern sin(arg);
				175	extern cos(arg);
				176	extern atan2(arg1 arg2);
				177
				178	atan2(sin(.4), cos(42))
				179	</pre>
				180	</div>
				181
				182	<p>A more interesting example is included in Chapter 6 where we write a little
				183	Kaleidoscope application that <a href="OCamlLangImpl6.html#example">displays
				184	a Mandelbrot Set</a> at various levels of magnification.</p>
				185
				186	<p>Lets dive into the implementation of this language!</p>
				187
				188	</div>
				189
				190	<!-- *********************************************************************** -->
				191	<div class="doc_section"><a name="lexer">The Lexer</a></div>
				192	<!-- *********************************************************************** -->
				193
				194	<div class="doc_text">
				195
				196	<p>When it comes to implementing a language, the first thing needed is
				197	the ability to process a text file and recognize what it says. The traditional
				198	way to do this is to use a "<a
				199	href="http://en.wikipedia.org/wiki/Lexical_analysis">lexer</a>" (aka 'scanner')
				200	to break the input up into "tokens". Each token returned by the lexer includes
				201	a token code and potentially some metadata (e.g. the numeric value of a number).
				202	First, we define the possibilities:
				203	</p>
				204
				205	<div class="doc_code">
				206	<pre>
				207	(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
				208	* these others for known things. *)
				209	type token =
				210	(* commands *)
				211	\| Def \| Extern
				212
				213	(* primary *)
				214	\| Ident of string \| Number of float
				215
				216	(* unknown *)
				217	\| Kwd of char
				218	</pre>
				219	</div>
				220
				221	<p>Each token returned by our lexer will be one of the token variant values.
Erick Tryzelaar	d564686	2008-03-30 19:14:31 +0000	[diff] [blame]	222	An unknown character like '+' will be returned as <tt>Token.Kwd '+'</tt>. If
				223	the curr token is an identifier, the value will be <tt>Token.Ident s</tt>. If
				224	the current token is a numeric literal (like 1.0), the value will be
				225	<tt>Token.Number 1.0</tt>.
Erick Tryzelaar	9ba8a57	2008-03-27 08:18:07 +0000	[diff] [blame]	226	</p>
				227
				228	<p>The actual implementation of the lexer is a collection of functions driven
Erick Tryzelaar	d564686	2008-03-30 19:14:31 +0000	[diff] [blame]	229	by a function named <tt>Lexer.lex</tt>. The <tt>Lexer.lex</tt> function is
				230	called to return the next token from standard input. We will use
Erick Tryzelaar	9ba8a57	2008-03-27 08:18:07 +0000	[diff] [blame]	231	<a href="http://caml.inria.fr/pub/docs/manual-camlp4/index.html">Camlp4</a>
				232	to simplify the tokenization of the standard input. Its definition starts
				233	as:</p>
				234
				235	<div class="doc_code">
				236	<pre>
				237	(*===----------------------------------------------------------------------===
				238	* Lexer
				239	===----------------------------------------------------------------------===)
				240
				241	let rec lex = parser
				242	(* Skip any whitespace. *)
				243	\| [< ' (' ' \| '\n' \| '\r' \| '\t'); stream >] -> lex stream
				244	</pre>
				245	</div>
				246
				247	<p>
Erick Tryzelaar	d564686	2008-03-30 19:14:31 +0000	[diff] [blame]	248	<tt>Lexer.lex</tt> works by recursing over a <tt>char Stream.t</tt> to read
Erick Tryzelaar	9ba8a57	2008-03-27 08:18:07 +0000	[diff] [blame]	249	characters one at a time from the standard input. It eats them as it recognizes
Erick Tryzelaar	d564686	2008-03-30 19:14:31 +0000	[diff] [blame]	250	them and stores them in in a <tt>Token.token</tt> variant. The first thing that
				251	it has to do is ignore whitespace between tokens. This is accomplished with the
Erick Tryzelaar	9ba8a57	2008-03-27 08:18:07 +0000	[diff] [blame]	252	recursive call above.</p>
				253
Erick Tryzelaar	d564686	2008-03-30 19:14:31 +0000	[diff] [blame]	254	<p>The next thing <tt>Lexer.lex</tt> needs to do is recognize identifiers and
Erick Tryzelaar	9ba8a57	2008-03-27 08:18:07 +0000	[diff] [blame]	255	specific keywords like "def". Kaleidoscope does this with this a pattern match
				256	and a helper function.<p>
				257
				258	<div class="doc_code">
				259	<pre>
				260	(* identifier: [a-zA-Z][a-zA-Z0-9] *)
				261	\| [< ' ('A' .. 'Z' \| 'a' .. 'z' as c); stream >] ->
				262	let buffer = Buffer.create 1 in
				263	Buffer.add_char buffer c;
				264	lex_ident buffer stream
				265
				266	...
				267
				268	and lex_ident buffer = parser
				269	\| [< ' ('A' .. 'Z' \| 'a' .. 'z' \| '0' .. '9' as c); stream >] ->
				270	Buffer.add_char buffer c;
				271	lex_ident buffer stream
				272	\| [< stream=lex >] ->
				273	match Buffer.contents buffer with
				274	\| "def" -> [< 'Token.Def; stream >]
				275	\| "extern" -> [< 'Token.Extern; stream >]
				276	\| id -> [< 'Token.Ident id; stream >]
				277	</pre>
				278	</div>
				279
				280	Numeric values are similar:</p>
				281
				282	<div class="doc_code">
				283	<pre>
				284	(* number: [0-9.]+ *)
				285	\| [< ' ('0' .. '9' as c); stream >] ->
				286	let buffer = Buffer.create 1 in
				287	Buffer.add_char buffer c;
				288	lex_number buffer stream
				289
				290	...
				291
				292	and lex_number buffer = parser
				293	\| [< ' ('0' .. '9' \| '.' as c); stream >] ->
				294	Buffer.add_char buffer c;
				295	lex_number buffer stream
				296	\| [< stream=lex >] ->
				297	[< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
				298	</pre>
				299	</div>
				300
				301	<p>This is all pretty straight-forward code for processing input. When reading
				302	a numeric value from input, we use the ocaml <tt>float_of_string</tt> function
Erick Tryzelaar	d564686	2008-03-30 19:14:31 +0000	[diff] [blame]	303	to convert it to a numeric value that we store in <tt>Token.Number</tt>. Note
				304	that this isn't doing sufficient error checking: it will raise <tt>Failure</tt>
Erick Tryzelaar	9ba8a57	2008-03-27 08:18:07 +0000	[diff] [blame]	305	if the string "1.23.45.67". Feel free to extend it :). Next we handle
				306	comments:
				307	</p>
				308
				309	<div class="doc_code">
				310	<pre>
				311	(* Comment until end of line. *)
				312	\| [< ' ('#'); stream >] ->
				313	lex_comment stream
				314
				315	...
				316
				317	and lex_comment = parser
				318	\| [< ' ('\n'); stream=lex >] -> stream
				319	\| [< 'c; e=lex_comment >] -> e
				320	\| [< >] -> [< >]
				321	</pre>
				322	</div>
				323
				324	<p>We handle comments by skipping to the end of the line and then return the
				325	next token. Finally, if the input doesn't match one of the above cases, it is
				326	either an operator character like '+' or the end of the file. These are handled
				327	with this code:</p>
				328
				329	<div class="doc_code">
				330	<pre>
				331	(* Otherwise, just return the character as its ascii value. *)
				332	\| [< 'c; stream >] ->
				333	[< 'Token.Kwd c; lex stream >]
				334
				335	(* end of stream. *)
				336	\| [< >] -> [< >]
				337	</pre>
				338	</div>
				339
				340	<p>With this, we have the complete lexer for the basic Kaleidoscope language
				341	(the <a href="OCamlLangImpl2.html#code">full code listing</a> for the Lexer is
				342	available in the <a href="OCamlLangImpl2.html">next chapter</a> of the
				343	tutorial). Next we'll <a href="OCamlLangImpl2.html">build a simple parser that
				344	uses this to build an Abstract Syntax Tree</a>. When we have that, we'll
				345	include a driver so that you can use the lexer and parser together.
				346	</p>
				347
				348	<a href="OCamlLangImpl2.html">Next: Implementing a Parser and AST</a>
				349	</div>
				350
				351	<!-- *********************************************************************** -->
				352	<hr>
				353	<address>
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				358
				359	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
				360	<a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
				361	<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
				362	Last modified: $Date: 2007-10-17 11:05:13 -0700 (Wed, 17 Oct 2007) $
				363	</address>
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