docs/tutorial/OCamlLangImpl1.rst

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Kaleidoscope: Tutorial Introduction and the Lexer
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.. contents::
   :local:

Tutorial Introduction
=====================

Welcome to the "Implementing a language with LLVM" tutorial. This
tutorial runs through the 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. The
code in this tutorial can also be used as a playground to hack on other
LLVM specific things.

The goal of this tutorial is to progressively unveil our language,
describing how it is built up over time. This will let us cover a fairly
broad range of language design and LLVM-specific usage issues, showing
and explaining the code for it all along the way, without overwhelming
you with tons of details up front.

It is useful to point out ahead of time that this tutorial is really
about teaching compiler techniques and LLVM specifically, *not* about
teaching modern and sane software engineering principles. In practice,
this means that we'll take a number of shortcuts to simplify the
exposition. For example, the code leaks memory, uses global variables
all over the place, doesn't use nice design patterns like
`visitors <http://en.wikipedia.org/wiki/Visitor_pattern>`_, etc... but
it is very simple. If you dig in and use the code as a basis for future
projects, fixing these deficiencies shouldn't be hard.

I've tried to put this tutorial together in a way that makes chapters
easy to skip over if you are already familiar with or are uninterested
in the various pieces. The structure of the tutorial is:

-  `Chapter #1 <#language>`_: Introduction to the Kaleidoscope
   language, and the definition of its Lexer - This shows where we are
   going and the basic functionality that we want it to do. In order to
   make this tutorial maximally understandable and hackable, we choose
   to implement everything in Objective Caml instead of using lexer and
   parser generators. LLVM obviously works just fine with such tools,
   feel free to use one if you prefer.
-  `Chapter #2 <OCamlLangImpl2.html>`_: Implementing a Parser and
   AST - With the lexer in place, we can talk about parsing techniques
   and basic AST construction. This tutorial describes recursive descent
   parsing and operator precedence parsing. Nothing in Chapters 1 or 2
   is LLVM-specific, the code doesn't even link in LLVM at this point.
   :)
-  `Chapter #3 <OCamlLangImpl3.html>`_: Code generation to LLVM IR -
   With the AST ready, we can show off how easy generation of LLVM IR
   really is.
-  `Chapter #4 <OCamlLangImpl4.html>`_: Adding JIT and Optimizer
   Support - Because a lot of people are interested in using LLVM as a
   JIT, we'll dive right into it and show you the 3 lines it takes to
   add JIT support. LLVM is also useful in many other ways, but this is
   one simple and "sexy" way to shows off its power. :)
-  `Chapter #5 <OCamlLangImpl5.html>`_: Extending the Language:
   Control Flow - With the language up and running, we show how to
   extend it with control flow operations (if/then/else and a 'for'
   loop). This gives us a chance to talk about simple SSA construction
   and control flow.
-  `Chapter #6 <OCamlLangImpl6.html>`_: Extending the Language:
   User-defined Operators - This is a silly but fun chapter that talks
   about extending the language to let the user program define their own
   arbitrary unary and binary operators (with assignable precedence!).
   This lets us build a significant piece of the "language" as library
   routines.
-  `Chapter #7 <OCamlLangImpl7.html>`_: Extending the Language:
   Mutable Variables - This chapter talks about adding user-defined
   local variables along with an assignment operator. The interesting
   part about this is how easy and trivial it is to construct SSA form
   in LLVM: no, LLVM does *not* require your front-end to construct SSA
   form!
-  `Chapter #8 <OCamlLangImpl8.html>`_: Conclusion and other useful
   LLVM tidbits - This chapter wraps up the series by talking about
   potential ways to extend the language, but also includes a bunch of
   pointers to info about "special topics" like adding garbage
   collection support, exceptions, debugging, support for "spaghetti
   stacks", and a bunch of other tips and tricks.

By the end of the tutorial, we'll have written a bit less than 700 lines
of non-comment, non-blank, lines of code. With this small amount of
code, we'll have built up a very reasonable compiler for a non-trivial
language including a hand-written lexer, parser, AST, as well as code
generation support with a JIT compiler. While other systems may have
interesting "hello world" tutorials, I think the breadth of this
tutorial is a great testament to the strengths of LLVM and why you
should consider it if you're interested in language or compiler design.

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,
compilers don't need to be scary creatures - it can be a lot of fun to
play with languages!

The Basic Language
==================

This tutorial will be illustrated with a toy language that we'll call
"`Kaleidoscope <http://en.wikipedia.org/wiki/Kaleidoscope>`_" (derived
from "meaning beautiful, form, and view"). 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.

Because we want to keep things simple, the only datatype in Kaleidoscope
is a 64-bit floating point type (aka 'float' in O'Caml 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
`Fibonacci numbers: <http://en.wikipedia.org/wiki/Fibonacci_number>`_

::

    # 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)

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:

::

    extern sin(arg);
    extern cos(arg);
    extern atan2(arg1 arg2);

    atan2(sin(.4), cos(42))

A more interesting example is included in Chapter 6 where we write a
little Kaleidoscope application that `displays a Mandelbrot
Set <OCamlLangImpl6.html#example>`_ at various levels of magnification.

Lets dive into the implementation of this language!

The Lexer
=========

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
"`lexer <http://en.wikipedia.org/wiki/Lexical_analysis>`_" (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:

.. code-block:: ocaml

    (* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
     * these others for known things. *)
    type token =
      (* commands *)
      | Def | Extern

      (* primary *)
      | Ident of string | Number of float

      (* unknown *)
      | Kwd of char

Each token returned by our lexer will be one of the token variant
values. An unknown character like '+' will be returned as
``Token.Kwd '+'``. If the curr token is an identifier, the value will be
``Token.Ident s``. If the current token is a numeric literal (like 1.0),
the value will be ``Token.Number 1.0``.

The actual implementation of the lexer is a collection of functions
driven by a function named ``Lexer.lex``. The ``Lexer.lex`` function is
called to return the next token from standard input. We will use
`Camlp4 <http://caml.inria.fr/pub/docs/manual-camlp4/index.html>`_ to
simplify the tokenization of the standard input. Its definition starts
as:

.. code-block:: ocaml

    (*===----------------------------------------------------------------------===
     * Lexer
     *===----------------------------------------------------------------------===*)

    let rec lex = parser
      (* Skip any whitespace. *)
      | [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream

``Lexer.lex`` works by recursing over a ``char Stream.t`` to read
characters one at a time from the standard input. It eats them as it
recognizes them and stores them in in a ``Token.token`` variant. The
first thing that it has to do is ignore whitespace between tokens. This
is accomplished with the recursive call above.

The next thing ``Lexer.lex`` needs to do is recognize identifiers and
specific keywords like "def". Kaleidoscope does this with a pattern
match and a helper function.

.. code-block:: ocaml

      (* identifier: [a-zA-Z][a-zA-Z0-9] *)
      | [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
          let buffer = Buffer.create 1 in
          Buffer.add_char buffer c;
          lex_ident buffer stream

    ...

    and lex_ident buffer = parser
      | [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
          Buffer.add_char buffer c;
          lex_ident buffer stream
      | [< stream=lex >] ->
          match Buffer.contents buffer with
          | "def" -> [< 'Token.Def; stream >]
          | "extern" -> [< 'Token.Extern; stream >]
          | id -> [< 'Token.Ident id; stream >]

Numeric values are similar:

.. code-block:: ocaml

      (* number: [0-9.]+ *)
      | [< ' ('0' .. '9' as c); stream >] ->
          let buffer = Buffer.create 1 in
          Buffer.add_char buffer c;
          lex_number buffer stream

    ...

    and lex_number buffer = parser
      | [< ' ('0' .. '9' | '.' as c); stream >] ->
          Buffer.add_char buffer c;
          lex_number buffer stream
      | [< stream=lex >] ->
          [< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]

This is all pretty straight-forward code for processing input. When
reading a numeric value from input, we use the ocaml ``float_of_string``
function to convert it to a numeric value that we store in
``Token.Number``. Note that this isn't doing sufficient error checking:
it will raise ``Failure`` if the string "1.23.45.67". Feel free to
extend it :). Next we handle comments:

.. code-block:: ocaml

      (* Comment until end of line. *)
      | [< ' ('#'); stream >] ->
          lex_comment stream

    ...

    and lex_comment = parser
      | [< ' ('\n'); stream=lex >] -> stream
      | [< 'c; e=lex_comment >] -> e
      | [< >] -> [< >]

We handle comments by skipping to the end of the line and then return
the next token. 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:

.. code-block:: ocaml

      (* Otherwise, just return the character as its ascii value. *)
      | [< 'c; stream >] ->
          [< 'Token.Kwd c; lex stream >]

      (* end of stream. *)
      | [< >] -> [< >]

With this, we have the complete lexer for the basic Kaleidoscope
language (the `full code listing <OCamlLangImpl2.html#code>`_ for the
Lexer is available in the `next chapter <OCamlLangImpl2.html>`_ of the
tutorial). Next we'll `build a simple parser that uses this to build an
Abstract Syntax Tree <OCamlLangImpl2.html>`_. When we have that, we'll
include a driver so that you can use the lexer and parser together.

`Next: Implementing a Parser and AST <OCamlLangImpl2.html>`_