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annotate docs/tutorial/OCamlLangImpl1.rst @ 134:3a76565eade5 LLVM5.0.1

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1 =================================================
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2 Kaleidoscope: Tutorial Introduction and the Lexer
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3 =================================================
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4
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5 .. contents::
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6 :local:
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7
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8 Tutorial Introduction
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9 =====================
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10
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11 Welcome to the "Implementing a language with LLVM" tutorial. This
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12 tutorial runs through the implementation of a simple language, showing
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13 how fun and easy it can be. This tutorial will get you up and started as
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14 well as help to build a framework you can extend to other languages. The
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15 code in this tutorial can also be used as a playground to hack on other
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16 LLVM specific things.
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17
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18 The goal of this tutorial is to progressively unveil our language,
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19 describing how it is built up over time. This will let us cover a fairly
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20 broad range of language design and LLVM-specific usage issues, showing
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21 and explaining the code for it all along the way, without overwhelming
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22 you with tons of details up front.
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23
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24 It is useful to point out ahead of time that this tutorial is really
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25 about teaching compiler techniques and LLVM specifically, *not* about
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26 teaching modern and sane software engineering principles. In practice,
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27 this means that we'll take a number of shortcuts to simplify the
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28 exposition. For example, the code leaks memory, uses global variables
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29 all over the place, doesn't use nice design patterns like
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30 `visitors <http://en.wikipedia.org/wiki/Visitor_pattern>`_, etc... but
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31 it is very simple. If you dig in and use the code as a basis for future
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32 projects, fixing these deficiencies shouldn't be hard.
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33
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34 I've tried to put this tutorial together in a way that makes chapters
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35 easy to skip over if you are already familiar with or are uninterested
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36 in the various pieces. The structure of the tutorial is:
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37
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38 - `Chapter #1 <#language>`_: Introduction to the Kaleidoscope
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39 language, and the definition of its Lexer - This shows where we are
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40 going and the basic functionality that we want it to do. In order to
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41 make this tutorial maximally understandable and hackable, we choose
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42 to implement everything in Objective Caml instead of using lexer and
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43 parser generators. LLVM obviously works just fine with such tools,
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44 feel free to use one if you prefer.
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45 - `Chapter #2 <OCamlLangImpl2.html>`_: Implementing a Parser and
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46 AST - With the lexer in place, we can talk about parsing techniques
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47 and basic AST construction. This tutorial describes recursive descent
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48 parsing and operator precedence parsing. Nothing in Chapters 1 or 2
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49 is LLVM-specific, the code doesn't even link in LLVM at this point.
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50 :)
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51 - `Chapter #3 <OCamlLangImpl3.html>`_: Code generation to LLVM IR -
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52 With the AST ready, we can show off how easy generation of LLVM IR
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53 really is.
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54 - `Chapter #4 <OCamlLangImpl4.html>`_: Adding JIT and Optimizer
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55 Support - Because a lot of people are interested in using LLVM as a
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56 JIT, we'll dive right into it and show you the 3 lines it takes to
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57 add JIT support. LLVM is also useful in many other ways, but this is
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58 one simple and "sexy" way to shows off its power. :)
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59 - `Chapter #5 <OCamlLangImpl5.html>`_: Extending the Language:
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60 Control Flow - With the language up and running, we show how to
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61 extend it with control flow operations (if/then/else and a 'for'
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62 loop). This gives us a chance to talk about simple SSA construction
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63 and control flow.
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64 - `Chapter #6 <OCamlLangImpl6.html>`_: Extending the Language:
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65 User-defined Operators - This is a silly but fun chapter that talks
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66 about extending the language to let the user program define their own
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67 arbitrary unary and binary operators (with assignable precedence!).
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68 This lets us build a significant piece of the "language" as library
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69 routines.
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70 - `Chapter #7 <OCamlLangImpl7.html>`_: Extending the Language:
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71 Mutable Variables - This chapter talks about adding user-defined
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72 local variables along with an assignment operator. The interesting
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73 part about this is how easy and trivial it is to construct SSA form
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74 in LLVM: no, LLVM does *not* require your front-end to construct SSA
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75 form!
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76 - `Chapter #8 <OCamlLangImpl8.html>`_: Conclusion and other useful
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77 LLVM tidbits - This chapter wraps up the series by talking about
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78 potential ways to extend the language, but also includes a bunch of
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79 pointers to info about "special topics" like adding garbage
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80 collection support, exceptions, debugging, support for "spaghetti
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81 stacks", and a bunch of other tips and tricks.
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82
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83 By the end of the tutorial, we'll have written a bit less than 700 lines
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84 of non-comment, non-blank, lines of code. With this small amount of
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85 code, we'll have built up a very reasonable compiler for a non-trivial
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86 language including a hand-written lexer, parser, AST, as well as code
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87 generation support with a JIT compiler. While other systems may have
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88 interesting "hello world" tutorials, I think the breadth of this
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89 tutorial is a great testament to the strengths of LLVM and why you
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90 should consider it if you're interested in language or compiler design.
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91
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92 A note about this tutorial: we expect you to extend the language and
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93 play with it on your own. Take the code and go crazy hacking away at it,
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94 compilers don't need to be scary creatures - it can be a lot of fun to
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95 play with languages!
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96
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97 The Basic Language
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98 ==================
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99
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100 This tutorial will be illustrated with a toy language that we'll call
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101 "`Kaleidoscope <http://en.wikipedia.org/wiki/Kaleidoscope>`_" (derived
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102 from "meaning beautiful, form, and view"). Kaleidoscope is a procedural
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103 language that allows you to define functions, use conditionals, math,
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104 etc. Over the course of the tutorial, we'll extend Kaleidoscope to
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105 support the if/then/else construct, a for loop, user defined operators,
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106 JIT compilation with a simple command line interface, etc.
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107
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108 Because we want to keep things simple, the only datatype in Kaleidoscope
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109 is a 64-bit floating point type (aka 'float' in OCaml parlance). As
0
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110 such, all values are implicitly double precision and the language
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111 doesn't require type declarations. This gives the language a very nice
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112 and simple syntax. For example, the following simple example computes
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113 `Fibonacci numbers: <http://en.wikipedia.org/wiki/Fibonacci_number>`_
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114
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115 ::
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116
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117 # Compute the x'th fibonacci number.
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118 def fib(x)
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119 if x < 3 then
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120 1
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121 else
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122 fib(x-1)+fib(x-2)
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123
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124 # This expression will compute the 40th number.
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125 fib(40)
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parents:
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126
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127 We also allow Kaleidoscope to call into standard library functions (the
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parents:
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128 LLVM JIT makes this completely trivial). This means that you can use the
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parents:
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129 'extern' keyword to define a function before you use it (this is also
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parents:
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130 useful for mutually recursive functions). For example:
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parents:
diff changeset
131
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parents:
diff changeset
132 ::
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parents:
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133
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parents:
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134 extern sin(arg);
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parents:
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135 extern cos(arg);
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parents:
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136 extern atan2(arg1 arg2);
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parents:
diff changeset
137
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parents:
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138 atan2(sin(.4), cos(42))
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139
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140 A more interesting example is included in Chapter 6 where we write a
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parents:
diff changeset
141 little Kaleidoscope application that `displays a Mandelbrot
100
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parents: 0
diff changeset
142 Set <OCamlLangImpl6.html#kicking-the-tires>`_ at various levels of magnification.
0
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143
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parents:
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144 Lets dive into the implementation of this language!
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145
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146 The Lexer
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parents:
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147 =========
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148
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parents:
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149 When it comes to implementing a language, the first thing needed is the
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parents:
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150 ability to process a text file and recognize what it says. The
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parents:
diff changeset
151 traditional way to do this is to use a
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152 "`lexer <http://en.wikipedia.org/wiki/Lexical_analysis>`_" (aka
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parents:
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153 'scanner') to break the input up into "tokens". Each token returned by
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parents:
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154 the lexer includes a token code and potentially some metadata (e.g. the
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parents:
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155 numeric value of a number). First, we define the possibilities:
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parents:
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156
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parents:
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157 .. code-block:: ocaml
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parents:
diff changeset
158
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parents:
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159 (* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
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parents:
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160 * these others for known things. *)
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parents:
diff changeset
161 type token =
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parents:
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162 (* commands *)
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parents:
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163 | Def | Extern
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parents:
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164
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parents:
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165 (* primary *)
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parents:
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166 | Ident of string | Number of float
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parents:
diff changeset
167
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parents:
diff changeset
168 (* unknown *)
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parents:
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169 | Kwd of char
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parents:
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170
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parents:
diff changeset
171 Each token returned by our lexer will be one of the token variant
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parents:
diff changeset
172 values. An unknown character like '+' will be returned as
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parents:
diff changeset
173 ``Token.Kwd '+'``. If the curr token is an identifier, the value will be
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parents:
diff changeset
174 ``Token.Ident s``. If the current token is a numeric literal (like 1.0),
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parents:
diff changeset
175 the value will be ``Token.Number 1.0``.
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parents:
diff changeset
176
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parents:
diff changeset
177 The actual implementation of the lexer is a collection of functions
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parents:
diff changeset
178 driven by a function named ``Lexer.lex``. The ``Lexer.lex`` function is
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parents:
diff changeset
179 called to return the next token from standard input. We will use
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parents:
diff changeset
180 `Camlp4 <http://caml.inria.fr/pub/docs/manual-camlp4/index.html>`_ to
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parents:
diff changeset
181 simplify the tokenization of the standard input. Its definition starts
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
182 as:
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parents:
diff changeset
183
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parents:
diff changeset
184 .. code-block:: ocaml
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parents:
diff changeset
185
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parents:
diff changeset
186 (*===----------------------------------------------------------------------===
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parents:
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187 * Lexer
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parents:
diff changeset
188 *===----------------------------------------------------------------------===*)
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parents:
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189
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parents:
diff changeset
190 let rec lex = parser
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parents:
diff changeset
191 (* Skip any whitespace. *)
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parents:
diff changeset
192 | [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
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parents:
diff changeset
193
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parents:
diff changeset
194 ``Lexer.lex`` works by recursing over a ``char Stream.t`` to read
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parents:
diff changeset
195 characters one at a time from the standard input. It eats them as it
134
3a76565eade5 update 5.0.1
mir3636
parents: 120
diff changeset
196 recognizes them and stores them in a ``Token.token`` variant. The
0
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parents:
diff changeset
197 first thing that it has to do is ignore whitespace between tokens. This
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parents:
diff changeset
198 is accomplished with the recursive call above.
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parents:
diff changeset
199
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parents:
diff changeset
200 The next thing ``Lexer.lex`` needs to do is recognize identifiers and
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parents:
diff changeset
201 specific keywords like "def". Kaleidoscope does this with a pattern
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parents:
diff changeset
202 match and a helper function.
95c75e76d11b LLVM 3.4
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parents:
diff changeset
203
95c75e76d11b LLVM 3.4
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parents:
diff changeset
204 .. code-block:: ocaml
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parents:
diff changeset
205
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parents:
diff changeset
206 (* identifier: [a-zA-Z][a-zA-Z0-9] *)
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parents:
diff changeset
207 | [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
208 let buffer = Buffer.create 1 in
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
209 Buffer.add_char buffer c;
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
210 lex_ident buffer stream
95c75e76d11b LLVM 3.4
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parents:
diff changeset
211
95c75e76d11b LLVM 3.4
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parents:
diff changeset
212 ...
95c75e76d11b LLVM 3.4
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parents:
diff changeset
213
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parents:
diff changeset
214 and lex_ident buffer = parser
95c75e76d11b LLVM 3.4
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parents:
diff changeset
215 | [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
95c75e76d11b LLVM 3.4
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parents:
diff changeset
216 Buffer.add_char buffer c;
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
217 lex_ident buffer stream
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
218 | [< stream=lex >] ->
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
219 match Buffer.contents buffer with
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
220 | "def" -> [< 'Token.Def; stream >]
95c75e76d11b LLVM 3.4
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parents:
diff changeset
221 | "extern" -> [< 'Token.Extern; stream >]
95c75e76d11b LLVM 3.4
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parents:
diff changeset
222 | id -> [< 'Token.Ident id; stream >]
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parents:
diff changeset
223
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parents:
diff changeset
224 Numeric values are similar:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
225
95c75e76d11b LLVM 3.4
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parents:
diff changeset
226 .. code-block:: ocaml
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
227
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parents:
diff changeset
228 (* number: [0-9.]+ *)
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parents:
diff changeset
229 | [< ' ('0' .. '9' as c); stream >] ->
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
230 let buffer = Buffer.create 1 in
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
231 Buffer.add_char buffer c;
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
232 lex_number buffer stream
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
233
95c75e76d11b LLVM 3.4
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parents:
diff changeset
234 ...
95c75e76d11b LLVM 3.4
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parents:
diff changeset
235
95c75e76d11b LLVM 3.4
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parents:
diff changeset
236 and lex_number buffer = parser
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
237 | [< ' ('0' .. '9' | '.' as c); stream >] ->
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
238 Buffer.add_char buffer c;
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
239 lex_number buffer stream
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
240 | [< stream=lex >] ->
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
241 [< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
242
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
243 This is all pretty straight-forward code for processing input. When
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
244 reading a numeric value from input, we use the ocaml ``float_of_string``
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
245 function to convert it to a numeric value that we store in
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
246 ``Token.Number``. Note that this isn't doing sufficient error checking:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
247 it will raise ``Failure`` if the string "1.23.45.67". Feel free to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
248 extend it :). Next we handle comments:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
249
95c75e76d11b LLVM 3.4
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parents:
diff changeset
250 .. code-block:: ocaml
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parents:
diff changeset
251
95c75e76d11b LLVM 3.4
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parents:
diff changeset
252 (* Comment until end of line. *)
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
253 | [< ' ('#'); stream >] ->
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
254 lex_comment stream
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parents:
diff changeset
255
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parents:
diff changeset
256 ...
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parents:
diff changeset
257
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parents:
diff changeset
258 and lex_comment = parser
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259 | [< ' ('\n'); stream=lex >] -> stream
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260 | [< 'c; e=lex_comment >] -> e
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261 | [< >] -> [< >]
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262
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263 We handle comments by skipping to the end of the line and then return
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264 the next token. Finally, if the input doesn't match one of the above
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265 cases, it is either an operator character like '+' or the end of the
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266 file. These are handled with this code:
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267
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268 .. code-block:: ocaml
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269
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270 (* Otherwise, just return the character as its ascii value. *)
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271 | [< 'c; stream >] ->
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272 [< 'Token.Kwd c; lex stream >]
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273
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274 (* end of stream. *)
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275 | [< >] -> [< >]
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276
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277 With this, we have the complete lexer for the basic Kaleidoscope
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278 language (the `full code listing <OCamlLangImpl2.html#full-code-listing>`_ for the
0
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279 Lexer is available in the `next chapter <OCamlLangImpl2.html>`_ of the
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280 tutorial). Next we'll `build a simple parser that uses this to build an
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281 Abstract Syntax Tree <OCamlLangImpl2.html>`_. When we have that, we'll
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282 include a driver so that you can use the lexer and parser together.
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283
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284 `Next: Implementing a Parser and AST <OCamlLangImpl2.html>`_
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285