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=================================================
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Kaleidoscope: Tutorial Introduction and the Lexer
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=================================================
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.. contents::
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:local:
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Tutorial Introduction
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=====================
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Welcome to the "Implementing a language with LLVM" tutorial. This
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tutorial runs through the implementation of a simple language, showing
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how fun and easy it can be. This tutorial will get you up and started as
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well as help to build a framework you can extend to other languages. The
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code in this tutorial can also be used as a playground to hack on other
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LLVM specific things.
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The goal of this tutorial is to progressively unveil our language,
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describing how it is built up over time. This will let us cover a fairly
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broad range of language design and LLVM-specific usage issues, showing
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and explaining the code for it all along the way, without overwhelming
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you with tons of details up front.
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It is useful to point out ahead of time that this tutorial is really
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about teaching compiler techniques and LLVM specifically, *not* about
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teaching modern and sane software engineering principles. In practice,
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this means that we'll take a number of shortcuts to simplify the
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exposition. For example, the code leaks memory, uses global variables
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all over the place, doesn't use nice design patterns like
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`visitors <http://en.wikipedia.org/wiki/Visitor_pattern>`_, etc... but
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it is very simple. If you dig in and use the code as a basis for future
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projects, fixing these deficiencies shouldn't be hard.
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I've tried to put this tutorial together in a way that makes chapters
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easy to skip over if you are already familiar with or are uninterested
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in the various pieces. The structure of the tutorial is:
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- `Chapter #1 <#language>`_: Introduction to the Kaleidoscope
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language, and the definition of its Lexer - This shows where we are
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going and the basic functionality that we want it to do. In order to
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make this tutorial maximally understandable and hackable, we choose
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to implement everything in C++ instead of using lexer and parser
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generators. LLVM obviously works just fine with such tools, feel free
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to use one if you prefer.
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- `Chapter #2 <LangImpl2.html>`_: Implementing a Parser and AST -
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With the lexer in place, we can talk about parsing techniques and
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basic AST construction. This tutorial describes recursive descent
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parsing and operator precedence parsing. Nothing in Chapters 1 or 2
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is LLVM-specific, the code doesn't even link in LLVM at this point.
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:)
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- `Chapter #3 <LangImpl3.html>`_: Code generation to LLVM IR - With
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the AST ready, we can show off how easy generation of LLVM IR really
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is.
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- `Chapter #4 <LangImpl4.html>`_: Adding JIT and Optimizer Support
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- Because a lot of people are interested in using LLVM as a JIT,
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we'll dive right into it and show you the 3 lines it takes to add JIT
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support. LLVM is also useful in many other ways, but this is one
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simple and "sexy" way to show off its power. :)
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- `Chapter #5 <LangImpl5.html>`_: Extending the Language: Control
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Flow - With the language up and running, we show how to extend it
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with control flow operations (if/then/else and a 'for' loop). This
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gives us a chance to talk about simple SSA construction and control
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flow.
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- `Chapter #6 <LangImpl6.html>`_: Extending the Language:
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User-defined Operators - This is a silly but fun chapter that talks
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about extending the language to let the user program define their own
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arbitrary unary and binary operators (with assignable precedence!).
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This lets us build a significant piece of the "language" as library
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routines.
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- `Chapter #7 <LangImpl7.html>`_: Extending the Language: Mutable
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Variables - This chapter talks about adding user-defined local
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variables along with an assignment operator. The interesting part
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about this is how easy and trivial it is to construct SSA form in
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LLVM: no, LLVM does *not* require your front-end to construct SSA
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form!
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- `Chapter #8 <LangImpl8.html>`_: Extending the Language: Debug
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Information - Having built a decent little programming language with
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control flow, functions and mutable variables, we consider what it
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takes to add debug information to standalone executables. This debug
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information will allow you to set breakpoints in Kaleidoscope
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functions, print out argument variables, and call functions - all
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from within the debugger!
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- `Chapter #9 <LangImpl8.html>`_: Conclusion and other useful LLVM
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tidbits - This chapter wraps up the series by talking about
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potential ways to extend the language, but also includes a bunch of
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pointers to info about "special topics" like adding garbage
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collection support, exceptions, debugging, support for "spaghetti
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stacks", and a bunch of other tips and tricks.
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By the end of the tutorial, we'll have written a bit less than 1000 lines
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of non-comment, non-blank, lines of code. With this small amount of
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code, we'll have built up a very reasonable compiler for a non-trivial
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language including a hand-written lexer, parser, AST, as well as code
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generation support with a JIT compiler. While other systems may have
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interesting "hello world" tutorials, I think the breadth of this
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tutorial is a great testament to the strengths of LLVM and why you
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should consider it if you're interested in language or compiler design.
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A note about this tutorial: we expect you to extend the language and
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play with it on your own. Take the code and go crazy hacking away at it,
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compilers don't need to be scary creatures - it can be a lot of fun to
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play with languages!
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The Basic Language
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==================
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This tutorial will be illustrated with a toy language that we'll call
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"`Kaleidoscope <http://en.wikipedia.org/wiki/Kaleidoscope>`_" (derived
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from "meaning beautiful, form, and view"). Kaleidoscope is a procedural
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language that allows you to define functions, use conditionals, math,
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etc. Over the course of the tutorial, we'll extend Kaleidoscope to
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support the if/then/else construct, a for loop, user defined operators,
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JIT compilation with a simple command line interface, etc.
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Because we want to keep things simple, the only datatype in Kaleidoscope
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is a 64-bit floating point type (aka 'double' in C parlance). As such,
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all values are implicitly double precision and the language doesn't
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require type declarations. This gives the language a very nice and
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simple syntax. For example, the following simple example computes
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`Fibonacci numbers: <http://en.wikipedia.org/wiki/Fibonacci_number>`_
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::
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# Compute the x'th fibonacci number.
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def fib(x)
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if x < 3 then
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1
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else
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fib(x-1)+fib(x-2)
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# This expression will compute the 40th number.
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fib(40)
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We also allow Kaleidoscope to call into standard library functions (the
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LLVM JIT makes this completely trivial). This means that you can use the
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'extern' keyword to define a function before you use it (this is also
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useful for mutually recursive functions). For example:
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::
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extern sin(arg);
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extern cos(arg);
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extern atan2(arg1 arg2);
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atan2(sin(.4), cos(42))
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A more interesting example is included in Chapter 6 where we write a
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little Kaleidoscope application that `displays a Mandelbrot
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Set <LangImpl6.html#example>`_ at various levels of magnification.
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Lets dive into the implementation of this language!
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The Lexer
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=========
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When it comes to implementing a language, the first thing needed is the
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ability to process a text file and recognize what it says. The
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traditional way to do this is to use a
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"`lexer <http://en.wikipedia.org/wiki/Lexical_analysis>`_" (aka
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'scanner') to break the input up into "tokens". Each token returned by
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the lexer includes a token code and potentially some metadata (e.g. the
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numeric value of a number). First, we define the possibilities:
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.. code-block:: c++
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// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
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// of these for known things.
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enum Token {
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tok_eof = -1,
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// commands
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tok_def = -2, tok_extern = -3,
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// primary
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tok_identifier = -4, tok_number = -5,
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};
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static std::string IdentifierStr; // Filled in if tok_identifier
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static double NumVal; // Filled in if tok_number
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Each token returned by our lexer will either be one of the Token enum
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values or it will be an 'unknown' character like '+', which is returned
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as its ASCII value. If the current token is an identifier, the
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``IdentifierStr`` global variable holds the name of the identifier. If
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the current token is a numeric literal (like 1.0), ``NumVal`` holds its
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value. Note that we use global variables for simplicity, this is not the
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best choice for a real language implementation :).
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The actual implementation of the lexer is a single function named
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``gettok``. The ``gettok`` function is called to return the next token
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from standard input. Its definition starts as:
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.. code-block:: c++
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/// gettok - Return the next token from standard input.
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static int gettok() {
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static int LastChar = ' ';
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// Skip any whitespace.
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while (isspace(LastChar))
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LastChar = getchar();
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``gettok`` works by calling the C ``getchar()`` function to read
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characters one at a time from standard input. It eats them as it
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recognizes them and stores the last character read, but not processed,
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in LastChar. The first thing that it has to do is ignore whitespace
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between tokens. This is accomplished with the loop above.
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The next thing ``gettok`` needs to do is recognize identifiers and
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specific keywords like "def". Kaleidoscope does this with this simple
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loop:
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.. code-block:: c++
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if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
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IdentifierStr = LastChar;
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while (isalnum((LastChar = getchar())))
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IdentifierStr += LastChar;
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if (IdentifierStr == "def") return tok_def;
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if (IdentifierStr == "extern") return tok_extern;
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return tok_identifier;
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}
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Note that this code sets the '``IdentifierStr``' global whenever it
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lexes an identifier. Also, since language keywords are matched by the
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same loop, we handle them here inline. Numeric values are similar:
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.. code-block:: c++
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if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
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std::string NumStr;
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do {
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NumStr += LastChar;
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LastChar = getchar();
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} while (isdigit(LastChar) || LastChar == '.');
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NumVal = strtod(NumStr.c_str(), 0);
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return tok_number;
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}
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This is all pretty straight-forward code for processing input. When
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reading a numeric value from input, we use the C ``strtod`` function to
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convert it to a numeric value that we store in ``NumVal``. Note that
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this isn't doing sufficient error checking: it will incorrectly read
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"1.23.45.67" and handle it as if you typed in "1.23". Feel free to
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extend it :). Next we handle comments:
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.. code-block:: c++
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if (LastChar == '#') {
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// Comment until end of line.
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do LastChar = getchar();
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while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
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if (LastChar != EOF)
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return gettok();
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}
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We handle comments by skipping to the end of the line and then return
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the next token. Finally, if the input doesn't match one of the above
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cases, it is either an operator character like '+' or the end of the
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file. These are handled with this code:
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.. code-block:: c++
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// Check for end of file. Don't eat the EOF.
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if (LastChar == EOF)
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return tok_eof;
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// Otherwise, just return the character as its ascii value.
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int ThisChar = LastChar;
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LastChar = getchar();
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return ThisChar;
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}
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With this, we have the complete lexer for the basic Kaleidoscope
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language (the `full code listing <LangImpl2.html#code>`_ for the Lexer
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is available in the `next chapter <LangImpl2.html>`_ of the tutorial).
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Next we'll `build a simple parser that uses this to build an Abstract
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Syntax Tree <LangImpl2.html>`_. When we have that, we'll include a
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driver so that you can use the lexer and parser together.
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`Next: Implementing a Parser and AST <LangImpl2.html>`_
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