llvm/docs/tutorial/MyFirstLanguageFrontend/LangImpl01.rst

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Kaleidoscope: Kaleidoscope Introduction and the Lexer
=====================================================

.. contents::
   :local:

The Kaleidoscope Language
=========================

This tutorial is illustrated with a toy language called
"`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, debug info, etc.

We want to keep things simple, so the only datatype in Kaleidoscope
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, 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 really easy. 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 <LangImpl06.html#kicking-the-tires>`_ at various levels of magnification.

Let's 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:: c++

    // 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

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
``IdentifierStr`` global variable holds the name of the identifier. If
the current token is a numeric literal (like 1.0), ``NumVal`` holds its
value. We use global variables for simplicity, but this is not the
best choice for a real language implementation :).

The actual implementation of the lexer is a single function named
``gettok``. The ``gettok`` function is called to return the next token
from standard input. Its definition starts as:

.. code-block:: c++

    /// gettok - Return the next token from standard input.
    static int gettok() {
      static int LastChar = ' ';

      // Skip any whitespace.
      while (isspace(LastChar))
        LastChar = getchar();

``gettok`` works by calling the C ``getchar()`` 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.

The next thing ``gettok`` needs to do is recognize identifiers and
specific keywords like "def". Kaleidoscope does this with this simple
loop:

.. code-block:: c++

      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;
      }

Note that this code sets the '``IdentifierStr``' 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:

.. code-block:: c++

      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;
      }

This is all pretty straightforward code for processing input. When
reading a numeric value from input, we use the C ``strtod`` function to
convert it to a numeric value that we store in ``NumVal``. Note that
this isn't doing sufficient error checking: it will incorrectly read
"1.23.45.67" and handle it as if you typed in "1.23". Feel free to
extend it!  Next we handle comments:

.. code-block:: c++

      if (LastChar == '#') {
        // Comment until end of line.
        do
          LastChar = getchar();
        while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');

        if (LastChar != EOF)
          return gettok();
      }

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:: c++

      // 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;
    }

With this, we have the complete lexer for the basic Kaleidoscope
language (the `full code listing <LangImpl02.html#full-code-listing>`_ for the Lexer
is available in the `next chapter <LangImpl02.html>`_ of the tutorial).
Next we'll `build a simple parser that uses this to build an Abstract
Syntax Tree <LangImpl02.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 <LangImpl02.html>`_