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195 lines
6.6 KiB
195 lines
6.6 KiB
=====================================================
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Kaleidoscope: Kaleidoscope Introduction and the Lexer
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=====================================================
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.. contents::
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:local:
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The Kaleidoscope Language
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=========================
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This tutorial is illustrated with a toy language called
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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, debug info, etc.
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We want to keep things simple, so 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 really easy. 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 <LangImpl06.html#kicking-the-tires>`_ at various levels of magnification.
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Let's 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,
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tok_extern = -3,
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// primary
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tok_identifier = -4,
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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. We use global variables for simplicity, but 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")
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return tok_def;
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if (IdentifierStr == "extern")
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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 straightforward 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
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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 <LangImpl02.html#full-code-listing>`_ for the Lexer
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is available in the `next chapter <LangImpl02.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 <LangImpl02.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 <LangImpl02.html>`_
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