PLY (Python Lex-Yacc)                   Version 1.2  (November 27, 2002)

David M. Beazley
Department of Computer Science
University of Chicago
Chicago, IL  60637
beazley@cs.uchicago.edu

Copyright (C) 2001   David M. Beazley

$Header: /home/stever/bk/newmem2/ext/ply/README 1.1 03/06/06 14:53:34-00:00 stever@ $

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA

See the file COPYING for a complete copy of the LGPL.

Introduction
============

PLY is a 100% Python implementation of the common parsing tools lex
and yacc.   Although several other parsing tools are available for
Python, there are several reasons why you might want to consider PLY:

 -  The tools are very closely modeled after traditional lex/yacc.
    If you know how to use these tools in C, you will find PLY
    to be similar.

 -  PLY provides *very* extensive error reporting and diagnostic 
    information to assist in parser construction.  The original
    implementation was developed for instructional purposes.  As
    a result, the system tries to identify the most common types
    of errors made by novice users.  

 -  PLY provides full support for empty productions, error recovery,
    precedence specifiers, and moderately ambiguous grammars.

 -  Parsing is based on LR-parsing which is fast, memory efficient, 
    better suited to large grammars, and which has a number of nice
    properties when dealing with syntax errors and other parsing problems.
    Currently, PLY builds its parsing tables using the SLR algorithm which 
    is slightly weaker than LALR(1) used in traditional yacc. 

 -  Like John Aycock's excellent SPARK toolkit, PLY uses Python 
    reflection to build lexers and parsers.  This greatly simplifies 
    the task of parser construction since it reduces the number of files
    and eliminates the need to run a separate lex/yacc tool before
    running your program.

 -  PLY can be used to build parsers for "real" programming languages.
    Although it is not ultra-fast due to its Python implementation,
    PLY can be used to parse grammars consisting of several hundred
    rules (as might be found for a language like C).  The lexer and LR 
    parser are also reasonably efficient when parsing typically
    sized programs.

The original version of PLY was developed for an Introduction to
Compilers course where students used it to build a compiler for a
simple Pascal-like language.  Their compiler had to include lexical
analysis, parsing, type checking, type inference, and generation of
assembly code for the SPARC processor.  Because of this, the current
implementation has been extensively tested and debugged.  In addition,
most of the API and error checking steps have been adapted to address
common usability problems.

How to Use
==========

PLY consists of two files : lex.py and yacc.py.  To use the system,
simply copy these files to your project and import them like standard
Python modules.

The file doc/ply.html contains complete documentation on how to use
the system.

The example directory contains several different examples including a
PLY specification for ANSI C as given in K&R 2nd Ed.   Note: To use
the examples, you will need to copy the lex.py and yacc.py files to
the example directory.

A simple example is found at the end of this document

Requirements
============
PLY requires the use of Python 2.0 or greater.  It should work on
just about any platform.

Resources
=========

More information about PLY can be obtained on the PLY webpage at:

     http://systems.cs.uchicago.edu/ply

For a detailed overview of parsing theory, consult the excellent
book "Compilers : Principles, Techniques, and Tools" by Aho, Sethi, and
Ullman.  The topics found in "Lex & Yacc" by Levine, Mason, and Brown
may also be useful.

Given that this is the first release, I welcome your comments on how
to improve the current implementation.  See the TODO file for things that
still need to be done.

Acknowledgments
===============

A special thanks is in order for all of the students in CS326 who
suffered through about 25 different versions of these tools :-).

Example
=======

Here is a simple example showing a PLY implementation of a calculator with variables.

# -----------------------------------------------------------------------------
# calc.py
#
# A simple calculator with variables.
# -----------------------------------------------------------------------------

tokens = (
    'NAME','NUMBER',
    'PLUS','MINUS','TIMES','DIVIDE','EQUALS',
    'LPAREN','RPAREN',
    )

# Tokens

t_PLUS    = r'\+'
t_MINUS   = r'-'
t_TIMES   = r'\*'
t_DIVIDE  = r'/'
t_EQUALS  = r'='
t_LPAREN  = r'\('
t_RPAREN  = r'\)'
t_NAME    = r'[a-zA-Z_][a-zA-Z0-9_]*'

def t_NUMBER(t):
    r'\d+'
    try:
        t.value = int(t.value)
    except ValueError:
        print "Integer value too large", t.value
        t.value = 0
    return t

# Ignored characters
t_ignore = " \t"

def t_newline(t):
    r'\n+'
    t.lineno += t.value.count("\n")
    
def t_error(t):
    print "Illegal character '%s'" % t.value[0]
    t.skip(1)
    
# Build the lexer
import lex
lex.lex()

# Precedence rules for the arithmetic operators
precedence = (
    ('left','PLUS','MINUS'),
    ('left','TIMES','DIVIDE'),
    ('right','UMINUS'),
    )

# dictionary of names (for storing variables)
names = { }

def p_statement_assign(t):
    'statement : NAME EQUALS expression'
    names[t[1]] = t[3]

def p_statement_expr(t):
    'statement : expression'
    print t[1]

def p_expression_binop(t):
    '''expression : expression PLUS expression
                  | expression MINUS expression
                  | expression TIMES expression
                  | expression DIVIDE expression'''
    if t[2] == '+'  : t[0] = t[1] + t[3]
    elif t[2] == '-': t[0] = t[1] - t[3]
    elif t[2] == '*': t[0] = t[1] * t[3]
    elif t[2] == '/': t[0] = t[1] / t[3]

def p_expression_uminus(t):
    'expression : MINUS expression %prec UMINUS'
    t[0] = -t[2]

def p_expression_group(t):
    'expression : LPAREN expression RPAREN'
    t[0] = t[2]

def p_expression_number(t):
    'expression : NUMBER'
    t[0] = t[1]

def p_expression_name(t):
    'expression : NAME'
    try:
        t[0] = names[t[1]]
    except LookupError:
        print "Undefined name '%s'" % t[1]
        t[0] = 0

def p_error(t):
    print "Syntax error at '%s'" % t.value

import yacc
yacc.yacc()

while 1:
    try:
        s = raw_input('calc > ')
    except EOFError:
        break
    yacc.parse(s)