Fixed README
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David Beazley
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README.txt
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README.txt
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SLY (Sly Lex-Yacc) Version 0.3
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Copyright (C) 2016-2018
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David M. Beazley (Dabeaz LLC)
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All rights reserved.
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Redistribution and use in source and binary forms, with or without
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modification, are permitted provided that the following conditions are
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met:
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* Redistributions of source code must retain the above copyright notice,
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this list of conditions and the following disclaimer.
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* Redistributions in binary form must reproduce the above copyright notice,
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this list of conditions and the following disclaimer in the documentation
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and/or other materials provided with the distribution.
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* Neither the name of the David Beazley or Dabeaz LLC may be used to
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endorse or promote products derived from this software without
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specific prior written permission.
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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CAUTION
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=======
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THIS IS A WORK IN PROGRESS. NO OFFICIAL RELEASE HAS BEEN MADE.
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USE AT YOUR OWN RISK.
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Requirements
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============
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SLY requires the use of Python 3.6 or greater. Older versions
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of Python are not supported.
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Introduction
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============
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SLY is a 100% Python implementation of the lex and yacc tools
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commonly used to write parsers and compilers. Parsing is
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based on the same LALR(1) algorithm used by many yacc tools.
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Here are a few notable features:
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- SLY provides *very* extensive error reporting and diagnostic
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information to assist in parser construction. The original
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implementation was developed for instructional purposes. As
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a result, the system tries to identify the most common types
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of errors made by novice users.
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- SLY provides full support for empty productions, error recovery,
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precedence specifiers, and moderately ambiguous grammars.
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- SLY uses various Python metaprogramming features to specify
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lexers and parsers. There are no generated files or extra
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steps involved. You simply write Python code and run it.
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- SLY can be used to build parsers for "real" programming languages.
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Although it is not ultra-fast due to its Python implementation,
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SLY can be used to parse grammars consisting of several hundred
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rules (as might be found for a language like C).
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SLY originates from the PLY project (http://www.dabeaz.com/ply/index.html).
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However, it's been modernized a bit. In fact, don't expect any code
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previously written for PLY to work. That said, most of the things
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that were possible in PLY are also possible in SLY.
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An Example
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==========
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SLY is probably best illustrated by an example. Here's what it
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looks like to write a parser that can evaluate simple arithmetic
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expressions and store variables:
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# -----------------------------------------------------------------------------
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# calc.py
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# -----------------------------------------------------------------------------
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from sly import Lexer, Parser
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class CalcLexer(Lexer):
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tokens = { NAME, NUMBER, PLUS, TIMES, MINUS, DIVIDE, ASSIGN, LPAREN, RPAREN }
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ignore = ' \t'
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# Tokens
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NAME = r'[a-zA-Z_][a-zA-Z0-9_]*'
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NUMBER = r'\d+'
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# Special symbols
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PLUS = r'\+'
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MINUS = r'-'
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TIMES = r'\*'
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DIVIDE = r'/'
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ASSIGN = r'='
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LPAREN = r'\('
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RPAREN = r'\)'
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# Ignored pattern
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ignore_newline = r'\n+'
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# Extra action for newlines
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def ignore_newline(self, t):
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self.lineno += t.value.count('\n')
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def error(self, t):
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print("Illegal character '%s'" % t.value[0])
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self.index += 1
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class CalcParser(Parser):
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tokens = CalcLexer.tokens
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precedence = (
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('left', PLUS, MINUS),
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('left', TIMES, DIVIDE),
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('right', UMINUS),
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)
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def __init__(self):
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self.names = { }
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@_('NAME ASSIGN expr')
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def statement(self, p):
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self.names[p.NAME] = p.expr
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@_('expr')
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def statement(self, p):
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print(p.expr)
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@_('expr PLUS expr')
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def expr(self, p):
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return p.expr0 + p.expr1
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@_('expr MINUS expr')
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def expr(self, p):
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return p.expr0 - p.expr1
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@_('expr TIMES expr')
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def expr(self, p):
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return p.expr0 * p.expr1
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@_('expr DIVIDE expr')
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def expr(self, p):
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return p.expr0 / p.expr1
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@_('MINUS expr %prec UMINUS')
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def expr(self, p):
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return -p.expr
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@_('LPAREN expr RPAREN')
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def expr(self, p):
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return p.expr
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@_('NUMBER')
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def expr(self, p):
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return int(p.NUMBER)
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@_('NAME')
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def expr(self, p):
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try:
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return self.names[p.NAME]
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except LookupError:
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print(f'Undefined name {p.NAME!r}')
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return 0
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if __name__ == '__main__':
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lexer = CalcLexer()
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parser = CalcParser()
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while True:
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try:
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text = input('calc > ')
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except EOFError:
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break
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if text:
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parser.parse(lexer.tokenize(text))
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Documentation
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=============
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Further documentation can be found at https://sly.readthedocs.io/en/latest
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Resources
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=========
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For a detailed overview of parsing theory, consult the excellent
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book "Compilers : Principles, Techniques, and Tools" by Aho, Sethi, and
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Ullman. The topics found in "Lex & Yacc" by Levine, Mason, and Brown
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may also be useful.
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The GitHub page for SLY can be found at:
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https://github.com/dabeaz/sly
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Please direct bug reports and pull requests to the GitHub page.
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To contact me directly, send email to dave@dabeaz.com or contact
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me on Twitter (@dabeaz).
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-- Dave
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