SLY (Sly Lex-Yacc)
==================

SLY is a 100% Python implementation of the lex and yacc tools
commonly used to write parsers and compilers.  Parsing is
based on the same LALR(1) algorithm used by many yacc tools.
Here are a few notable features:

-  SLY 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.  

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

-  SLY uses various Python metaprogramming features to specify
   lexers and parsers.  There are no generated files or extra
   steps involved. You simply write Python code and run it.

-  SLY can be used to build parsers for "real" programming languages.
   Although it is not ultra-fast due to its Python implementation,
   SLY can be used to parse grammars consisting of several hundred
   rules (as might be found for a language like C).  

SLY originates from the `PLY project <http://www.dabeaz.com/ply/index.html>`_.
However, it's been modernized a bit.  In fact, don't expect any code
previously written for PLY to work. That said, most of the things 
that were possible in PLY are also possible in SLY. 

SLY is a modern library for performing lexing and parsing. It
implements the LALR(1) parsing algorithm, commonly used for
parsing and compiling various programming languages. 

Important Notice : October 11, 2022
-----------------------------------
The SLY project is no longer making package-installable releases.
It's fully functional, but if choose to use it, you should
vendor the code into your application. SLY has zero-dependencies.
Although I am semi-retiring the project, I will respond to
bug reports and still may decide to make future changes to it
depending on my mood. I'd like to thank everyone who
has contributed to it over the years. --Dave

Requirements
------------

SLY requires the use of Python 3.6 or greater.  Older versions
of Python are not supported.

An Example
----------

SLY is probably best illustrated by an example.  Here's what it
looks like to write a parser that can evaluate simple arithmetic
expressions and store variables:

.. code:: python

    # -----------------------------------------------------------------------------
    # calc.py
    # -----------------------------------------------------------------------------

    from sly import Lexer, Parser

    class CalcLexer(Lexer):
        tokens = { NAME, NUMBER, PLUS, TIMES, MINUS, DIVIDE, ASSIGN, LPAREN, RPAREN }
        ignore = ' \t'

        # Tokens
        NAME = r'[a-zA-Z_][a-zA-Z0-9_]*'
        NUMBER = r'\d+'

        # Special symbols
        PLUS = r'\+'
        MINUS = r'-'
        TIMES = r'\*'
        DIVIDE = r'/'
        ASSIGN = r'='
        LPAREN = r'\('
        RPAREN = r'\)'

        # Ignored pattern
        ignore_newline = r'\n+'

        # Extra action for newlines
        def ignore_newline(self, t):
            self.lineno += t.value.count('\n')

        def error(self, t):
            print("Illegal character '%s'" % t.value[0])
            self.index += 1

    class CalcParser(Parser):
        tokens = CalcLexer.tokens

        precedence = (
            ('left', PLUS, MINUS),
            ('left', TIMES, DIVIDE),
            ('right', UMINUS),
            )

        def __init__(self):
            self.names = { }

        @_('NAME ASSIGN expr')
        def statement(self, p):
            self.names[p.NAME] = p.expr

        @_('expr')
        def statement(self, p):
            print(p.expr)

        @_('expr PLUS expr')
        def expr(self, p):
            return p.expr0 + p.expr1

        @_('expr MINUS expr')
        def expr(self, p):
            return p.expr0 - p.expr1

        @_('expr TIMES expr')
        def expr(self, p):
            return p.expr0 * p.expr1

        @_('expr DIVIDE expr')
        def expr(self, p):
            return p.expr0 / p.expr1

        @_('MINUS expr %prec UMINUS')
        def expr(self, p):
            return -p.expr

        @_('LPAREN expr RPAREN')
        def expr(self, p):
            return p.expr

        @_('NUMBER')
        def expr(self, p):
            return int(p.NUMBER)

        @_('NAME')
        def expr(self, p):
            try:
                return self.names[p.NAME]
            except LookupError:
                print(f'Undefined name {p.NAME!r}')
                return 0

    if __name__ == '__main__':
        lexer = CalcLexer()
        parser = CalcParser()
        while True:
            try:
                text = input('calc > ')
            except EOFError:
                break
            if text:
                parser.parse(lexer.tokenize(text))

Documentation
-------------

Further documentation can be found at `https://sly.readthedocs.io/en/latest <https://sly.readthedocs.io/en/latest>`_.

Talks
-----

* `Reinventing the Parser Generator <https://www.youtube.com/watch?v=zJ9z6Ge-vXs>`_, talk by David Beazley at PyCon 2018, Cleveland.

Resources
---------

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.

The GitHub page for SLY can be found at:

     ``https://github.com/dabeaz/sly``

Please direct bug reports and pull requests to the GitHub page.
To contact me directly, send email to dave@dabeaz.com or contact
me on Twitter (@dabeaz).
 
-- Dave

P.S.
----

You should come take a `course <https://www.dabeaz.com/courses.html>`_!