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<html>
<head>
<title>PLY Internals</title>
</head>
<body bgcolor="#ffffff">
<h1>PLY Internals</h1>
<b>
David M. Beazley <br>
dave@dabeaz.com<br>
</b>
<p>
<b>PLY Version: 3.0</b>
<p>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#internal_nn1">Introduction</a>
<li><a href="#internal_nn2">Grammar Class</a>
<li><a href="#internal_nn3">Productions</a>
<li><a href="#internal_nn4">LRItems</a>
<li><a href="#internal_nn5">LRTable</a>
<li><a href="#internal_nn6">LRGeneratedTable</a>
<li><a href="#internal_nn7">LRParser</a>
<li><a href="#internal_nn8">ParserReflect</a>
<li><a href="#internal_nn9">High-level operation</a>
</ul>
</div>
<!-- INDEX -->
<H2><a name="internal_nn1"></a>1. Introduction</H2>
This document describes classes and functions that make up the internal
operation of PLY. Using this programming interface, it is possible to
manually build an parser using a different interface specification
than what PLY normally uses. For example, you could build a gramar
from information parsed in a completely different input format. Some of
these objects may be useful for building more advanced parsing engines
such as GLR.
<p>
It should be stressed that using PLY at this level is not for the
faint of heart. Generally, it's assumed that you know a bit of
the underlying compiler theory and how an LR parser is put together.
<H2><a name="internal_nn2"></a>2. Grammar Class</H2>
The file <tt>ply.yacc</tt> defines a class <tt>Grammar</tt> that
is used to hold and manipulate information about a grammar
specification. It encapsulates the same basic information
about a grammar that is put into a YACC file including
the list of tokens, precedence rules, and grammar rules.
Various operations are provided to perform different validations
on the grammar. In addition, there are operations to compute
the first and follow sets that are needed by the various table
generation algorithms.
<p>
<tt><b>Grammar(terminals)</b></tt>
<blockquote>
Creates a new grammar object. <tt>terminals</tt> is a list of strings
specifying the terminals for the grammar. An instance <tt>g</tt> of
<tt>Grammar</tt> has the following methods:
</blockquote>
<p>
<b><tt>g.set_precedence(term,assoc,level)</tt></b>
<blockquote>
Sets the precedence level and associativity for a given terminal <tt>term</tt>.
<tt>assoc</tt> is one of <tt>'right'</tt>,
<tt>'left'</tt>, or <tt>'nonassoc'</tt> and <tt>level</tt> is a positive integer. The higher
the value of <tt>level</tt>, the higher the precedence. Here is an example of typical
precedence settings:
<pre>
g.set_precedence('PLUS', 'left',1)
g.set_precedence('MINUS', 'left',1)
g.set_precedence('TIMES', 'left',2)
g.set_precedence('DIVIDE','left',2)
g.set_precedence('UMINUS','left',3)
</pre>
This method must be called prior to adding any productions to the
grammar with <tt>g.add_production()</tt>. The precedence of individual grammar
rules is determined by the precedence of the right-most terminal.
</blockquote>
<p>
<b><tt>g.add_production(name,syms,func=None,file='',line=0)</tt></b>
<blockquote>
Adds a new grammar rule. <tt>name</tt> is the name of the rule,
<tt>syms</tt> is a list of symbols making up the right hand
side of the rule, <tt>func</tt> is the function to call when
reducing the rule. <tt>file</tt> and <tt>line</tt> specify
the filename and line number of the rule and are used for
generating error messages.
<p>
The list of symbols in <tt>syms</tt> may include character
literals and <tt>%prec</tt> specifiers. Here are some
examples:
<pre>
g.add_production('expr',['expr','PLUS','term'],func,file,line)
g.add_production('expr',['expr','"+"','term'],func,file,line)
g.add_production('expr',['MINUS','expr','%prec','UMINUS'],func,file,line)
</pre>
<p>
If any kind of error is detected, a <tt>GrammarError</tt> exception
is raised with a message indicating the reason for the failure.
</blockquote>
<p>
<b><tt>g.set_start(start=None)</tt></b>
<blockquote>
Sets the starting rule for the grammar. <tt>start</tt> is a string
specifying the name of the start rule. If <tt>start</tt> is omitted,
the first grammar rule added with <tt>add_production()</tt> is taken to be
the starting rule. This method must always be called after all
productions have been added.
</blockquote>
<p>
<b><tt>g.find_unreachable()</tt></b>
<blockquote>
Diagnostic function. Returns a list of all unreachable non-terminals
defined in the grammar. This is used to identify inactive parts of
the grammar specification.
</blockquote>
<p>
<b><tt>g.infinite_cycle()</tt></b>
<blockquote>
Diagnostic function. Returns a list of all non-terminals in the
grammar that result in an infinite cycle. This condition occurs if
there is no way for a grammar rule to expand to a string containing
only terminal symbols.
</blockquote>
<p>
<b><tt>g.undefined_symbols()</tt></b>
<blockquote>
Diagnostic function. Returns a list of tuples <tt>(name, prod)</tt>
corresponding to undefined symbols in the grammar. <tt>name</tt> is the
name of the undefined symbol and <tt>prod</tt> is an instance of
<tt>Production</tt> which has information about the production rule
where the undefined symbol was used.
</blockquote>
<p>
<b><tt>g.unused_terminals()</tt></b>
<blockquote>
Diagnostic function. Returns a list of terminals that were defined,
but never used in the grammar.
</blockquote>
<p>
<b><tt>g.unused_rules()</tt></b>
<blockquote>
Diagnostic function. Returns a list of <tt>Production</tt> instances
corresponding to production rules that were defined in the grammar,
but never used anywhere. This is slightly different
than <tt>find_unreachable()</tt>.
</blockquote>
<p>
<b><tt>g.unused_precedence()</tt></b>
<blockquote>
Diagnostic function. Returns a list of tuples <tt>(term, assoc)</tt>
corresponding to precedence rules that were set, but never used the
grammar. <tt>term</tt> is the terminal name and <tt>assoc</tt> is the
precedence associativity (e.g., <tt>'left'</tt>, <tt>'right'</tt>,
or <tt>'nonassoc'</tt>.
</blockquote>
<p>
<b><tt>g.compute_first()</tt></b>
<blockquote>
Compute all of the first sets for all symbols in the grammar. Returns a dictionary
mapping symbol names to a list of all first symbols.
</blockquote>
<p>
<b><tt>g.compute_follow()</tt></b>
<blockquote>
Compute all of the follow sets for all non-terminals in the grammar.
The follow set is the set of all possible symbols that might follow a
given non-terminal. Returns a dictionary mapping non-terminal names
to a list of symbols.
</blockquote>
<p>
<b><tt>g.build_lritems()</tt></b>
<blockquote>
Calculates all of the LR items for all productions in the grammar. This
step is required before using the grammar for any kind of table generation.
See the section on LR items below.
</blockquote>
<p>
The following attributes are set by the above methods and may be useful
in code that works with the grammar. All of these attributes should be
assumed to be read-only. Changing their values directly will likely
break the grammar.
<p>
<b><tt>g.Productions</tt></b>
<blockquote>
A list of all productions added. The first entry is reserved for
a production representing the starting rule. The objects in this list
are instances of the <tt>Production</tt> class, described shortly.
</blockquote>
<p>
<b><tt>g.Prodnames</tt></b>
<blockquote>
A dictionary mapping the names of nonterminals to a list of all
productions of that nonterminal.
</blockquote>
<p>
<b><tt>g.Terminals</tt></b>
<blockquote>
A dictionary mapping the names of terminals to a list of the
production numbers where they are used.
</blockquote>
<p>
<b><tt>g.Nonterminals</tt></b>
<blockquote>
A dictionary mapping the names of nonterminals to a list of the
production numbers where they are used.
</blockquote>
<p>
<b><tt>g.First</tt></b>
<blockquote>
A dictionary representing the first sets for all grammar symbols. This is
computed and returned by the <
|
