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12632Sstever@eecs.umich.edu<html> 22632Sstever@eecs.umich.edu<head> 32632Sstever@eecs.umich.edu<title>PLY (Python Lex-Yacc)</title> 42632Sstever@eecs.umich.edu</head> 52632Sstever@eecs.umich.edu<body bgcolor="#ffffff"> 62632Sstever@eecs.umich.edu 72632Sstever@eecs.umich.edu<h1>PLY (Python Lex-Yacc)</h1> 84479Sbinkertn@umich.edu 92632Sstever@eecs.umich.edu<b> 102632Sstever@eecs.umich.eduDavid M. Beazley <br> 114479Sbinkertn@umich.eduDepartment of Computer Science <br> 122632Sstever@eecs.umich.eduUniversity of Chicago <br> 132632Sstever@eecs.umich.eduChicago, IL 60637 <br> 142632Sstever@eecs.umich.edubeazley@cs.uchicago.edu <br> 154479Sbinkertn@umich.edu</b> 162632Sstever@eecs.umich.edu 174479Sbinkertn@umich.edu<p> 184479Sbinkertn@umich.eduDocumentation version: $Header: /home/stever/bk/newmem2/ext/ply/doc/ply.html 1.1 03/06/06 14:53:34-00:00 stever@ $ 194479Sbinkertn@umich.edu 204479Sbinkertn@umich.edu<h2>Introduction</h2> 214479Sbinkertn@umich.edu 224479Sbinkertn@umich.eduPLY is a Python-only implementation of the popular compiler 234479Sbinkertn@umich.educonstruction tools lex and yacc. The implementation borrows ideas 244479Sbinkertn@umich.edufrom a number of previous efforts; most notably John Aycock's SPARK 254479Sbinkertn@umich.edutoolkit. However, the overall flavor of the implementation is more 264479Sbinkertn@umich.educlosely modeled after the C version of lex and yacc. The other 274479Sbinkertn@umich.edusignificant feature of PLY is that it provides extensive input 284479Sbinkertn@umich.eduvalidation and error reporting--much more so than other Python parsing 294479Sbinkertn@umich.edutools. 304479Sbinkertn@umich.edu 314479Sbinkertn@umich.edu<p> 324479Sbinkertn@umich.eduEarly versions of PLY were developed to support the Introduction to 334479Sbinkertn@umich.eduCompilers Course at the University of Chicago. In this course, 344479Sbinkertn@umich.edustudents built a fully functional compiler for a simple Pascal-like 354479Sbinkertn@umich.edulanguage. Their compiler, implemented entirely in Python, had to 364479Sbinkertn@umich.eduinclude lexical analysis, parsing, type checking, type inference, 374479Sbinkertn@umich.edunested scoping, and code generation for the SPARC processor. 384479Sbinkertn@umich.eduApproximately 30 different compiler implementations were completed in 394479Sbinkertn@umich.eduthis course. Most of PLY's interface and operation has been motivated by common 404479Sbinkertn@umich.eduusability problems encountered by students. 414479Sbinkertn@umich.edu 424479Sbinkertn@umich.edu<p> 434479Sbinkertn@umich.eduBecause PLY was primarily developed as an instructional tool, you will 444479Sbinkertn@umich.edufind it to be <em>MUCH</em> more picky about token and grammar rule 454479Sbinkertn@umich.eduspecification than most other Python parsing tools. In part, this 464479Sbinkertn@umich.eduadded formality is meant to catch common programming mistakes made by 474479Sbinkertn@umich.edunovice users. However, advanced users will also find such features to 484479Sbinkertn@umich.edube useful when building complicated grammars for real programming 494479Sbinkertn@umich.edulanguages. It should also be noted that PLY does not provide much in the way 504479Sbinkertn@umich.eduof bells and whistles (e.g., automatic construction of abstract syntax trees, 514479Sbinkertn@umich.edutree traversal, etc.). Instead, you will find a bare-bones, yet 524479Sbinkertn@umich.edufully capable lex/yacc implementation written entirely in Python. 534479Sbinkertn@umich.edu 544479Sbinkertn@umich.edu<p> 554479Sbinkertn@umich.eduThe rest of this document assumes that you are somewhat familar with 564479Sbinkertn@umich.eduparsing theory, syntax directed translation, and automatic tools such 574479Sbinkertn@umich.eduas lex and yacc. If you are unfamilar with these topics, you will 584479Sbinkertn@umich.eduprobably want to consult an introductory text such as "Compilers: 594479Sbinkertn@umich.eduPrinciples, Techniques, and Tools", by Aho, Sethi, and Ullman. "Lex 604479Sbinkertn@umich.eduand Yacc" by John Levine may also be handy. 614479Sbinkertn@umich.edu 624479Sbinkertn@umich.edu<h2>PLY Overview</h2> 634479Sbinkertn@umich.edu 644479Sbinkertn@umich.eduPLY consists of two separate tools; <tt>lex.py</tt> and 654479Sbinkertn@umich.edu<tt>yacc.py</tt>. <tt>lex.py</tt> is used to break input text into a 664479Sbinkertn@umich.educollection of tokens specified by a collection of regular expression 674479Sbinkertn@umich.edurules. <tt>yacc.py</tt> is used to recognize language syntax that has 684479Sbinkertn@umich.edubeen specified in the form of a context free grammar. Currently, 694479Sbinkertn@umich.edu<tt>yacc.py</tt> uses LR parsing and generates its parsing tables 704479Sbinkertn@umich.eduusing the SLR algorithm. LALR(1) parsing may be supported in a future 714479Sbinkertn@umich.edurelease. 724479Sbinkertn@umich.edu 734479Sbinkertn@umich.edu<p> 744479Sbinkertn@umich.eduThe two tools are meant to work together. Specifically, 754479Sbinkertn@umich.edu<tt>lex.py</tt> provides an external interface in the form of a 764479Sbinkertn@umich.edu<tt>token()</tt> function that returns the next valid token on the 774479Sbinkertn@umich.eduinput stream. <tt>yacc.py</tt> calls this repeatedly to retrieve 784479Sbinkertn@umich.edutokens and invoke grammar rules. The output of <tt>yacc.py</tt> is 794479Sbinkertn@umich.eduoften an Abstract Syntax Tree (AST). However, this is entirely up to 804479Sbinkertn@umich.eduthe user. If desired, <tt>yacc.py</tt> can also be used to implement 814479Sbinkertn@umich.edusimple one-pass compilers. 824479Sbinkertn@umich.edu 834479Sbinkertn@umich.edu<p> 844479Sbinkertn@umich.eduLike its Unix counterpart, <tt>yacc.py</tt> provides most of the 854479Sbinkertn@umich.edufeatures you expect including extensive error checking, grammar 864479Sbinkertn@umich.eduvalidation, support for empty productions, error tokens, and ambiguity 874479Sbinkertn@umich.eduresolution via precedence rules. The primary difference between 884479Sbinkertn@umich.edu<tt>yacc.py</tt> and <tt>yacc</tt> is the use of SLR parsing instead 894479Sbinkertn@umich.eduof LALR(1). Although this slightly restricts the types of grammars 904479Sbinkertn@umich.eduthan can be successfully parsed, it is sufficiently powerful to handle most 914479Sbinkertn@umich.edukinds of normal programming language constructs. 922632Sstever@eecs.umich.edu 932632Sstever@eecs.umich.edu<p> 942632Sstever@eecs.umich.eduFinally, it is important to note that PLY relies on reflection 952632Sstever@eecs.umich.edu(introspection) to build its lexers and parsers. Unlike traditional 962632Sstever@eecs.umich.edulex/yacc which require a special input file that is converted into a 974479Sbinkertn@umich.eduseparate source file, the specifications given to PLY <em>are</em> 982632Sstever@eecs.umich.eduvalid Python programs. This means that there are no extra source 992632Sstever@eecs.umich.edufiles nor is there a special compiler construction step (e.g., running 1002632Sstever@eecs.umich.eduyacc to generate Python code for the compiler). 1014479Sbinkertn@umich.edu 1024479Sbinkertn@umich.edu<h2>Lex Example</h2> 1034479Sbinkertn@umich.edu 1042632Sstever@eecs.umich.edu<tt>lex.py</tt> is used to write tokenizers. To do this, each token 1052632Sstever@eecs.umich.edumust be defined by a regular expression rule. The following file 1062632Sstever@eecs.umich.eduimplements a very simple lexer for tokenizing simple integer expressions: 1074479Sbinkertn@umich.edu 1084479Sbinkertn@umich.edu<blockquote> 1094479Sbinkertn@umich.edu<pre> 1104479Sbinkertn@umich.edu# ------------------------------------------------------------ 1112632Sstever@eecs.umich.edu# calclex.py 1122632Sstever@eecs.umich.edu# 1132632Sstever@eecs.umich.edu# tokenizer for a simple expression evaluator for 1142632Sstever@eecs.umich.edu# numbers and +,-,*,/ 1154479Sbinkertn@umich.edu# ------------------------------------------------------------ 1164479Sbinkertn@umich.eduimport lex 1174479Sbinkertn@umich.edu 1184479Sbinkertn@umich.edu# List of token names. This is always required 1194479Sbinkertn@umich.edutokens = ( 1204479Sbinkertn@umich.edu 'NUMBER', 1214479Sbinkertn@umich.edu 'PLUS', 1224479Sbinkertn@umich.edu 'MINUS', 1234479Sbinkertn@umich.edu 'TIMES', 1244479Sbinkertn@umich.edu 'DIVIDE', 1254479Sbinkertn@umich.edu 'LPAREN', 1264479Sbinkertn@umich.edu 'RPAREN', 1274479Sbinkertn@umich.edu) 1284479Sbinkertn@umich.edu 1292632Sstever@eecs.umich.edu# Regular expression rules for simple tokens 1302632Sstever@eecs.umich.edut_PLUS = r'\+' 1314479Sbinkertn@umich.edut_MINUS = r'-' 1324479Sbinkertn@umich.edut_TIMES = r'\*' 1332632Sstever@eecs.umich.edut_DIVIDE = r'/' 1342632Sstever@eecs.umich.edut_LPAREN = r'\(' 1352632Sstever@eecs.umich.edut_RPAREN = r'\)' 1362632Sstever@eecs.umich.edu 1372632Sstever@eecs.umich.edu# A regular expression rule with some action code 1382632Sstever@eecs.umich.edudef t_NUMBER(t): 1392632Sstever@eecs.umich.edu r'\d+' 1402632Sstever@eecs.umich.edu try: 1412632Sstever@eecs.umich.edu t.value = int(t.value) 1424479Sbinkertn@umich.edu except ValueError: 1432632Sstever@eecs.umich.edu print "Line %d: Number %s is too large!" % (t.lineno,t.value) 1442632Sstever@eecs.umich.edu t.value = 0 1452632Sstever@eecs.umich.edu return t 1462632Sstever@eecs.umich.edu 1472632Sstever@eecs.umich.edu# Define a rule so we can track line numbers 1484479Sbinkertn@umich.edudef t_newline(t): 1494479Sbinkertn@umich.edu r'\n+' 1502632Sstever@eecs.umich.edu t.lineno += len(t.value) 1512632Sstever@eecs.umich.edu 1524479Sbinkertn@umich.edu# A string containing ignored characters (spaces and tabs) 1534479Sbinkertn@umich.edut_ignore = ' \t' 1544479Sbinkertn@umich.edu 1554479Sbinkertn@umich.edu# Error handling rule 1564479Sbinkertn@umich.edudef t_error(t): 1574479Sbinkertn@umich.edu print "Illegal character '%s'" % t.value[0] 1584479Sbinkertn@umich.edu t.skip(1) 1594479Sbinkertn@umich.edu 1604479Sbinkertn@umich.edu# Build the lexer 1614479Sbinkertn@umich.edulex.lex() 1624479Sbinkertn@umich.edu 1634479Sbinkertn@umich.edu# Test it out 1644479Sbinkertn@umich.edudata = ''' 1654479Sbinkertn@umich.edu3 + 4 * 10 1664479Sbinkertn@umich.edu + -20 *2 1674479Sbinkertn@umich.edu''' 1684479Sbinkertn@umich.edu 1694479Sbinkertn@umich.edu# Give the lexer some input 1704479Sbinkertn@umich.edulex.input(data) 1714479Sbinkertn@umich.edu 1724479Sbinkertn@umich.edu# Tokenize 1734479Sbinkertn@umich.eduwhile 1: 1744479Sbinkertn@umich.edu tok = lex.token() 1754479Sbinkertn@umich.edu if not tok: break # No more input 1764479Sbinkertn@umich.edu print tok 1774479Sbinkertn@umich.edu</pre> 1784479Sbinkertn@umich.edu</blockquote> 1794479Sbinkertn@umich.edu 1804479Sbinkertn@umich.eduIn the example, the <tt>tokens</tt> list defines all of the possible 1814479Sbinkertn@umich.edutoken names that can be produced by the lexer. This list is always required 1824479Sbinkertn@umich.eduand is used to perform a variety of validation checks. Following the <tt>tokens</tt> 1834479Sbinkertn@umich.edulist, regular expressions are written for each token. Each of these 1844479Sbinkertn@umich.edurules are defined by making declarations with a special prefix <tt>t_</tt> to indicate that it 1854479Sbinkertn@umich.edudefines a token. For simple tokens, the regular expression can 1864479Sbinkertn@umich.edube specified as strings such as this (note: Python raw strings are used since they are the 1874479Sbinkertn@umich.edumost convenient way to write regular expression strings): 1884479Sbinkertn@umich.edu 1894479Sbinkertn@umich.edu<blockquote> 1904479Sbinkertn@umich.edu<pre> 1914479Sbinkertn@umich.edut_PLUS = r'\+' 1924479Sbinkertn@umich.edu</pre> 1934479Sbinkertn@umich.edu</blockquote> 1944479Sbinkertn@umich.edu 1954479Sbinkertn@umich.eduIn this case, the name following the <tt>t_</tt> must exactly match one of the 1964479Sbinkertn@umich.edunames supplied in <tt>tokens</tt>. If some kind of action needs to be performed, 1974479Sbinkertn@umich.edua token rule can be specified as a function. For example: 1984479Sbinkertn@umich.edu 1994479Sbinkertn@umich.edu<blockquote> 2004479Sbinkertn@umich.edu<pre> 2014479Sbinkertn@umich.edudef t_NUMBER(t): 2024479Sbinkertn@umich.edu r'\d+' 2034479Sbinkertn@umich.edu try: 2044479Sbinkertn@umich.edu t.value = int(t.value) 2054479Sbinkertn@umich.edu except ValueError: 2064479Sbinkertn@umich.edu print "Number %s is too large!" % t.value 2074479Sbinkertn@umich.edu t.value = 0 2084479Sbinkertn@umich.edu return t 2094479Sbinkertn@umich.edu</pre> 2104479Sbinkertn@umich.edu</blockquote> 2114479Sbinkertn@umich.edu 2124479Sbinkertn@umich.eduIn this case, the regular expression rule is specified in the function documentation string. 2132632Sstever@eecs.umich.eduThe function always takes a single argument which is an instance of 2142632Sstever@eecs.umich.edu<tt>LexToken</tt>. This object has attributes of <tt>t.type</tt> which is the token type, 2152632Sstever@eecs.umich.edu<tt>t.value</tt> which is the lexeme, and <tt>t.lineno</tt> which is the current line number. 2162632Sstever@eecs.umich.eduBy default, <tt>t.type</tt> is set to the name following the <tt>t_</tt> prefix. The action 2172632Sstever@eecs.umich.edufunction can modify the contents of the <tt>LexToken</tt> object as appropriate. However, 2182632Sstever@eecs.umich.eduwhen it is done, the resulting token should be returned. If no value is returned by the action 2192632Sstever@eecs.umich.edufunction, the token is simply discarded and the next token read. 2202632Sstever@eecs.umich.edu 2212632Sstever@eecs.umich.edu<p> 2224479Sbinkertn@umich.eduThe rule <tt>t_newline()</tt> illustrates a regular expression rule 2232632Sstever@eecs.umich.edufor a discarded token. In this case, a rule is written to match 2242632Sstever@eecs.umich.edunewlines so that proper line number tracking can be performed. 2252632Sstever@eecs.umich.eduBy returning no value, the function causes the newline character to be 2262632Sstever@eecs.umich.edudiscarded. 2272632Sstever@eecs.umich.edu 2282632Sstever@eecs.umich.edu<p> 2292632Sstever@eecs.umich.eduThe special <tt>t_ignore</tt> rule is reserved by <tt>lex.py</tt> for characters 2302632Sstever@eecs.umich.eduthat should be completely ignored in the input stream. 2312632Sstever@eecs.umich.eduUsually this is used to skip over whitespace and other non-essential characters. 2322632Sstever@eecs.umich.eduAlthough it is possible to define a regular expression rule for whitespace in a manner 2332632Sstever@eecs.umich.edusimilar to <tt>t_newline()</tt>, the use of <tt>t_ignore</tt> provides substantially better 2342632Sstever@eecs.umich.edulexing performance because it is handled as a special case and is checked in a much 2352632Sstever@eecs.umich.edumore efficient manner than the normal regular expression rules. 2362632Sstever@eecs.umich.edu 2372632Sstever@eecs.umich.edu<p> 2382632Sstever@eecs.umich.eduFinally, the <tt>t_error()</tt> 2392632Sstever@eecs.umich.edufunction is used to handle lexing errors that occur when illegal 2402632Sstever@eecs.umich.educharacters are detected. In this case, the <tt>t.value</tt> attribute contains the 2412632Sstever@eecs.umich.edurest of the input string that has not been tokenized. In the example, we simply print 2422632Sstever@eecs.umich.eduthe offending character and skip ahead one character by calling <tt>t.skip(1)</tt>. 2432632Sstever@eecs.umich.edu 2442632Sstever@eecs.umich.edu<p> 2452632Sstever@eecs.umich.eduTo build the lexer, the function <tt>lex.lex()</tt> is used. This function 2462632Sstever@eecs.umich.eduuses Python reflection (or introspection) to read the the regular expression rules 2472632Sstever@eecs.umich.eduout of the calling context and build the lexer. Once the lexer has been built, two functions can 2482632Sstever@eecs.umich.edube used to control the lexer. 2492632Sstever@eecs.umich.edu 2502632Sstever@eecs.umich.edu<ul> 2512632Sstever@eecs.umich.edu<li><tt>lex.input(data)</tt>. Reset the lexer and store a new input string. 2522632Sstever@eecs.umich.edu<li><tt>lex.token()</tt>. Return the next token. Returns a special <tt>LexToken</tt> instance on success or 2532632Sstever@eecs.umich.eduNone if the end of the input text has been reached. 2542632Sstever@eecs.umich.edu</ul> 2552632Sstever@eecs.umich.edu 2564479Sbinkertn@umich.eduThe code at the bottom of the example shows how the lexer is actually used. When executed, 2572632Sstever@eecs.umich.eduthe following output will be produced: 2582632Sstever@eecs.umich.edu 2592632Sstever@eecs.umich.edu<blockquote> 2602632Sstever@eecs.umich.edu<pre> 2612632Sstever@eecs.umich.edu$ python example.py 2622632Sstever@eecs.umich.eduLexToken(NUMBER,3,2) 2632632Sstever@eecs.umich.eduLexToken(PLUS,'+',2) 2644479Sbinkertn@umich.eduLexToken(NUMBER,4,2) 2652632Sstever@eecs.umich.eduLexToken(TIMES,'*',2) 2662632Sstever@eecs.umich.eduLexToken(NUMBER,10,2) 2672632Sstever@eecs.umich.eduLexToken(PLUS,'+',3) 2682632Sstever@eecs.umich.eduLexToken(MINUS,'-',3) 2694479Sbinkertn@umich.eduLexToken(NUMBER,20,3) 2704479Sbinkertn@umich.eduLexToken(TIMES,'*',3) 2714479Sbinkertn@umich.eduLexToken(NUMBER,2,3) 2724479Sbinkertn@umich.edu</pre> 2734479Sbinkertn@umich.edu</blockquote> 2744479Sbinkertn@umich.edu 2754479Sbinkertn@umich.edu<h2>Lex Implementation Notes</h2> 2762632Sstever@eecs.umich.edu 2772632Sstever@eecs.umich.edu<ul> 2782632Sstever@eecs.umich.edu<li><tt>lex.py</tt> uses the <tt>re</tt> module to do its patten matching. When building the master regular expression, 2792632Sstever@eecs.umich.edurules are added in the following order: 2802632Sstever@eecs.umich.edu<p> 2812632Sstever@eecs.umich.edu<ol> 2822632Sstever@eecs.umich.edu<li>All tokens defined by functions are added in the same order as they appear in the lexer file. 2832632Sstever@eecs.umich.edu<li>Tokens defined by strings are added by sorting them in order of decreasing regular expression length (longer expressions 2842632Sstever@eecs.umich.eduare added first). 2852632Sstever@eecs.umich.edu</ol> 2862632Sstever@eecs.umich.edu<p> 2872632Sstever@eecs.umich.eduWithout this ordering, it can be difficult to correctly match certain types of tokens. For example, if you 2882632Sstever@eecs.umich.eduwanted to have separate tokens for "=" and "==", you need to make sure that "==" is checked first. By sorting regular 2892632Sstever@eecs.umich.eduexpressions in order of decreasing length, this problem is solved for rules defined as strings. For functions, 2902632Sstever@eecs.umich.eduthe order can be explicitly controlled since rules appearing first are checked first. 2912632Sstever@eecs.umich.edu 2922632Sstever@eecs.umich.edu<P> 2934479Sbinkertn@umich.edu<li>The lexer requires input to be supplied as a single input string. Since most machines have more than enough memory, this 2944479Sbinkertn@umich.edurarely presents a performance concern. However, it means that the lexer currently can't be used with streaming data 2954479Sbinkertn@umich.edusuch as open files or sockets. This limitation is primarily a side-effect of using the <tt>re</tt> module. 2964479Sbinkertn@umich.edu 2974479Sbinkertn@umich.edu<p> 2984479Sbinkertn@umich.edu<li> 2994479Sbinkertn@umich.eduTo handle reserved words, it is usually easier to just match an identifier and do a special name lookup in a function 3004479Sbinkertn@umich.edulike this: 3014479Sbinkertn@umich.edu 3024479Sbinkertn@umich.edu<blockquote> 3034479Sbinkertn@umich.edu<pre> 3044479Sbinkertn@umich.edureserved = { 3054479Sbinkertn@umich.edu 'if' : 'IF', 3064479Sbinkertn@umich.edu 'then' : 'THEN', 3074479Sbinkertn@umich.edu 'else' : 'ELSE', 3084479Sbinkertn@umich.edu 'while' : 'WHILE', 3094479Sbinkertn@umich.edu ... 3104479Sbinkertn@umich.edu} 3114479Sbinkertn@umich.edu 3124479Sbinkertn@umich.edudef t_ID(t): 3134479Sbinkertn@umich.edu r'[a-zA-Z_][a-zA-Z_0-9]*' 3144479Sbinkertn@umich.edu t.type = reserved.get(t.value,'ID') # Check for reserved words 3154479Sbinkertn@umich.edu return t 3164479Sbinkertn@umich.edu</pre> 3174479Sbinkertn@umich.edu</blockquote> 3184479Sbinkertn@umich.edu 3194479Sbinkertn@umich.edu<p> 3204479Sbinkertn@umich.edu<li>The lexer requires tokens to be defined as class instances with <tt>t.type</tt>, <tt>t.value</tt>, and <tt>t.lineno</tt> 3214479Sbinkertn@umich.eduattributes. By default, tokens are created as instances of the <tt>LexToken</tt> class defined internally to <tt>lex.py</tt>. 3224479Sbinkertn@umich.eduIf desired, you can create new kinds of tokens provided that they have the three required attributes. However, 3234479Sbinkertn@umich.eduin practice, it is probably safer to stick with the default. 3244479Sbinkertn@umich.edu 3254479Sbinkertn@umich.edu<p> 3264479Sbinkertn@umich.edu<li>The only safe attribute for assigning token properties is <tt>t.value</tt>. In some cases, you may want to attach 3274479Sbinkertn@umich.edua number of different properties to a token (e.g., symbol table entries for identifiers). To do this, replace <tt>t.value</tt> 3284479Sbinkertn@umich.eduwith a tuple or class instance. For example: 3294479Sbinkertn@umich.edu 3304479Sbinkertn@umich.edu<blockquote> 3314479Sbinkertn@umich.edu<pre> 3324479Sbinkertn@umich.edudef t_ID(t): 3334479Sbinkertn@umich.edu ... 3344479Sbinkertn@umich.edu # For identifiers, create a (lexeme, symtab) tuple 3354479Sbinkertn@umich.edu t.value = (t.value, symbol_lookup(t.value)) 3364479Sbinkertn@umich.edu ... 3374479Sbinkertn@umich.edu return t 3384479Sbinkertn@umich.edu</pre> 3394479Sbinkertn@umich.edu</blockquote> 3404479Sbinkertn@umich.edu 3414479Sbinkertn@umich.eduAlthough allowed, do NOT assign additional attributes to the token object. For example, 3424479Sbinkertn@umich.edu<blockquote> 3434479Sbinkertn@umich.edu<pre> 3444479Sbinkertn@umich.edudef t_ID(t): 3454479Sbinkertn@umich.edu ... 3464479Sbinkertn@umich.edu # Bad implementation of above 3474479Sbinkertn@umich.edu t.symtab = symbol_lookup(t.value) 3484479Sbinkertn@umich.edu ... 3494479Sbinkertn@umich.edu</pre> 3504479Sbinkertn@umich.edu</blockquote> 3514479Sbinkertn@umich.edu 3524479Sbinkertn@umich.eduThe reason you don't want to do this is that the <tt>yacc.py</tt> 3534479Sbinkertn@umich.edumodule only provides public access to the <tt>t.value</tt> attribute of each token. 3544479Sbinkertn@umich.eduTherefore, any other attributes you assign are inaccessible (if you are familiar 3554479Sbinkertn@umich.eduwith the internals of C lex/yacc, <tt>t.value</tt> is the same as <tt>yylval.tok</tt>). 3564479Sbinkertn@umich.edu 3574479Sbinkertn@umich.edu<p> 3584479Sbinkertn@umich.edu<li>To track line numbers, the lexer internally maintains a line 3594479Sbinkertn@umich.edunumber variable. Each token automatically gets the value of the 3604479Sbinkertn@umich.educurrent line number in the <tt>t.lineno</tt> attribute. To modify the 3614479Sbinkertn@umich.educurrent line number, simply change the <tt>t.lineno</tt> attribute 3624479Sbinkertn@umich.eduin a function rule (as previously shown for 3632632Sstever@eecs.umich.edu<tt>t_newline()</tt>). Even if the resulting token is discarded, 3642632Sstever@eecs.umich.educhanges to the line number remain in effect for subsequent tokens. 3652632Sstever@eecs.umich.edu 3662632Sstever@eecs.umich.edu<p> 3672632Sstever@eecs.umich.edu<li>To support multiple scanners in the same application, the <tt>lex.lex()</tt> function 3682632Sstever@eecs.umich.eduactually returns a special <tt>Lexer</tt> object. This object has two methods 3692632Sstever@eecs.umich.edu<tt>input()</tt> and <tt>token()</tt> that can be used to supply input and get tokens. For example: 3702632Sstever@eecs.umich.edu 3712632Sstever@eecs.umich.edu<blockquote> 3722632Sstever@eecs.umich.edu<pre> 3732632Sstever@eecs.umich.edulexer = lex.lex() 3742632Sstever@eecs.umich.edulexer.input(sometext) 3754479Sbinkertn@umich.eduwhile 1: 3764479Sbinkertn@umich.edu tok = lexer.token() 3772632Sstever@eecs.umich.edu if not tok: break 3782632Sstever@eecs.umich.edu print tok 3792632Sstever@eecs.umich.edu</pre> 3802632Sstever@eecs.umich.edu</blockquote> 3812632Sstever@eecs.umich.edu 3822632Sstever@eecs.umich.eduThe functions <tt>lex.input()</tt> and <tt>lex.token()</tt> are bound to the <tt>input()</tt> 3832632Sstever@eecs.umich.eduand <tt>token()</tt> methods of the last lexer created by the lex module. 3842632Sstever@eecs.umich.edu 3852632Sstever@eecs.umich.edu 3862632Sstever@eecs.umich.edu<p> 3872632Sstever@eecs.umich.edu<li>To reduce compiler startup time and improve performance, the lexer can be built in optimized mode as follows: 3882632Sstever@eecs.umich.edu 3892632Sstever@eecs.umich.edu<blockquote> 3902632Sstever@eecs.umich.edu<pre> 3914479Sbinkertn@umich.edulex.lex(optimize=1) 3922632Sstever@eecs.umich.edu</pre> 3934479Sbinkertn@umich.edu</blockquote> 3944479Sbinkertn@umich.edu 3954479Sbinkertn@umich.eduWhen used, most error checking and validation is disabled. This provides a slight performance 3962632Sstever@eecs.umich.edugain while tokenizing and tends to chop a few tenths of a second off startup time. Since it disables 3972632Sstever@eecs.umich.eduerror checking, this mode is not the default and is not recommended during development. However, once 3982632Sstever@eecs.umich.eduyou have your compiler fully working, it is usually safe to disable the error checks. 3992632Sstever@eecs.umich.edu 4002632Sstever@eecs.umich.edu<p> 4012632Sstever@eecs.umich.edu<li>You can enable some additional debugging by building the lexer like this: 4024479Sbinkertn@umich.edu 4032632Sstever@eecs.umich.edu<blockquote> 4042632Sstever@eecs.umich.edu<pre> 4052632Sstever@eecs.umich.edulex.lex(debug=1) 4062632Sstever@eecs.umich.edu</pre> 4074479Sbinkertn@umich.edu</blockquote> 4082632Sstever@eecs.umich.edu 4092632Sstever@eecs.umich.edu<p> 4102632Sstever@eecs.umich.edu<li>To help you debug your lexer, <tt>lex.py</tt> comes with a simple main program which will either 4112632Sstever@eecs.umich.edutokenize input read from standard input or from a file. To use it, simply put this in your lexer: 4122632Sstever@eecs.umich.edu 4132632Sstever@eecs.umich.edu<blockquote> 4142632Sstever@eecs.umich.edu<pre> 4152632Sstever@eecs.umich.eduif __name__ == '__main__': 4162632Sstever@eecs.umich.edu lex.runmain() 4172632Sstever@eecs.umich.edu</pre> 4182632Sstever@eecs.umich.edu</blockquote> 4192632Sstever@eecs.umich.edu 4202632Sstever@eecs.umich.eduThen, run you lexer as a main program such as <tt>python mylex.py</tt> 4212632Sstever@eecs.umich.edu 4222632Sstever@eecs.umich.edu<p> 4232632Sstever@eecs.umich.edu<li>Since the lexer is written entirely in Python, its performance is 4242632Sstever@eecs.umich.edulargely determined by that of the Python <tt>re</tt> module. Although 4252632Sstever@eecs.umich.eduthe lexer has been written to be as efficient as possible, it's not 4262632Sstever@eecs.umich.edublazingly fast when used on very large input files. Sorry. If 4272632Sstever@eecs.umich.eduperformance is concern, you might consider upgrading to the most 4282632Sstever@eecs.umich.edurecent version of Python, creating a hand-written lexer, or offloading 4292632Sstever@eecs.umich.eduthe lexer into a C extension module. In defense of <tt>lex.py</tt>, 4302632Sstever@eecs.umich.eduit's performance is not <em>that</em> bad when used on reasonably 4312632Sstever@eecs.umich.edusized input files. For instance, lexing a 4700 line C program with 4322632Sstever@eecs.umich.edu32000 input tokens takes about 20 seconds on a 200 Mhz PC. Obviously, 4332632Sstever@eecs.umich.eduit will run much faster on a more speedy machine. 4342632Sstever@eecs.umich.edu 4352632Sstever@eecs.umich.edu</ul> 4362632Sstever@eecs.umich.edu 4374479Sbinkertn@umich.edu<h2>Parsing basics</h2> 4384479Sbinkertn@umich.edu 4392632Sstever@eecs.umich.edu<tt>yacc.py</tt> is used to parse language syntax. Before showing an 4404479Sbinkertn@umich.eduexample, there are a few important bits of background that must be 4414479Sbinkertn@umich.edumentioned. First, <tt>syntax</tt> is usually specified in terms of a 4424479Sbinkertn@umich.educontext free grammar (CFG). For example, if you wanted to parse 4434479Sbinkertn@umich.edusimple arithmetic expressions, you might first write an unambiguous 4444479Sbinkertn@umich.edugrammar specification like this: 4454479Sbinkertn@umich.edu 4464479Sbinkertn@umich.edu<blockquote> 4474479Sbinkertn@umich.edu<pre> 4484479Sbinkertn@umich.eduexpression : expression + term 4494479Sbinkertn@umich.edu | expression - term 4504479Sbinkertn@umich.edu | term 4514479Sbinkertn@umich.edu 4524479Sbinkertn@umich.eduterm : term * factor 4534479Sbinkertn@umich.edu | term / factor 4544479Sbinkertn@umich.edu | factor 4554479Sbinkertn@umich.edu 4564479Sbinkertn@umich.edufactor : NUMBER 4574479Sbinkertn@umich.edu | ( expression ) 4582632Sstever@eecs.umich.edu</pre> 4592632Sstever@eecs.umich.edu</blockquote> 4602632Sstever@eecs.umich.edu 4612632Sstever@eecs.umich.eduNext, the semantic behavior of a language is often specified using a 4622632Sstever@eecs.umich.edutechnique known as syntax directed translation. In syntax directed 4634479Sbinkertn@umich.edutranslation, attributes are attached to each symbol in a given grammar 4642632Sstever@eecs.umich.edurule along with an action. Whenever a particular grammar rule is 4652632Sstever@eecs.umich.edurecognized, the action describes what to do. For example, given the 4662632Sstever@eecs.umich.eduexpression grammar above, you might write the specification for a 4672632Sstever@eecs.umich.edusimple calculator like this: 4682632Sstever@eecs.umich.edu 4692632Sstever@eecs.umich.edu<blockquote> 4704479Sbinkertn@umich.edu<pre> 4714479Sbinkertn@umich.eduGrammar Action 4724479Sbinkertn@umich.edu-------------------------------- -------------------------------------------- 4734479Sbinkertn@umich.eduexpression0 : expression1 + term expression0.val = expression1.val + term.val 4744479Sbinkertn@umich.edu | expression1 - term expression0.val = expression1.val - term.val 4754479Sbinkertn@umich.edu | term expression0.val = term.val 4764479Sbinkertn@umich.edu 4774479Sbinkertn@umich.eduterm0 : term1 * factor term0.val = term1.val * factor.val 4782632Sstever@eecs.umich.edu | term1 / factor term0.val = term1.val / factor.val 4792632Sstever@eecs.umich.edu | factor term0.val = factor.val 4804479Sbinkertn@umich.edu 4814479Sbinkertn@umich.edufactor : NUMBER factor.val = int(NUMBER.lexval) 4824479Sbinkertn@umich.edu | ( expression ) factor.val = expression.val 4834479Sbinkertn@umich.edu</pre> 4842632Sstever@eecs.umich.edu</blockquote> 4852632Sstever@eecs.umich.edu 4862632Sstever@eecs.umich.eduFinally, Yacc uses a parsing technique known as LR-parsing or shift-reduce parsing. LR parsing is a 4874479Sbinkertn@umich.edubottom up technique that tries to recognize the right-hand-side of various grammar rules. 4884479Sbinkertn@umich.eduWhenever a valid right-hand-side is found in the input, the appropriate action code is triggered and the 4894479Sbinkertn@umich.edugrammar symbols are replaced by the grammar symbol on the left-hand-side. 4904479Sbinkertn@umich.edu 4914479Sbinkertn@umich.edu<p> 4924479Sbinkertn@umich.eduLR parsing is commonly implemented by shifting grammar symbols onto a stack and looking at the stack and the next 4934479Sbinkertn@umich.eduinput token for patterns. The details of the algorithm can be found in a compiler text, but the 4944479Sbinkertn@umich.edufollowing example illustrates the steps that are performed if you wanted to parse the expression 4954479Sbinkertn@umich.edu<tt>3 + 5 * (10 - 20)</tt> using the grammar defined above: 4964479Sbinkertn@umich.edu 4974479Sbinkertn@umich.edu<blockquote> 4984479Sbinkertn@umich.edu<pre> 4994479Sbinkertn@umich.eduStep Symbol Stack Input Tokens Action 5004479Sbinkertn@umich.edu---- --------------------- --------------------- ------------------------------- 5014479Sbinkertn@umich.edu1 $ 3 + 5 * ( 10 - 20 )$ Shift 3 5024479Sbinkertn@umich.edu2 $ 3 + 5 * ( 10 - 20 )$ Reduce factor : NUMBER 5034479Sbinkertn@umich.edu3 $ factor + 5 * ( 10 - 20 )$ Reduce term : factor 5044479Sbinkertn@umich.edu4 $ term + 5 * ( 10 - 20 )$ Reduce expr : term 5054479Sbinkertn@umich.edu5 $ expr + 5 * ( 10 - 20 )$ Shift + 5064479Sbinkertn@umich.edu6 $ expr + 5 * ( 10 - 20 )$ Shift 5 5074479Sbinkertn@umich.edu7 $ expr + 5 * ( 10 - 20 )$ Reduce factor : NUMBER 5084479Sbinkertn@umich.edu8 $ expr + factor * ( 10 - 20 )$ Reduce term : factor 5094479Sbinkertn@umich.edu9 $ expr + term * ( 10 - 20 )$ Shift * 5104479Sbinkertn@umich.edu10 $ expr + term * ( 10 - 20 )$ Shift ( 5114479Sbinkertn@umich.edu11 $ expr + term * ( 10 - 20 )$ Shift 10 5124479Sbinkertn@umich.edu12 $ expr + term * ( 10 - 20 )$ Reduce factor : NUMBER 5134479Sbinkertn@umich.edu13 $ expr + term * ( factor - 20 )$ Reduce term : factor 5144479Sbinkertn@umich.edu14 $ expr + term * ( term - 20 )$ Reduce expr : term 5154479Sbinkertn@umich.edu15 $ expr + term * ( expr - 20 )$ Shift - 5162632Sstever@eecs.umich.edu16 $ expr + term * ( expr - 20 )$ Shift 20 5172632Sstever@eecs.umich.edu17 $ expr + term * ( expr - 20 )$ Reduce factor : NUMBER 5184479Sbinkertn@umich.edu18 $ expr + term * ( expr - factor )$ Reduce term : factor 5194479Sbinkertn@umich.edu19 $ expr + term * ( expr - term )$ Reduce expr : expr - term 5204479Sbinkertn@umich.edu20 $ expr + term * ( expr )$ Shift ) 5214479Sbinkertn@umich.edu21 $ expr + term * ( expr ) $ Reduce factor : (expr) 5224479Sbinkertn@umich.edu22 $ expr + term * factor $ Reduce term : term * factor 5234479Sbinkertn@umich.edu23 $ expr + term $ Reduce expr : expr + term 5244479Sbinkertn@umich.edu24 $ expr $ Reduce expr 5254479Sbinkertn@umich.edu25 $ $ Success! 5264479Sbinkertn@umich.edu</pre> 5274479Sbinkertn@umich.edu</blockquote> 5284479Sbinkertn@umich.edu 5294479Sbinkertn@umich.eduWhen parsing the expression, an underlying state machine and the current input token determine what to do next. 5304479Sbinkertn@umich.eduIf the next token looks like part of a valid grammar rule (based on other items on the stack), it is generally shifted 5314479Sbinkertn@umich.eduonto the stack. If the top of the stack contains a valid right-hand-side of a grammar rule, it is 5324479Sbinkertn@umich.eduusually "reduced" and the symbols replaced with the symbol on the left-hand-side. When this reduction occurs, the 5334479Sbinkertn@umich.eduappropriate action is triggered (if defined). If the input token can't be shifted and the top of stack doesn't match 5344479Sbinkertn@umich.eduany grammar rules, a syntax error has occurred and the parser must take some kind of recovery step (or bail out). 5354479Sbinkertn@umich.edu 5364479Sbinkertn@umich.edu<p> 5374479Sbinkertn@umich.eduIt is important to note that the underlying implementation is actually built around a large finite-state machine 5384479Sbinkertn@umich.eduand some tables. The construction of these tables is quite complicated and beyond the scope of this discussion. 5394479Sbinkertn@umich.eduHowever, subtle details of this process explain why, in the example above, the parser chooses to shift a token 5404479Sbinkertn@umich.eduonto the stack in step 9 rather than reducing the rule <tt>expr : expr + term</tt>. 5414479Sbinkertn@umich.edu 5422632Sstever@eecs.umich.edu<h2>Yacc example</h2> 5432632Sstever@eecs.umich.edu 5444479Sbinkertn@umich.eduSuppose you wanted to make a grammar for simple arithmetic expressions as previously described. Here is 5454479Sbinkertn@umich.eduhow you would do it with <tt>yacc.py</tt>: 5464479Sbinkertn@umich.edu 5474479Sbinkertn@umich.edu<blockquote> 5484479Sbinkertn@umich.edu<pre> 5494479Sbinkertn@umich.edu# Yacc example 5504479Sbinkertn@umich.edu 5514479Sbinkertn@umich.eduimport yacc 5524479Sbinkertn@umich.edu 5534479Sbinkertn@umich.edu# Get the token map from the lexer. This is required. 5544479Sbinkertn@umich.edufrom calclex import tokens 5554479Sbinkertn@umich.edu 5564479Sbinkertn@umich.edudef p_expression_plus(t): 5574479Sbinkertn@umich.edu 'expression : expression PLUS term' 5584479Sbinkertn@umich.edu t[0] = t[1] + t[3] 5594479Sbinkertn@umich.edu 5604479Sbinkertn@umich.edudef p_expression_minus(t): 5614479Sbinkertn@umich.edu 'expression : expression MINUS term' 5624479Sbinkertn@umich.edu t[0] = t[1] - t[3] 5634479Sbinkertn@umich.edu 5644479Sbinkertn@umich.edudef p_expression_term(t): 5654479Sbinkertn@umich.edu 'expression : term' 5664479Sbinkertn@umich.edu t[0] = t[1] 5674479Sbinkertn@umich.edu 5684479Sbinkertn@umich.edudef p_term_times(t): 5694479Sbinkertn@umich.edu 'term : term TIMES factor' 5704479Sbinkertn@umich.edu t[0] = t[1] * t[3] 5714479Sbinkertn@umich.edu 5724479Sbinkertn@umich.edudef p_term_div(t): 5734479Sbinkertn@umich.edu 'term : term DIVIDE factor' 5744479Sbinkertn@umich.edu t[0] = t[1] / t[3] 5754479Sbinkertn@umich.edu 5764479Sbinkertn@umich.edudef p_term_factor(t): 5774479Sbinkertn@umich.edu 'term : factor' 5784479Sbinkertn@umich.edu t[0] = t[1] 5794479Sbinkertn@umich.edu 5804479Sbinkertn@umich.edudef p_factor_num(t): 5814479Sbinkertn@umich.edu 'factor : NUMBER' 5824479Sbinkertn@umich.edu t[0] = t[1] 5834479Sbinkertn@umich.edu 5844479Sbinkertn@umich.edudef p_factor_expr(t): 5854479Sbinkertn@umich.edu 'factor : LPAREN expression RPAREN' 5864479Sbinkertn@umich.edu t[0] = t[2] 5874479Sbinkertn@umich.edu 5884479Sbinkertn@umich.edu# Error rule for syntax errors 5894479Sbinkertn@umich.edudef p_error(t): 5904479Sbinkertn@umich.edu print "Syntax error in input!" 5914479Sbinkertn@umich.edu 5924479Sbinkertn@umich.edu# Build the parser 5934479Sbinkertn@umich.eduyacc.yacc() 5944479Sbinkertn@umich.edu 5954479Sbinkertn@umich.eduwhile 1: 5964479Sbinkertn@umich.edu try: 5974479Sbinkertn@umich.edu s = raw_input('calc > ') 5984479Sbinkertn@umich.edu except EOFError: 5994479Sbinkertn@umich.edu break 6004479Sbinkertn@umich.edu if not s: continue 6014479Sbinkertn@umich.edu result = yacc.parse(s) 6024479Sbinkertn@umich.edu print result 6034479Sbinkertn@umich.edu</pre> 6044479Sbinkertn@umich.edu</blockquote> 6054479Sbinkertn@umich.edu 6064479Sbinkertn@umich.eduIn this example, each grammar rule is defined by a Python function where the docstring to that function contains the 6074479Sbinkertn@umich.eduappropriate context-free grammar specification (an idea borrowed from John Aycock's SPARK toolkit). Each function accepts a single 6084479Sbinkertn@umich.eduargument <tt>t</tt> that is a sequence containing the values of each grammar symbol in the corresponding rule. The values of 6094479Sbinkertn@umich.edu<tt>t[i]</tt> are mapped to grammar symbols as shown here: 6104479Sbinkertn@umich.edu 6114479Sbinkertn@umich.edu<blockquote> 6124479Sbinkertn@umich.edu<pre> 6134479Sbinkertn@umich.edudef p_expression_plus(t): 6144479Sbinkertn@umich.edu 'expression : expression PLUS term' 6154479Sbinkertn@umich.edu # ^ ^ ^ ^ 6162632Sstever@eecs.umich.edu # t[0] t[1] t[2] t[3] 6172632Sstever@eecs.umich.edu 6182632Sstever@eecs.umich.edu t[0] = t[1] + t[3] 6192632Sstever@eecs.umich.edu</pre> 6202632Sstever@eecs.umich.edu</blockquote> 6212632Sstever@eecs.umich.edu 6222632Sstever@eecs.umich.eduFor tokens, the "value" in the corresponding <tt>t[i]</tt> is the 6232632Sstever@eecs.umich.edu<em>same</em> as the value of the <tt>t.value</tt> attribute assigned 6242632Sstever@eecs.umich.eduin the lexer module. For non-terminals, the value is determined by 6252632Sstever@eecs.umich.eduwhatever is placed in <tt>t[0]</tt> when rules are reduced. This 6262632Sstever@eecs.umich.eduvalue can be anything at all. However, it probably most common for 6272632Sstever@eecs.umich.eduthe value to be a simple Python type, a tuple, or an instance. In this example, we 6284479Sbinkertn@umich.eduare relying on the fact that the <tt>NUMBER</tt> token stores an integer value in its value 6294479Sbinkertn@umich.edufield. All of the other rules simply perform various types of integer operations and store 6304479Sbinkertn@umich.eduthe result. 6314479Sbinkertn@umich.edu 6324479Sbinkertn@umich.edu<p> 6334479Sbinkertn@umich.eduThe first rule defined in the yacc specification determines the starting grammar 6342632Sstever@eecs.umich.edusymbol (in this case, a rule for <tt>expression</tt> appears first). Whenever 6352632Sstever@eecs.umich.eduthe starting rule is reduced by the parser and no more input is available, parsing 6364479Sbinkertn@umich.edustops and the final value is returned (this value will be whatever the top-most rule 6374479Sbinkertn@umich.eduplaced in <tt>t[0]</tt>). 6384479Sbinkertn@umich.edu 6394479Sbinkertn@umich.edu<p>The <tt>p_error(t)</tt> rule is defined to catch syntax errors. See the error handling section 6404479Sbinkertn@umich.edubelow for more detail. 6412632Sstever@eecs.umich.edu 6422632Sstever@eecs.umich.edu<p> 6432632Sstever@eecs.umich.eduTo build the parser, call the <tt>yacc.yacc()</tt> function. This function 6444479Sbinkertn@umich.edulooks at the module and attempts to construct all of the LR parsing tables for the grammar 6454479Sbinkertn@umich.eduyou have specified. The first time <tt>yacc.yacc()</tt> is invoked, you will get a message 6464479Sbinkertn@umich.edusuch as this: 6474479Sbinkertn@umich.edu 6484479Sbinkertn@umich.edu<blockquote> 6494479Sbinkertn@umich.edu<pre> 6504479Sbinkertn@umich.edu$ python calcparse.py 6512632Sstever@eecs.umich.eduyacc: Generating SLR parsing table... 6522632Sstever@eecs.umich.educalc > 6532632Sstever@eecs.umich.edu</pre> 6544479Sbinkertn@umich.edu</blockquote> 6554479Sbinkertn@umich.edu 6564479Sbinkertn@umich.eduSince table construction is relatively expensive (especially for large 6572632Sstever@eecs.umich.edugrammars), the resulting parsing table is written to the current 6582632Sstever@eecs.umich.edudirectory in a file called <tt>parsetab.py</tt>. In addition, a 6592632Sstever@eecs.umich.edudebugging file called <tt>parser.out</tt> is created. On subsequent 6604479Sbinkertn@umich.eduexecutions, <tt>yacc</tt> will reload the table from 6614479Sbinkertn@umich.edu<tt>parsetab.py</tt> unless it has detected a change in the underlying 6624479Sbinkertn@umich.edugrammar (in which case the tables and <tt>parsetab.py</tt> file are 6634479Sbinkertn@umich.eduregenerated). 6644479Sbinkertn@umich.edu 6652632Sstever@eecs.umich.edu<p> 6662632Sstever@eecs.umich.eduIf any errors are detected in your grammar specification, <tt>yacc.py</tt> will produce 6672632Sstever@eecs.umich.edudiagnostic messages and possibly raise an exception. Some of the errors that can be detected include: 6684479Sbinkertn@umich.edu 6694479Sbinkertn@umich.edu<ul> 6704479Sbinkertn@umich.edu<li>Duplicated function names (if more than one rule function have the same name in the grammar file). 6714479Sbinkertn@umich.edu<li>Shift/reduce and reduce/reduce conflicts generated by ambiguous grammars. 6724479Sbinkertn@umich.edu<li>Badly specified grammar rules. 6734479Sbinkertn@umich.edu<li>Infinite recursion (rules that can never terminate). 6744479Sbinkertn@umich.edu<li>Unused rules and tokens 6754479Sbinkertn@umich.edu<li>Undefined rules and tokens 6764479Sbinkertn@umich.edu</ul> 6774479Sbinkertn@umich.edu 6784479Sbinkertn@umich.eduThe next few sections now discuss a few finer points of grammar construction. 6794479Sbinkertn@umich.edu 6804479Sbinkertn@umich.edu<h2>Combining Grammar Rule Functions</h2> 6814479Sbinkertn@umich.edu 6824479Sbinkertn@umich.eduWhen grammar rules are similar, they can be combined into a single function. 6834479Sbinkertn@umich.eduFor example, consider the two rules in our earlier example: 6844479Sbinkertn@umich.edu 6854479Sbinkertn@umich.edu<blockquote> 6864479Sbinkertn@umich.edu<pre> 6874479Sbinkertn@umich.edudef p_expression_plus(t): 6884479Sbinkertn@umich.edu 'expression : expression PLUS term' 6894479Sbinkertn@umich.edu t[0] = t[1] + t[3] 6904479Sbinkertn@umich.edu 6914479Sbinkertn@umich.edudef p_expression_minus(t): 6924479Sbinkertn@umich.edu 'expression : expression MINUS term' 6934479Sbinkertn@umich.edu t[0] = t[1] - t[3] 6944479Sbinkertn@umich.edu</pre> 6954479Sbinkertn@umich.edu</blockquote> 6964479Sbinkertn@umich.edu 6974479Sbinkertn@umich.eduInstead of writing two functions, you might write a single function like this: 6984479Sbinkertn@umich.edu 6994479Sbinkertn@umich.edu<blockquote> 7004479Sbinkertn@umich.edu<pre> 7014479Sbinkertn@umich.edudef p_expression(t): 7024479Sbinkertn@umich.edu '''expression : expression PLUS term 7034479Sbinkertn@umich.edu | expression MINUS term''' 7044479Sbinkertn@umich.edu if t[2] == '+': 7054479Sbinkertn@umich.edu t[0] = t[1] + t[3] 7064479Sbinkertn@umich.edu elif t[2] == '-': 7074479Sbinkertn@umich.edu t[0] = t[1] - t[3] 7082632Sstever@eecs.umich.edu</pre> 7094479Sbinkertn@umich.edu</blockquote> 7104479Sbinkertn@umich.edu 7114479Sbinkertn@umich.eduIn general, the doc string for any given function can contain multiple grammar rules. So, it would 7124479Sbinkertn@umich.eduhave also been legal (although possibly confusing) to write this: 7134479Sbinkertn@umich.edu 7144479Sbinkertn@umich.edu<blockquote> 7154479Sbinkertn@umich.edu<pre> 7164479Sbinkertn@umich.edudef p_binary_operators(t): 7174479Sbinkertn@umich.edu '''expression : expression PLUS term 7184479Sbinkertn@umich.edu | expression MINUS term 7194479Sbinkertn@umich.edu term : term TIMES factor 7204479Sbinkertn@umich.edu | term DIVIDE factor''' 7214479Sbinkertn@umich.edu if t[2] == '+': 7224479Sbinkertn@umich.edu t[0] = t[1] + t[3] 7234479Sbinkertn@umich.edu elif t[2] == '-': 7244479Sbinkertn@umich.edu t[0] = t[1] - t[3] 7254479Sbinkertn@umich.edu elif t[2] == '*': 7264479Sbinkertn@umich.edu t[0] = t[1] * t[3] 7274479Sbinkertn@umich.edu elif t[2] == '/': 7284479Sbinkertn@umich.edu t[0] = t[1] / t[3] 7294479Sbinkertn@umich.edu</pre> 7304479Sbinkertn@umich.edu</blockquote> 7314479Sbinkertn@umich.edu 7324479Sbinkertn@umich.eduWhen combining grammar rules into a single function, it is usually a good idea for all of the rules to have 7334479Sbinkertn@umich.edua similar structure (e.g., the same number of terms). Otherwise, the corresponding action code may be more 7344479Sbinkertn@umich.educomplicated than necessary. 7354479Sbinkertn@umich.edu 7364479Sbinkertn@umich.edu<h2>Empty Productions</h2> 7372632Sstever@eecs.umich.edu 7382632Sstever@eecs.umich.edu<tt>yacc.py</tt> can handle empty productions by defining a rule like this: 7392632Sstever@eecs.umich.edu 7402632Sstever@eecs.umich.edu<blockquote> 7412632Sstever@eecs.umich.edu<pre> 7422632Sstever@eecs.umich.edudef p_empty(t): 7432632Sstever@eecs.umich.edu 'empty :' 7442632Sstever@eecs.umich.edu pass 7454479Sbinkertn@umich.edu</pre> 7464479Sbinkertn@umich.edu</blockquote> 7474479Sbinkertn@umich.edu 7484479Sbinkertn@umich.eduNow to use the empty production, simply use 'empty' as a symbol. For example: 7494479Sbinkertn@umich.edu 7504479Sbinkertn@umich.edu<blockquote> 7514479Sbinkertn@umich.edu<pre> 7524479Sbinkertn@umich.edudef p_optitem(t): 7534479Sbinkertn@umich.edu 'optitem : item' 7544479Sbinkertn@umich.edu ' | empty' 7554479Sbinkertn@umich.edu ... 7564479Sbinkertn@umich.edu</pre> 7574479Sbinkertn@umich.edu</blockquote> 7584479Sbinkertn@umich.edu 7594479Sbinkertn@umich.edu<h2>Dealing With Ambiguous Grammars</h2> 7604479Sbinkertn@umich.edu 7614479Sbinkertn@umich.eduThe expression grammar given in the earlier example has been written in a special format to eliminate ambiguity. 7624479Sbinkertn@umich.eduHowever, in many situations, it is extremely difficult or awkward to write grammars in this format. A 7634479Sbinkertn@umich.edumuch more natural way to express the grammar is in a more compact form like this: 7644479Sbinkertn@umich.edu 7654479Sbinkertn@umich.edu<blockquote> 7664479Sbinkertn@umich.edu<pre> 7674479Sbinkertn@umich.eduexpression : expression PLUS expression 7684479Sbinkertn@umich.edu | expression MINUS expression 7694479Sbinkertn@umich.edu | expression TIMES expression 7704479Sbinkertn@umich.edu | expression DIVIDE expression 7714479Sbinkertn@umich.edu | LPAREN expression RPAREN 7724479Sbinkertn@umich.edu | NUMBER 7734479Sbinkertn@umich.edu</pre> 7744479Sbinkertn@umich.edu</blockquote> 7754479Sbinkertn@umich.edu 7764479Sbinkertn@umich.eduUnfortunately, this grammar specification is ambiguous. For example, if you are parsing the string 7774479Sbinkertn@umich.edu"3 * 4 + 5", there is no way to tell how the operators are supposed to be grouped. 7784479Sbinkertn@umich.eduFor example, does this expression mean "(3 * 4) + 5" or is it "3 * (4+5)"? 7794479Sbinkertn@umich.edu 7804479Sbinkertn@umich.edu<p> 7814479Sbinkertn@umich.eduWhen an ambiguous grammar is given to <tt>yacc.py</tt> it will print messages about "shift/reduce conflicts" 7824479Sbinkertn@umich.eduor a "reduce/reduce conflicts". A shift/reduce conflict is caused when the parser generator can't decide 7834479Sbinkertn@umich.eduwhether or not to reduce a rule or shift a symbol on the parsing stack. For example, consider 7844479Sbinkertn@umich.eduthe string "3 * 4 + 5" and the internal parsing stack: 7854479Sbinkertn@umich.edu 7864479Sbinkertn@umich.edu<blockquote> 7874479Sbinkertn@umich.edu<pre> 7884479Sbinkertn@umich.eduStep Symbol Stack Input Tokens Action 7894479Sbinkertn@umich.edu---- --------------------- --------------------- ------------------------------- 7904479Sbinkertn@umich.edu1 $ 3 * 4 + 5$ Shift 3 7914479Sbinkertn@umich.edu2 $ 3 * 4 + 5$ Reduce : expression : NUMBER 7924479Sbinkertn@umich.edu3 $ expr * 4 + 5$ Shift * 7934479Sbinkertn@umich.edu4 $ expr * 4 + 5$ Shift 4 7944479Sbinkertn@umich.edu5 $ expr * 4 + 5$ Reduce: expression : NUMBER 7954479Sbinkertn@umich.edu6 $ expr * expr + 5$ SHIFT/REDUCE CONFLICT ???? 7964479Sbinkertn@umich.edu</pre> 7974479Sbinkertn@umich.edu</blockquote> 7984479Sbinkertn@umich.edu 7994479Sbinkertn@umich.eduIn this case, when the parser reaches step 6, it has two options. One is the reduce the 8004479Sbinkertn@umich.edurule <tt>expr : expr * expr</tt> on the stack. The other option is to shift the 8014479Sbinkertn@umich.edutoken <tt>+</tt> on the stack. Both options are perfectly legal from the rules 8024479Sbinkertn@umich.eduof the context-free-grammar. 8034479Sbinkertn@umich.edu 8044479Sbinkertn@umich.edu<p> 8054479Sbinkertn@umich.eduBy default, all shift/reduce conflicts are resolved in favor of shifting. Therefore, in the above 8064479Sbinkertn@umich.eduexample, the parser will always shift the <tt>+</tt> instead of reducing. Although this 8074479Sbinkertn@umich.edustrategy works in many cases (including the ambiguous if-then-else), it is not enough for arithmetic 8084479Sbinkertn@umich.eduexpressions. In fact, in the above example, the decision to shift <tt>+</tt> is completely wrong---we should have 8094479Sbinkertn@umich.edureduced <tt>expr * expr</tt> since multiplication has higher precedence than addition. 8104479Sbinkertn@umich.edu 8114479Sbinkertn@umich.edu<p>To resolve ambiguity, especially in expression grammars, <tt>yacc.py</tt> allows individual 8124479Sbinkertn@umich.edutokens to be assigned a precedence level and associativity. This is done by adding a variable 8134479Sbinkertn@umich.edu<tt>precedence</tt> to the grammar file like this: 8144479Sbinkertn@umich.edu 8154479Sbinkertn@umich.edu<blockquote> 8164479Sbinkertn@umich.edu<pre> 8174479Sbinkertn@umich.eduprecedence = ( 8184479Sbinkertn@umich.edu ('left', 'PLUS', 'MINUS'), 8194479Sbinkertn@umich.edu ('left', 'TIMES', 'DIVIDE'), 8204479Sbinkertn@umich.edu) 8214479Sbinkertn@umich.edu</pre> 8224479Sbinkertn@umich.edu</blockquote> 8234479Sbinkertn@umich.edu 8244479Sbinkertn@umich.eduThis declaration specifies that <tt>PLUS</tt>/<tt>MINUS</tt> have 8254479Sbinkertn@umich.eduthe same precedence level and are left-associative and that 8264479Sbinkertn@umich.edu<tt>TIMES</tt>/<tt>DIVIDE</tt> have the same precedence and are left-associative. 8274479Sbinkertn@umich.eduFurthermore, the declaration specifies that <tt>TIMES</tt>/<tt>DIVIDE</tt> have higher 8284479Sbinkertn@umich.eduprecedence than <tt>PLUS</tt>/<tt>MINUS</tt> (since they appear later in the 8294479Sbinkertn@umich.eduprecedence specification). 8304479Sbinkertn@umich.edu 8314479Sbinkertn@umich.edu<p> 8324479Sbinkertn@umich.eduThe precedence specification is used to attach a numerical precedence value and associativity direction 8334479Sbinkertn@umich.eduto each grammar rule. This is always determined by the precedence of the right-most terminal symbol. Therefore, 8344479Sbinkertn@umich.eduif PLUS/MINUS had a precedence of 1 and TIMES/DIVIDE had a precedence of 2, the grammar rules 8354479Sbinkertn@umich.eduwould have precedence values as follows: 8364479Sbinkertn@umich.edu 8374479Sbinkertn@umich.edu<blockquote> 8384479Sbinkertn@umich.edu<pre> 8394479Sbinkertn@umich.eduexpression : expression PLUS expression # prec = 1, left 8404479Sbinkertn@umich.edu | expression MINUS expression # prec = 1, left 8414479Sbinkertn@umich.edu | expression TIMES expression # prec = 2, left 8424479Sbinkertn@umich.edu | expression DIVIDE expression # prec = 2, left 8434479Sbinkertn@umich.edu | LPAREN expression RPAREN # prec = unknown 8444479Sbinkertn@umich.edu | NUMBER # prec = unknown 8454479Sbinkertn@umich.edu</pre> 8464479Sbinkertn@umich.edu</blockquote> 8474479Sbinkertn@umich.edu 8484479Sbinkertn@umich.eduWhen shift/reduce conflicts are encountered, the parser generator resolves the conflict by 8494479Sbinkertn@umich.edulooking at the precedence rules and associativity specifiers. 8504479Sbinkertn@umich.edu 8514479Sbinkertn@umich.edu<p> 8524479Sbinkertn@umich.edu<ol> 8534479Sbinkertn@umich.edu<li>If the current token has higher precedence, it is shifted. 8544479Sbinkertn@umich.edu<li>If the grammar rule on the stack has higher precedence, the rule is reduced. 8554479Sbinkertn@umich.edu<li>If the current token and the grammar rule have the same precedence, the 8564479Sbinkertn@umich.edurule is reduced for left associativity, whereas the token is shifted for right associativity. 8574479Sbinkertn@umich.edu<li>If nothing is known about the precedence, shift/reduce conflicts are resolved in 8584479Sbinkertn@umich.edufavor of shifting (the default). 8594479Sbinkertn@umich.edu</ol> 8604479Sbinkertn@umich.edu 8614479Sbinkertn@umich.edu<p> 8624479Sbinkertn@umich.eduWhen shift/reduce conflicts are resolved using the first three techniques (with the help of 8634479Sbinkertn@umich.eduprecedence rules), <tt>yacc.py</tt> will report no errors or conflicts in the grammar. 8644479Sbinkertn@umich.edu 8654479Sbinkertn@umich.edu<p> 8664479Sbinkertn@umich.eduOne problem with the precedence specifier technique is that it is sometimes necessary to 8674479Sbinkertn@umich.educhange the precedence of an operator in certain contents. For example, consider a unary-minus operator 8684479Sbinkertn@umich.eduin "3 + 4 * -5". Normally, unary minus has a very high precedence--being evaluated before the multiply. 8694479Sbinkertn@umich.eduHowever, in our precedence specifier, MINUS has a lower precedence than TIMES. To deal with this, 8704479Sbinkertn@umich.eduprecedence rules can be given for fictitious tokens like this: 8714479Sbinkertn@umich.edu 8724479Sbinkertn@umich.edu<blockquote> 8734479Sbinkertn@umich.edu<pre> 8744479Sbinkertn@umich.eduprecedence = ( 8754479Sbinkertn@umich.edu ('left', 'PLUS', 'MINUS'), 8764479Sbinkertn@umich.edu ('left', 'TIMES', 'DIVIDE'), 8774479Sbinkertn@umich.edu ('right', 'UMINUS'), # Unary minus operator 8784479Sbinkertn@umich.edu) 8794479Sbinkertn@umich.edu</pre> 8804479Sbinkertn@umich.edu</blockquote> 8814479Sbinkertn@umich.edu 8824479Sbinkertn@umich.eduNow, in the grammar file, we can write our unary minus rule like this: 8834479Sbinkertn@umich.edu 8844479Sbinkertn@umich.edu<blockquote> 8854479Sbinkertn@umich.edu<pre> 8864479Sbinkertn@umich.edudef p_expr_uminus(t): 8874479Sbinkertn@umich.edu 'expression : MINUS expression %prec UMINUS' 8884479Sbinkertn@umich.edu t[0] = -t[2] 8894479Sbinkertn@umich.edu</pre> 8904479Sbinkertn@umich.edu</blockquote> 8914479Sbinkertn@umich.edu 8924479Sbinkertn@umich.eduIn this case, <tt>%prec UMINUS</tt> overrides the default rule precedence--setting it to that 8934479Sbinkertn@umich.eduof UMINUS in the precedence specifier. 8944479Sbinkertn@umich.edu 8954479Sbinkertn@umich.edu<p> 8964479Sbinkertn@umich.eduIt is also possible to specify non-associativity in the <tt>precedence</tt> table. This would 8974479Sbinkertn@umich.edube used when you <em>don't</em> want operations to chain together. For example, suppose 8984479Sbinkertn@umich.eduyou wanted to support a comparison operators like <tt><</tt> and <tt>></tt> but you didn't want to allow 8994479Sbinkertn@umich.educombinations like <tt>a < b < c</tt>. To do this, simply specify a rule like this: 9004479Sbinkertn@umich.edu 9014479Sbinkertn@umich.edu<blockquote> 9024479Sbinkertn@umich.edu<pre> 9034479Sbinkertn@umich.eduprecedence = ( 9044479Sbinkertn@umich.edu ('nonassoc', 'LESSTHAN', 'GREATERTHAN'), # Nonassociative operators 9054479Sbinkertn@umich.edu ('left', 'PLUS', 'MINUS'), 9064479Sbinkertn@umich.edu ('left', 'TIMES', 'DIVIDE'), 9074479Sbinkertn@umich.edu ('right', 'UMINUS'), # Unary minus operator 9084479Sbinkertn@umich.edu) 9094479Sbinkertn@umich.edu</pre> 9104479Sbinkertn@umich.edu</blockquote> 9114479Sbinkertn@umich.edu 9124479Sbinkertn@umich.edu<p> 9134479Sbinkertn@umich.eduReduce/reduce conflicts are caused when there are multiple grammar 9144479Sbinkertn@umich.edurules that can be applied to a given set of symbols. This kind of 9154479Sbinkertn@umich.educonflict is almost always bad and is always resolved by picking the 9164479Sbinkertn@umich.edurule that appears first in the grammar file. Reduce/reduce conflicts 9174479Sbinkertn@umich.eduare almost always caused when different sets of grammar rules somehow 9184479Sbinkertn@umich.edugenerate the same set of symbols. For example: 9194479Sbinkertn@umich.edu 9204479Sbinkertn@umich.edu<blockquote> 9214479Sbinkertn@umich.edu<pre> 9224479Sbinkertn@umich.eduassignment : ID EQUALS NUMBER 9234479Sbinkertn@umich.edu | ID EQUALS expression 9244479Sbinkertn@umich.edu 9254479Sbinkertn@umich.eduexpression : expression PLUS expression 9264479Sbinkertn@umich.edu | expression MINUS expression 9274479Sbinkertn@umich.edu | expression TIMES expression 9284479Sbinkertn@umich.edu | expression DIVIDE expression 9294479Sbinkertn@umich.edu | LPAREN expression RPAREN 9304479Sbinkertn@umich.edu | NUMBER 9314479Sbinkertn@umich.edu</pre> 9324479Sbinkertn@umich.edu</blockquote> 9334479Sbinkertn@umich.edu 9344479Sbinkertn@umich.eduIn this case, a reduce/reduce conflict exists between these two rules: 9354479Sbinkertn@umich.edu 9364479Sbinkertn@umich.edu<blockquote> 9374479Sbinkertn@umich.edu<pre> 9384479Sbinkertn@umich.eduassignment : ID EQUALS NUMBER 9394479Sbinkertn@umich.eduexpression : NUMBER 9404479Sbinkertn@umich.edu</pre> 9414479Sbinkertn@umich.edu</blockquote> 9424479Sbinkertn@umich.edu 9434479Sbinkertn@umich.eduFor example, if you wrote "a = 5", the parser can't figure out if this 9444479Sbinkertn@umich.eduis supposed to reduced as <tt>assignment : ID EQUALS NUMBER</tt> or 9454479Sbinkertn@umich.eduwhether it's supposed to reduce the 5 as an expression and then reduce 9464479Sbinkertn@umich.eduthe rule <tt>assignment : ID EQUALS expression</tt>. 9474479Sbinkertn@umich.edu 9484479Sbinkertn@umich.edu<h2>The parser.out file</h2> 9494479Sbinkertn@umich.edu 9504479Sbinkertn@umich.eduTracking down shift/reduce and reduce/reduce conflicts is one of the finer pleasures of using an LR 9514479Sbinkertn@umich.eduparsing algorithm. To assist in debugging, <tt>yacc.py</tt> creates a debugging file called 9524479Sbinkertn@umich.edu'parser.out' when it generates the parsing table. The contents of this file look like the following: 9534479Sbinkertn@umich.edu 9544479Sbinkertn@umich.edu<blockquote> 9554479Sbinkertn@umich.edu<pre> 9564479Sbinkertn@umich.eduUnused terminals: 9574479Sbinkertn@umich.edu 9584479Sbinkertn@umich.edu 9594479Sbinkertn@umich.eduGrammar 9604479Sbinkertn@umich.edu 9614479Sbinkertn@umich.eduRule 1 expression -> expression PLUS expression 9624479Sbinkertn@umich.eduRule 2 expression -> expression MINUS expression 9634479Sbinkertn@umich.eduRule 3 expression -> expression TIMES expression 9644479Sbinkertn@umich.eduRule 4 expression -> expression DIVIDE expression 9654479Sbinkertn@umich.eduRule 5 expression -> NUMBER 9664479Sbinkertn@umich.eduRule 6 expression -> LPAREN expression RPAREN 9674479Sbinkertn@umich.edu 9684479Sbinkertn@umich.eduTerminals, with rules where they appear 9694479Sbinkertn@umich.edu 9704479Sbinkertn@umich.eduTIMES : 3 9714479Sbinkertn@umich.eduerror : 9724479Sbinkertn@umich.eduMINUS : 2 9734479Sbinkertn@umich.eduRPAREN : 6 9744479Sbinkertn@umich.eduLPAREN : 6 9754479Sbinkertn@umich.eduDIVIDE : 4 9764479Sbinkertn@umich.eduPLUS : 1 9774479Sbinkertn@umich.eduNUMBER : 5 9784479Sbinkertn@umich.edu 9794479Sbinkertn@umich.eduNonterminals, with rules where they appear 9804479Sbinkertn@umich.edu 9814479Sbinkertn@umich.eduexpression : 1 1 2 2 3 3 4 4 6 0 9824479Sbinkertn@umich.edu 9834479Sbinkertn@umich.edu 9844479Sbinkertn@umich.eduParsing method: SLR 9854479Sbinkertn@umich.edu 9864479Sbinkertn@umich.edu 9874479Sbinkertn@umich.edustate 0 9884479Sbinkertn@umich.edu 9894479Sbinkertn@umich.edu S' -> . expression 9904479Sbinkertn@umich.edu expression -> . expression PLUS expression 9914479Sbinkertn@umich.edu expression -> . expression MINUS expression 9924479Sbinkertn@umich.edu expression -> . expression TIMES expression 9934479Sbinkertn@umich.edu expression -> . expression DIVIDE expression 9944479Sbinkertn@umich.edu expression -> . NUMBER 9954479Sbinkertn@umich.edu expression -> . LPAREN expression RPAREN 9964479Sbinkertn@umich.edu 9974479Sbinkertn@umich.edu NUMBER shift and go to state 3 9984479Sbinkertn@umich.edu LPAREN shift and go to state 2 9994479Sbinkertn@umich.edu 10004479Sbinkertn@umich.edu 10014479Sbinkertn@umich.edustate 1 10024479Sbinkertn@umich.edu 10034479Sbinkertn@umich.edu S' -> expression . 10044479Sbinkertn@umich.edu expression -> expression . PLUS expression 10054479Sbinkertn@umich.edu expression -> expression . MINUS expression 10064479Sbinkertn@umich.edu expression -> expression . TIMES expression 10074479Sbinkertn@umich.edu expression -> expression . DIVIDE expression 10084479Sbinkertn@umich.edu 10094479Sbinkertn@umich.edu PLUS shift and go to state 6 10104479Sbinkertn@umich.edu MINUS shift and go to state 5 10114479Sbinkertn@umich.edu TIMES shift and go to state 4 10124479Sbinkertn@umich.edu DIVIDE shift and go to state 7 10134479Sbinkertn@umich.edu 10144479Sbinkertn@umich.edu 10154479Sbinkertn@umich.edustate 2 10164479Sbinkertn@umich.edu 10174479Sbinkertn@umich.edu expression -> LPAREN . expression RPAREN 10184479Sbinkertn@umich.edu expression -> . expression PLUS expression 10194479Sbinkertn@umich.edu expression -> . expression MINUS expression 10204479Sbinkertn@umich.edu expression -> . expression TIMES expression 10214479Sbinkertn@umich.edu expression -> . expression DIVIDE expression 10224479Sbinkertn@umich.edu expression -> . NUMBER 10234479Sbinkertn@umich.edu expression -> . LPAREN expression RPAREN 10244479Sbinkertn@umich.edu 10254479Sbinkertn@umich.edu NUMBER shift and go to state 3 10264479Sbinkertn@umich.edu LPAREN shift and go to state 2 10274479Sbinkertn@umich.edu 10284479Sbinkertn@umich.edu 10294479Sbinkertn@umich.edustate 3 10304479Sbinkertn@umich.edu 10314479Sbinkertn@umich.edu expression -> NUMBER . 10324479Sbinkertn@umich.edu 10334479Sbinkertn@umich.edu $ reduce using rule 5 10344479Sbinkertn@umich.edu PLUS reduce using rule 5 10354479Sbinkertn@umich.edu MINUS reduce using rule 5 10364479Sbinkertn@umich.edu TIMES reduce using rule 5 10374479Sbinkertn@umich.edu DIVIDE reduce using rule 5 10384479Sbinkertn@umich.edu RPAREN reduce using rule 5 10394479Sbinkertn@umich.edu 10404479Sbinkertn@umich.edu 10414479Sbinkertn@umich.edustate 4 10424479Sbinkertn@umich.edu 10434479Sbinkertn@umich.edu expression -> expression TIMES . expression 10444479Sbinkertn@umich.edu expression -> . expression PLUS expression 10454479Sbinkertn@umich.edu expression -> . expression MINUS expression 10464479Sbinkertn@umich.edu expression -> . expression TIMES expression 10474479Sbinkertn@umich.edu expression -> . expression DIVIDE expression 10484479Sbinkertn@umich.edu expression -> . NUMBER 10494479Sbinkertn@umich.edu expression -> . LPAREN expression RPAREN 10504479Sbinkertn@umich.edu 10514479Sbinkertn@umich.edu NUMBER shift and go to state 3 10524479Sbinkertn@umich.edu LPAREN shift and go to state 2 10534479Sbinkertn@umich.edu 10544479Sbinkertn@umich.edu 10554479Sbinkertn@umich.edustate 5 10564479Sbinkertn@umich.edu 10574479Sbinkertn@umich.edu expression -> expression MINUS . expression 10584479Sbinkertn@umich.edu expression -> . expression PLUS expression 10594479Sbinkertn@umich.edu expression -> . expression MINUS expression 10604479Sbinkertn@umich.edu expression -> . expression TIMES expression 10614479Sbinkertn@umich.edu expression -> . expression DIVIDE expression 10624479Sbinkertn@umich.edu expression -> . NUMBER 10634479Sbinkertn@umich.edu expression -> . LPAREN expression RPAREN 10644479Sbinkertn@umich.edu 10654479Sbinkertn@umich.edu NUMBER shift and go to state 3 10664479Sbinkertn@umich.edu LPAREN shift and go to state 2 10674479Sbinkertn@umich.edu 10684479Sbinkertn@umich.edu 10694479Sbinkertn@umich.edustate 6 10704479Sbinkertn@umich.edu 10714479Sbinkertn@umich.edu expression -> expression PLUS . expression 10724479Sbinkertn@umich.edu expression -> . expression PLUS expression 10734479Sbinkertn@umich.edu expression -> . expression MINUS expression 10744479Sbinkertn@umich.edu expression -> . expression TIMES expression 10754479Sbinkertn@umich.edu expression -> . expression DIVIDE expression 10764479Sbinkertn@umich.edu expression -> . NUMBER 10774479Sbinkertn@umich.edu expression -> . LPAREN expression RPAREN 10784479Sbinkertn@umich.edu 10794479Sbinkertn@umich.edu NUMBER shift and go to state 3 10804479Sbinkertn@umich.edu LPAREN shift and go to state 2 10814479Sbinkertn@umich.edu 10824479Sbinkertn@umich.edu 10834479Sbinkertn@umich.edustate 7 10844479Sbinkertn@umich.edu 10854479Sbinkertn@umich.edu expression -> expression DIVIDE . expression 10864479Sbinkertn@umich.edu expression -> . expression PLUS expression 10874479Sbinkertn@umich.edu expression -> . expression MINUS expression 10884479Sbinkertn@umich.edu expression -> . expression TIMES expression 10894479Sbinkertn@umich.edu expression -> . expression DIVIDE expression 10904479Sbinkertn@umich.edu expression -> . NUMBER 10914479Sbinkertn@umich.edu expression -> . LPAREN expression RPAREN 10924479Sbinkertn@umich.edu 10934479Sbinkertn@umich.edu NUMBER shift and go to state 3 10944479Sbinkertn@umich.edu LPAREN shift and go to state 2 10954479Sbinkertn@umich.edu 10964479Sbinkertn@umich.edu 10974479Sbinkertn@umich.edustate 8 10984479Sbinkertn@umich.edu 10994479Sbinkertn@umich.edu expression -> LPAREN expression . RPAREN 11004479Sbinkertn@umich.edu expression -> expression . PLUS expression 11014479Sbinkertn@umich.edu expression -> expression . MINUS expression 11024479Sbinkertn@umich.edu expression -> expression . TIMES expression 11034479Sbinkertn@umich.edu expression -> expression . DIVIDE expression 11044479Sbinkertn@umich.edu 11054479Sbinkertn@umich.edu RPAREN shift and go to state 13 11064479Sbinkertn@umich.edu PLUS shift and go to state 6 11074479Sbinkertn@umich.edu MINUS shift and go to state 5 11084479Sbinkertn@umich.edu TIMES shift and go to state 4 11094479Sbinkertn@umich.edu DIVIDE shift and go to state 7 11104479Sbinkertn@umich.edu 11114479Sbinkertn@umich.edu 11124479Sbinkertn@umich.edustate 9 11134479Sbinkertn@umich.edu 11144479Sbinkertn@umich.edu expression -> expression TIMES expression . 11154479Sbinkertn@umich.edu expression -> expression . PLUS expression 11164479Sbinkertn@umich.edu expression -> expression . MINUS expression 11174479Sbinkertn@umich.edu expression -> expression . TIMES expression 11184479Sbinkertn@umich.edu expression -> expression . DIVIDE expression 11194479Sbinkertn@umich.edu 11204479Sbinkertn@umich.edu $ reduce using rule 3 11214479Sbinkertn@umich.edu PLUS reduce using rule 3 11224479Sbinkertn@umich.edu MINUS reduce using rule 3 11234479Sbinkertn@umich.edu TIMES reduce using rule 3 11244479Sbinkertn@umich.edu DIVIDE reduce using rule 3 11254479Sbinkertn@umich.edu RPAREN reduce using rule 3 11264479Sbinkertn@umich.edu 11274479Sbinkertn@umich.edu ! PLUS [ shift and go to state 6 ] 11284479Sbinkertn@umich.edu ! MINUS [ shift and go to state 5 ] 11294479Sbinkertn@umich.edu ! TIMES [ shift and go to state 4 ] 11304479Sbinkertn@umich.edu ! DIVIDE [ shift and go to state 7 ] 11314479Sbinkertn@umich.edu 11324479Sbinkertn@umich.edustate 10 11334479Sbinkertn@umich.edu 11344479Sbinkertn@umich.edu expression -> expression MINUS expression . 11354479Sbinkertn@umich.edu expression -> expression . PLUS expression 11364479Sbinkertn@umich.edu expression -> expression . MINUS expression 11374479Sbinkertn@umich.edu expression -> expression . TIMES expression 11384479Sbinkertn@umich.edu expression -> expression . DIVIDE expression 11394479Sbinkertn@umich.edu 11404479Sbinkertn@umich.edu $ reduce using rule 2 11414479Sbinkertn@umich.edu PLUS reduce using rule 2 11424479Sbinkertn@umich.edu MINUS reduce using rule 2 11434479Sbinkertn@umich.edu RPAREN reduce using rule 2 11444479Sbinkertn@umich.edu TIMES shift and go to state 4 11454479Sbinkertn@umich.edu DIVIDE shift and go to state 7 11464479Sbinkertn@umich.edu 11474479Sbinkertn@umich.edu ! TIMES [ reduce using rule 2 ] 11484479Sbinkertn@umich.edu ! DIVIDE [ reduce using rule 2 ] 11494479Sbinkertn@umich.edu ! PLUS [ shift and go to state 6 ] 11504479Sbinkertn@umich.edu ! MINUS [ shift and go to state 5 ] 11514479Sbinkertn@umich.edu 11524479Sbinkertn@umich.edustate 11 11534479Sbinkertn@umich.edu 11544479Sbinkertn@umich.edu expression -> expression PLUS expression . 11554479Sbinkertn@umich.edu expression -> expression . PLUS expression 11564479Sbinkertn@umich.edu expression -> expression . MINUS expression 11574479Sbinkertn@umich.edu expression -> expression . TIMES expression 11584479Sbinkertn@umich.edu expression -> expression . DIVIDE expression 11594479Sbinkertn@umich.edu 11604479Sbinkertn@umich.edu $ reduce using rule 1 11614479Sbinkertn@umich.edu PLUS reduce using rule 1 11624479Sbinkertn@umich.edu MINUS reduce using rule 1 11634479Sbinkertn@umich.edu RPAREN reduce using rule 1 11644479Sbinkertn@umich.edu TIMES shift and go to state 4 11654479Sbinkertn@umich.edu DIVIDE shift and go to state 7 11664479Sbinkertn@umich.edu 11674479Sbinkertn@umich.edu ! TIMES [ reduce using rule 1 ] 11684479Sbinkertn@umich.edu ! DIVIDE [ reduce using rule 1 ] 11694479Sbinkertn@umich.edu ! PLUS [ shift and go to state 6 ] 11704479Sbinkertn@umich.edu ! MINUS [ shift and go to state 5 ] 11714479Sbinkertn@umich.edu 11724479Sbinkertn@umich.edustate 12 11734479Sbinkertn@umich.edu 11744479Sbinkertn@umich.edu expression -> expression DIVIDE expression . 11754479Sbinkertn@umich.edu expression -> expression . PLUS expression 11764479Sbinkertn@umich.edu expression -> expression . MINUS expression 11774479Sbinkertn@umich.edu expression -> expression . TIMES expression 11784479Sbinkertn@umich.edu expression -> expression . DIVIDE expression 11794479Sbinkertn@umich.edu 11804479Sbinkertn@umich.edu $ reduce using rule 4 11814479Sbinkertn@umich.edu PLUS reduce using rule 4 11824479Sbinkertn@umich.edu MINUS reduce using rule 4 11834479Sbinkertn@umich.edu TIMES reduce using rule 4 11844479Sbinkertn@umich.edu DIVIDE reduce using rule 4 11854479Sbinkertn@umich.edu RPAREN reduce using rule 4 11864479Sbinkertn@umich.edu 11874479Sbinkertn@umich.edu ! PLUS [ shift and go to state 6 ] 11884479Sbinkertn@umich.edu ! MINUS [ shift and go to state 5 ] 11894479Sbinkertn@umich.edu ! TIMES [ shift and go to state 4 ] 11904479Sbinkertn@umich.edu ! DIVIDE [ shift and go to state 7 ] 11914479Sbinkertn@umich.edu 11924479Sbinkertn@umich.edustate 13 11934479Sbinkertn@umich.edu 11944479Sbinkertn@umich.edu expression -> LPAREN expression RPAREN . 11954479Sbinkertn@umich.edu 11964479Sbinkertn@umich.edu $ reduce using rule 6 11974479Sbinkertn@umich.edu PLUS reduce using rule 6 11984479Sbinkertn@umich.edu MINUS reduce using rule 6 11994479Sbinkertn@umich.edu TIMES reduce using rule 6 12004479Sbinkertn@umich.edu DIVIDE reduce using rule 6 12014479Sbinkertn@umich.edu RPAREN reduce using rule 6 12024479Sbinkertn@umich.edu</pre> 12034479Sbinkertn@umich.edu</blockquote> 12044479Sbinkertn@umich.edu 12054479Sbinkertn@umich.eduIn the file, each state of the grammar is described. Within each state the "." indicates the current 12064479Sbinkertn@umich.edulocation of the parse within any applicable grammar rules. In addition, the actions for each valid 12074479Sbinkertn@umich.eduinput token are listed. When a shift/reduce or reduce/reduce conflict arises, rules <em>not</em> selected 12084479Sbinkertn@umich.eduare prefixed with an !. For example: 12094479Sbinkertn@umich.edu 12104479Sbinkertn@umich.edu<blockquote> 12114479Sbinkertn@umich.edu<pre> 12124479Sbinkertn@umich.edu ! TIMES [ reduce using rule 2 ] 12134479Sbinkertn@umich.edu ! DIVIDE [ reduce using rule 2 ] 12144479Sbinkertn@umich.edu ! PLUS [ shift and go to state 6 ] 12154479Sbinkertn@umich.edu ! MINUS [ shift and go to state 5 ] 12164479Sbinkertn@umich.edu</pre> 12174479Sbinkertn@umich.edu</blockquote> 12184479Sbinkertn@umich.edu 12194479Sbinkertn@umich.eduBy looking at these rules (and with a little practice), you can usually track down the source 12204479Sbinkertn@umich.eduof most parsing conflicts. It should also be stressed that not all shift-reduce conflicts are 12214479Sbinkertn@umich.edubad. However, the only way to be sure that they are resolved correctly is to look at <tt>parser.out</tt>. 12224479Sbinkertn@umich.edu 12234479Sbinkertn@umich.edu<h2>Syntax Error Handling</h2> 12244479Sbinkertn@umich.edu 12254479Sbinkertn@umich.eduWhen a syntax error occurs during parsing, the error is immediately 12264479Sbinkertn@umich.edudetected (i.e., the parser does not read any more tokens beyond the 12274479Sbinkertn@umich.edusource of the error). Error recovery in LR parsers is a delicate 12284479Sbinkertn@umich.edutopic that involves ancient rituals and black-magic. The recovery mechanism 12294479Sbinkertn@umich.eduprovided by <tt>yacc.py</tt> is comparable to Unix yacc so you may want 12304479Sbinkertn@umich.educonsult a book like O'Reilly's "Lex and Yacc" for some of the finer details. 12314479Sbinkertn@umich.edu 12324479Sbinkertn@umich.edu<p> 12334479Sbinkertn@umich.eduWhen a syntax error occurs, <tt>yacc.py</tt> performs the following steps: 12344479Sbinkertn@umich.edu 12354479Sbinkertn@umich.edu<ol> 12364479Sbinkertn@umich.edu<li>On the first occurrence of an error, the user-defined <tt>p_error()</tt> function 12374479Sbinkertn@umich.eduis called with the offending token as an argument. Afterwards, the parser enters 12384479Sbinkertn@umich.eduan "error-recovery" mode in which it will not make future calls to <tt>p_error()</tt> until it 12394479Sbinkertn@umich.eduhas successfully shifted at least 3 tokens onto the parsing stack. 12404479Sbinkertn@umich.edu 12414479Sbinkertn@umich.edu<p> 12424479Sbinkertn@umich.edu<li>If no recovery action is taken in <tt>p_error()</tt>, the offending lookahead token is replaced 12434479Sbinkertn@umich.eduwith a special <tt>error</tt> token. 12444479Sbinkertn@umich.edu 12454479Sbinkertn@umich.edu<p> 12464479Sbinkertn@umich.edu<li>If the offending lookahead token is already set to <tt>error</tt>, the top item of the parsing stack is 12474479Sbinkertn@umich.edudeleted. 12484479Sbinkertn@umich.edu 12494479Sbinkertn@umich.edu<p> 12504479Sbinkertn@umich.edu<li>If the entire parsing stack is unwound, the parser enters a restart state and attempts to start 12514479Sbinkertn@umich.eduparsing from its initial state. 12524479Sbinkertn@umich.edu 12534479Sbinkertn@umich.edu<p> 12544479Sbinkertn@umich.edu<li>If a grammar rule accepts <tt>error</tt> as a token, it will be 12554479Sbinkertn@umich.edushifted onto the parsing stack. 12564479Sbinkertn@umich.edu 12574479Sbinkertn@umich.edu<p> 12584479Sbinkertn@umich.edu<li>If the top item of the parsing stack is <tt>error</tt>, lookahead tokens will be discarded until the 12594479Sbinkertn@umich.eduparser can successfully shift a new symbol or reduce a rule involving <tt>error</tt>. 12604479Sbinkertn@umich.edu</ol> 12614479Sbinkertn@umich.edu 12624479Sbinkertn@umich.edu<h4>Recovery and resynchronization with error rules</h4> 12634479Sbinkertn@umich.edu 12644479Sbinkertn@umich.eduThe most well-behaved approach for handling syntax errors is to write grammar rules that include the <tt>error</tt> 12654479Sbinkertn@umich.edutoken. For example, suppose your language had a grammar rule for a print statement like this: 12664479Sbinkertn@umich.edu 12674479Sbinkertn@umich.edu<blockquote> 12684479Sbinkertn@umich.edu<pre> 12694479Sbinkertn@umich.edudef p_statement_print(t): 12704479Sbinkertn@umich.edu 'statement : PRINT expr SEMI' 12714479Sbinkertn@umich.edu ... 12724479Sbinkertn@umich.edu</pre> 12734479Sbinkertn@umich.edu</blockquote> 12744479Sbinkertn@umich.edu 12754479Sbinkertn@umich.eduTo account for the possibility of a bad expression, you might write an additional grammar rule like this: 12764479Sbinkertn@umich.edu 12774479Sbinkertn@umich.edu<blockquote> 12784479Sbinkertn@umich.edu<pre> 12794479Sbinkertn@umich.edudef p_statement_print_error(t): 12804479Sbinkertn@umich.edu 'statement : PRINT error SEMI' 12814479Sbinkertn@umich.edu print "Syntax error in print statement. Bad expression" 12824479Sbinkertn@umich.edu 12834479Sbinkertn@umich.edu</pre> 12844479Sbinkertn@umich.edu</blockquote> 12854479Sbinkertn@umich.edu 12864479Sbinkertn@umich.eduIn this case, the <tt>error</tt> token will match any sequence of 12874479Sbinkertn@umich.edutokens that might appear up to the first semicolon that is 12884479Sbinkertn@umich.eduencountered. Once the semicolon is reached, the rule will be 12894479Sbinkertn@umich.eduinvoked and the <tt>error</tt> token will go away. 12904479Sbinkertn@umich.edu 12914479Sbinkertn@umich.edu<p> 12924479Sbinkertn@umich.eduThis type of recovery is sometimes known as parser resynchronization. 12934479Sbinkertn@umich.eduThe <tt>error</tt> token acts as a wildcard for any bad input text and 12944479Sbinkertn@umich.eduthe token immediately following <tt>error</tt> acts as a 12954479Sbinkertn@umich.edusynchronization token. 12964479Sbinkertn@umich.edu 12974479Sbinkertn@umich.edu<p> 12984479Sbinkertn@umich.eduIt is important to note that the <tt>error</tt> token usually does not appear as the last token 12994479Sbinkertn@umich.eduon the right in an error rule. For example: 13004479Sbinkertn@umich.edu 13014479Sbinkertn@umich.edu<blockquote> 13024479Sbinkertn@umich.edu<pre> 13034479Sbinkertn@umich.edudef p_statement_print_error(t): 13044479Sbinkertn@umich.edu 'statement : PRINT error' 13054479Sbinkertn@umich.edu print "Syntax error in print statement. Bad expression" 13064479Sbinkertn@umich.edu</pre> 13074479Sbinkertn@umich.edu</blockquote> 13084479Sbinkertn@umich.edu 13094479Sbinkertn@umich.eduThis is because the first bad token encountered will cause the rule to 13104479Sbinkertn@umich.edube reduced--which may make it difficult to recover if more bad tokens 13114479Sbinkertn@umich.eduimmediately follow. 13124479Sbinkertn@umich.edu 13134479Sbinkertn@umich.edu<h4>Panic mode recovery</h4> 13142632Sstever@eecs.umich.edu 13152632Sstever@eecs.umich.eduAn alternative error recovery scheme is to enter a panic mode recovery in which tokens are 13162632Sstever@eecs.umich.edudiscarded to a point where the parser might be able to recover in some sensible manner. 13172632Sstever@eecs.umich.edu 13182632Sstever@eecs.umich.edu<p> 13194479Sbinkertn@umich.eduPanic mode recovery is implemented entirely in the <tt>p_error()</tt> function. For example, this 13202632Sstever@eecs.umich.edufunction starts discarding tokens until it reaches a closing '}'. Then, it restarts the 13212632Sstever@eecs.umich.eduparser in its initial state. 13224479Sbinkertn@umich.edu 13234479Sbinkertn@umich.edu<blockquote> 13244479Sbinkertn@umich.edu<pre> 13254479Sbinkertn@umich.edudef p_error(t): 13264479Sbinkertn@umich.edu print "Whoa. You are seriously hosed." 13274479Sbinkertn@umich.edu # Read ahead looking for a closing '}' 13284479Sbinkertn@umich.edu while 1: 13294479Sbinkertn@umich.edu tok = yacc.token() # Get the next token 13304479Sbinkertn@umich.edu if not tok or tok.type == 'RBRACE': break 13314479Sbinkertn@umich.edu yacc.restart() 13322632Sstever@eecs.umich.edu</pre> 13332632Sstever@eecs.umich.edu</blockquote> 13344479Sbinkertn@umich.edu 13354479Sbinkertn@umich.edu<p> 13362632Sstever@eecs.umich.eduThis function simply discards the bad token and tells the parser that the error was ok. 13372632Sstever@eecs.umich.edu 13382632Sstever@eecs.umich.edu<blockquote> 13394479Sbinkertn@umich.edu<pre> 13404479Sbinkertn@umich.edudef p_error(t): 13412632Sstever@eecs.umich.edu print "Syntax error at token", t.type 13422632Sstever@eecs.umich.edu # Just discard the token and tell the parser it's okay. 13432632Sstever@eecs.umich.edu yacc.errok() 13442632Sstever@eecs.umich.edu</pre> 13452632Sstever@eecs.umich.edu</blockquote> 13462632Sstever@eecs.umich.edu 13472632Sstever@eecs.umich.edu<P> 13482632Sstever@eecs.umich.eduWithin the <tt>p_error()</tt> function, three functions are available to control the behavior 13492632Sstever@eecs.umich.eduof the parser: 13502632Sstever@eecs.umich.edu<p> 13512632Sstever@eecs.umich.edu<ul> 13522632Sstever@eecs.umich.edu<li><tt>yacc.errok()</tt>. This resets the parser state so it doesn't think it's in error-recovery 13532632Sstever@eecs.umich.edumode. This will prevent an <tt>error</tt> token from being generated and will reset the internal 13542632Sstever@eecs.umich.eduerror counters so that the next syntax error will call <tt>p_error()</tt> again. 13552632Sstever@eecs.umich.edu 13562632Sstever@eecs.umich.edu<p> 13572632Sstever@eecs.umich.edu<li><tt>yacc.token()</tt>. This returns the next token on the input stream. 13582632Sstever@eecs.umich.edu 13594479Sbinkertn@umich.edu<p> 13604479Sbinkertn@umich.edu<li><tt>yacc.restart()</tt>. This discards the entire parsing stack and resets the parser 13614479Sbinkertn@umich.eduto its initial state. 13624479Sbinkertn@umich.edu</ul> 13634479Sbinkertn@umich.edu 13642632Sstever@eecs.umich.eduNote: these functions are only available when invoking <tt>p_error()</tt> and are not available 13652632Sstever@eecs.umich.eduat any other time. 13662632Sstever@eecs.umich.edu 13672632Sstever@eecs.umich.edu<p> 13682632Sstever@eecs.umich.eduTo supply the next lookahead token to the parser, <tt>p_error()</tt> can return a token. This might be 13692632Sstever@eecs.umich.eduuseful if trying to synchronize on special characters. For example: 13702632Sstever@eecs.umich.edu 13712632Sstever@eecs.umich.edu<blockquote> 13722632Sstever@eecs.umich.edu<pre> 13732632Sstever@eecs.umich.edudef p_error(t): 13742632Sstever@eecs.umich.edu # Read ahead looking for a terminating ";" 13752632Sstever@eecs.umich.edu while 1: 13762632Sstever@eecs.umich.edu tok = yacc.token() # Get the next token 13772632Sstever@eecs.umich.edu if not tok or tok.type == 'SEMI': break 13782632Sstever@eecs.umich.edu yacc.errok() 13792632Sstever@eecs.umich.edu 13802632Sstever@eecs.umich.edu # Return SEMI to the parser as the next lookahead token 13812632Sstever@eecs.umich.edu return tok 13822632Sstever@eecs.umich.edu</pre> 13832632Sstever@eecs.umich.edu</blockquote> 13842632Sstever@eecs.umich.edu 13852632Sstever@eecs.umich.edu<h4>General comments on error handling</h4> 13862632Sstever@eecs.umich.edu 13872632Sstever@eecs.umich.eduFor normal types of languages, error recovery with error rules and resynchronization characters is probably the most reliable 13884479Sbinkertn@umich.edutechnique. This is because you can instrument the grammar to catch errors at selected places where it is relatively easy 13894479Sbinkertn@umich.eduto recover and continue parsing. Panic mode recovery is really only useful in certain specialized applications where you might want 13904479Sbinkertn@umich.eduto discard huge portions of the input text to find a valid restart point. 13914479Sbinkertn@umich.edu 13924479Sbinkertn@umich.edu<h2>Line Number Tracking</h2> 13934479Sbinkertn@umich.edu 13942632Sstever@eecs.umich.edu<tt>yacc.py</tt> automatically tracks line numbers for all of the grammar symbols and tokens it processes. To retrieve the line 13952632Sstever@eecs.umich.edunumbers, two functions are used in grammar rules: 13962632Sstever@eecs.umich.edu 13972632Sstever@eecs.umich.edu<ul> 13982632Sstever@eecs.umich.edu<li><tt>t.lineno(num)</tt>. Return the starting line number for symbol <em>num</em> 13992632Sstever@eecs.umich.edu<li><tt>t.linespan(num)</tt>. Return a tuple (startline,endline) with the starting and ending line number for symbol <em>num</em>. 14002632Sstever@eecs.umich.edu</ul> 14012632Sstever@eecs.umich.edu 14022632Sstever@eecs.umich.eduFor example: 14032632Sstever@eecs.umich.edu 14042632Sstever@eecs.umich.edu<blockquote> 14052632Sstever@eecs.umich.edu<pre> 14062632Sstever@eecs.umich.edudef t_expression(t): 14072632Sstever@eecs.umich.edu 'expression : expression PLUS expression' 14082632Sstever@eecs.umich.edu t.lineno(1) # Line number of the left expression 14092632Sstever@eecs.umich.edu t.lineno(2) # line number of the PLUS operator 14102632Sstever@eecs.umich.edu t.lineno(3) # line number of the right expression 14112632Sstever@eecs.umich.edu ... 14122632Sstever@eecs.umich.edu start,end = t.linespan(3) # Start,end lines of the right expression 14132632Sstever@eecs.umich.edu 14142632Sstever@eecs.umich.edu</pre> 14152632Sstever@eecs.umich.edu</blockquote> 14162632Sstever@eecs.umich.edu 14172632Sstever@eecs.umich.eduSince line numbers are managed internally by the parser, there is usually no need to modify the line 14182632Sstever@eecs.umich.edunumbers. However, if you want to save the line numbers in a parse-tree node, you will need to make your own 14192632Sstever@eecs.umich.eduprivate copy. 14202632Sstever@eecs.umich.edu 14212632Sstever@eecs.umich.edu<h2>AST Construction</h2> 14222632Sstever@eecs.umich.edu 14232632Sstever@eecs.umich.edu<tt>yacc.py</tt> provides no special functions for constructing an abstract syntax tree. However, such 14242632Sstever@eecs.umich.educonstruction is easy enough to do on your own. Simply create a data structure for abstract syntax tree nodes 14252632Sstever@eecs.umich.eduand assign nodes to <tt>t[0]</tt> in each rule. 14262632Sstever@eecs.umich.edu 14272632Sstever@eecs.umich.eduFor example: 14282632Sstever@eecs.umich.edu 14292632Sstever@eecs.umich.edu<blockquote> 14302632Sstever@eecs.umich.edu<pre> 14312632Sstever@eecs.umich.educlass Expr: pass 14322632Sstever@eecs.umich.edu 14332632Sstever@eecs.umich.educlass BinOp(Expr): 14342632Sstever@eecs.umich.edu def __init__(self,left,op,right): 14352632Sstever@eecs.umich.edu self.type = "binop" 14362632Sstever@eecs.umich.edu self.left = left 14372632Sstever@eecs.umich.edu self.right = right 14382632Sstever@eecs.umich.edu self.op = op 14392632Sstever@eecs.umich.edu 14402632Sstever@eecs.umich.educlass Number(Expr): 14412632Sstever@eecs.umich.edu def __init__(self,value): 14422632Sstever@eecs.umich.edu self.type = "number" 14432632Sstever@eecs.umich.edu self.value = value 14444479Sbinkertn@umich.edu 14454479Sbinkertn@umich.edudef p_expression_binop(t): 14462632Sstever@eecs.umich.edu '''expression : expression PLUS expression 14472632Sstever@eecs.umich.edu | expression MINUS expression 14482632Sstever@eecs.umich.edu | expression TIMES expression 14494479Sbinkertn@umich.edu | expression DIVIDE expression''' 14504479Sbinkertn@umich.edu 14514479Sbinkertn@umich.edu t[0] = BinOp(t[1],t[2],t[3]) 14524479Sbinkertn@umich.edu 14534479Sbinkertn@umich.edudef p_expression_group(t): 14544479Sbinkertn@umich.edu 'expression : LPAREN expression RPAREN' 14554479Sbinkertn@umich.edu t[0] = t[2] 14562632Sstever@eecs.umich.edu 14572632Sstever@eecs.umich.edudef p_expression_number(t): 14582632Sstever@eecs.umich.edu 'expression : NUMBER' 14592632Sstever@eecs.umich.edu t[0] = Number(t[1]) 14602632Sstever@eecs.umich.edu</pre> 14612632Sstever@eecs.umich.edu</blockquote> 14622632Sstever@eecs.umich.edu 14632632Sstever@eecs.umich.eduTo simplify tree traversal, it may make sense to pick a very generic tree structure for your parse tree nodes. 14644479Sbinkertn@umich.eduFor example: 14652632Sstever@eecs.umich.edu 14662632Sstever@eecs.umich.edu<blockquote> 14672632Sstever@eecs.umich.edu<pre> 14682632Sstever@eecs.umich.educlass Node: 14694479Sbinkertn@umich.edu def __init__(self,type,children=None,leaf=None): 14702632Sstever@eecs.umich.edu self.type = type 14714479Sbinkertn@umich.edu if children: 14724479Sbinkertn@umich.edu self.children = children 14734479Sbinkertn@umich.edu else: 14742632Sstever@eecs.umich.edu self.children = [ ] 14754479Sbinkertn@umich.edu self.leaf = leaf 14764479Sbinkertn@umich.edu 14774479Sbinkertn@umich.edudef p_expression_binop(t): 14782632Sstever@eecs.umich.edu '''expression : expression PLUS expression 14794479Sbinkertn@umich.edu | expression MINUS expression 14804479Sbinkertn@umich.edu | expression TIMES expression 14814479Sbinkertn@umich.edu | expression DIVIDE expression''' 14822632Sstever@eecs.umich.edu 14834479Sbinkertn@umich.edu t[0] = Node("binop", [t[1],t[3]], t[2]) 14844479Sbinkertn@umich.edu</pre> 14854479Sbinkertn@umich.edu</blockquote> 14862632Sstever@eecs.umich.edu 14874479Sbinkertn@umich.edu<h2>Yacc implementation notes</h2> 14884479Sbinkertn@umich.edu 14894479Sbinkertn@umich.edu<ul> 14902632Sstever@eecs.umich.edu<li>By default, <tt>yacc.py</tt> relies on <tt>lex.py</tt> for tokenizing. However, an alternative tokenizer 14914479Sbinkertn@umich.educan be supplied as follows: 14924479Sbinkertn@umich.edu 14934479Sbinkertn@umich.edu<blockquote> 14942632Sstever@eecs.umich.edu<pre> 14954479Sbinkertn@umich.eduyacc.parse(lexer=x) 14964479Sbinkertn@umich.edu</pre> 14974479Sbinkertn@umich.edu</blockquote> 14982632Sstever@eecs.umich.eduin this case, <tt>x</tt> must be a Lexer object that minimally has a <tt>x.token()</tt> method for retrieving the next 14994479Sbinkertn@umich.edutoken. If an input string is given to <tt>yacc.parse()</tt>, the lexer must also have an <tt>x.input()</tt> method. 15002632Sstever@eecs.umich.edu 15012632Sstever@eecs.umich.edu<p> 15024479Sbinkertn@umich.edu<li>By default, the yacc generates tables in debugging mode (which produces the parser.out file and other output). 15032632Sstever@eecs.umich.eduTo disable this, use 15042632Sstever@eecs.umich.edu 15052632Sstever@eecs.umich.edu<blockquote> 15062632Sstever@eecs.umich.edu<pre> 15072632Sstever@eecs.umich.eduyacc.yacc(debug=0) 15084479Sbinkertn@umich.edu</pre> 15094479Sbinkertn@umich.edu</blockquote> 15104479Sbinkertn@umich.edu 15112632Sstever@eecs.umich.edu<p> 15122632Sstever@eecs.umich.edu<li>To change the name of the <tt>parsetab.py</tt> file, use: 15132632Sstever@eecs.umich.edu 15142632Sstever@eecs.umich.edu<blockquote> 15152632Sstever@eecs.umich.edu<pre> 15162632Sstever@eecs.umich.eduyacc.yacc(tabmodule="foo") 15172632Sstever@eecs.umich.edu</pre> 15182632Sstever@eecs.umich.edu</blockquote> 15192632Sstever@eecs.umich.edu 15202632Sstever@eecs.umich.edu<P> 15212632Sstever@eecs.umich.edu<li>To print copious amounts of debugging during parsing, use: 15222632Sstever@eecs.umich.edu 15234479Sbinkertn@umich.edu<blockquote> 15244479Sbinkertn@umich.edu<pre> 15254479Sbinkertn@umich.eduyacc.parse(debug=1) 15262632Sstever@eecs.umich.edu</pre> 15272632Sstever@eecs.umich.edu</blockquote> 15282632Sstever@eecs.umich.edu 15294479Sbinkertn@umich.edu<p> 15302632Sstever@eecs.umich.edu<li>The <tt>yacc.yacc()</tt> function really returns a parser object. If you want to support multiple 15312632Sstever@eecs.umich.eduparsers in the same application, do this: 15324479Sbinkertn@umich.edu 15334479Sbinkertn@umich.edu<blockquote> 15344479Sbinkertn@umich.edu<pre> 15352632Sstever@eecs.umich.edup = yacc.yacc() 15362632Sstever@eecs.umich.edu... 15372632Sstever@eecs.umich.edup.parse() 15384479Sbinkertn@umich.edu</pre> 15394479Sbinkertn@umich.edu</blockquote> 15402632Sstever@eecs.umich.edu 15414479Sbinkertn@umich.eduNote: The function <tt>yacc.parse()</tt> is bound to the last parser that was generated. 15422632Sstever@eecs.umich.edu 15432632Sstever@eecs.umich.edu<p> 15442632Sstever@eecs.umich.edu<li>Since the generation of the SLR tables is relatively expensive, previously generated tables are 15452632Sstever@eecs.umich.educached and reused if possible. The decision to regenerate the tables is determined by taking an MD5 15462632Sstever@eecs.umich.educhecksum of all grammar rules and precedence rules. Only in the event of a mismatch are the tables regenerated. 15472632Sstever@eecs.umich.edu 15484479Sbinkertn@umich.edu<p> 15494479Sbinkertn@umich.eduIt should be noted that table generation is reasonably efficient, even for grammars that involve around a 100 rules 15504479Sbinkertn@umich.eduand several hundred states. For more complex languages such as C, table generation may take 30-60 seconds on a slow 15514479Sbinkertn@umich.edumachine. Please be patient. 15524479Sbinkertn@umich.edu 15532632Sstever@eecs.umich.edu<p> 15542632Sstever@eecs.umich.edu<li>Since LR parsing is mostly driven by tables, the performance of the parser is largely independent of the 15552632Sstever@eecs.umich.edusize of the grammar. The biggest bottlenecks will be the lexer and the complexity of your grammar rules. 15562632Sstever@eecs.umich.edu</ul> 15572632Sstever@eecs.umich.edu 15584479Sbinkertn@umich.edu<h2>Parser and Lexer State Management</h2> 15594479Sbinkertn@umich.edu 15604479Sbinkertn@umich.eduIn advanced parsing applications, you may want to have multiple 15614479Sbinkertn@umich.eduparsers and lexers. Furthermore, the parser may want to control the 15622632Sstever@eecs.umich.edubehavior of the lexer in some way. 15632632Sstever@eecs.umich.edu 15642632Sstever@eecs.umich.edu<p> 15652632Sstever@eecs.umich.eduTo do this, it is important to note that both the lexer and parser are 15662632Sstever@eecs.umich.eduactually implemented as objects. These objects are returned by the 15672632Sstever@eecs.umich.edu<tt>lex()</tt> and <tt>yacc()</tt> functions respectively. For example: 15682632Sstever@eecs.umich.edu 15692632Sstever@eecs.umich.edu<blockquote> 15702632Sstever@eecs.umich.edu<pre> 15712632Sstever@eecs.umich.edulexer = lex.lex() # Return lexer object 15722632Sstever@eecs.umich.eduparser = yacc.yacc() # Return parser object 15734479Sbinkertn@umich.edu</pre> 15742632Sstever@eecs.umich.edu</blockquote> 15752632Sstever@eecs.umich.edu 15762632Sstever@eecs.umich.eduWithin lexer and parser rules, these objects are also available. In the lexer, 15772632Sstever@eecs.umich.eduthe "lexer" attribute of a token refers to the lexer object in use. For example: 15782632Sstever@eecs.umich.edu 15792632Sstever@eecs.umich.edu<blockquote> 15802632Sstever@eecs.umich.edu<pre> 15812632Sstever@eecs.umich.edudef t_NUMBER(t): 15822632Sstever@eecs.umich.edu r'\d+' 15832632Sstever@eecs.umich.edu ... 15842632Sstever@eecs.umich.edu print t.lexer # Show lexer object 15854479Sbinkertn@umich.edu</pre> 15862632Sstever@eecs.umich.edu</blockquote> 15872632Sstever@eecs.umich.edu 15882632Sstever@eecs.umich.eduIn the parser, the "lexer" and "parser" attributes refer to the lexer 15892632Sstever@eecs.umich.eduand parser objects respectively. 15902632Sstever@eecs.umich.edu 15912632Sstever@eecs.umich.edu<blockquote> 15922632Sstever@eecs.umich.edu<pre> 15932632Sstever@eecs.umich.edudef p_expr_plus(t): 15942632Sstever@eecs.umich.edu 'expr : expr PLUS expr' 15952632Sstever@eecs.umich.edu ... 15962632Sstever@eecs.umich.edu print t.parser # Show parser object 15972632Sstever@eecs.umich.edu print t.lexer # Show lexer object 15982632Sstever@eecs.umich.edu</pre> 15992632Sstever@eecs.umich.edu</blockquote> 16002632Sstever@eecs.umich.edu 16012632Sstever@eecs.umich.eduIf necessary, arbitrary attributes can be attached to the lexer or parser object. 16024479Sbinkertn@umich.eduFor example, if you wanted to have different parsing modes, you could attach a mode 16034479Sbinkertn@umich.eduattribute to the parser object and look at it later. 16042632Sstever@eecs.umich.edu 16052632Sstever@eecs.umich.edu<h2>Using Python's Optimized Mode</h2> 16062632Sstever@eecs.umich.edu 16072632Sstever@eecs.umich.eduBecause PLY uses information from doc-strings, parsing and lexing 16082632Sstever@eecs.umich.eduinformation must be gathered while running the Python interpreter in 16092632Sstever@eecs.umich.edunormal mode (i.e., not with the -O or -OO options). However, if you 16104479Sbinkertn@umich.eduspecify optimized mode like this: 16112632Sstever@eecs.umich.edu 16124479Sbinkertn@umich.edu<blockquote> 16132632Sstever@eecs.umich.edu<pre> 16142632Sstever@eecs.umich.edulex.lex(optimize=1) 16152632Sstever@eecs.umich.eduyacc.yacc(optimize=1) 16164479Sbinkertn@umich.edu</pre> 16172632Sstever@eecs.umich.edu</blockquote> 16182632Sstever@eecs.umich.edu 16192632Sstever@eecs.umich.eduthen PLY can later be used when Python runs in optimized mode. To make this work, 16202632Sstever@eecs.umich.edumake sure you first run Python in normal mode. Once the lexing and parsing tables 16212632Sstever@eecs.umich.eduhave been generated the first time, run Python in optimized mode. PLY will use 16222632Sstever@eecs.umich.eduthe tables without the need for doc strings. 16232632Sstever@eecs.umich.edu 16244479Sbinkertn@umich.edu<p> 16252632Sstever@eecs.umich.eduBeware: running PLY in optimized mode disables a lot of error 16262632Sstever@eecs.umich.educhecking. You should only do this when your project has stabilized 16274479Sbinkertn@umich.eduand you don't need to do any debugging. 16284479Sbinkertn@umich.edu 16294479Sbinkertn@umich.edu<h2>Where to go from here?</h2> 16304479Sbinkertn@umich.edu 16312632Sstever@eecs.umich.eduThe <tt>examples</tt> directory of the PLY distribution contains several simple examples. Please consult a 16322632Sstever@eecs.umich.educompilers textbook for the theory and underlying implementation details or LR parsing. 16332632Sstever@eecs.umich.edu 16342632Sstever@eecs.umich.edu</body> 16352632Sstever@eecs.umich.edu</html> 16362632Sstever@eecs.umich.edu 16372632Sstever@eecs.umich.edu 16382632Sstever@eecs.umich.edu 16394479Sbinkertn@umich.edu 16402632Sstever@eecs.umich.edu 16412632Sstever@eecs.umich.edu 16422632Sstever@eecs.umich.edu 16432632Sstever@eecs.umich.edu