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-<html>

-<head>

-<title>The Lemon Parser Generator</title>

-</head>

-<body bgcolor=white>

-<h1 align=center>The Lemon Parser Generator</h1>

-<p>Lemon is an LALR(1) parser generator for C or C++.

-It does the same job as ``bison'' and ``yacc''.

-But lemon is not another bison or yacc clone. It

-uses a different grammar syntax which is designed to

-reduce the number of coding errors. Lemon also uses a more

-sophisticated parsing engine that is faster than yacc and

-bison and which is both reentrant and thread-safe.

-Furthermore, Lemon implements features that can be used

-to eliminate resource leaks, making is suitable for use

-in long-running programs such as graphical user interfaces

-or embedded controllers.</p>

-<p>This document is an introduction to the Lemon

-parser generator.</p>

-<h2>Theory of Operation</h2>

-<p>The main goal of Lemon is to translate a context free grammar (CFG)

-for a particular language into C code that implements a parser for

-that language.

-The program has two inputs:

-<ul>

-<li>The grammar specification.

-<li>A parser template file.

-</ul>

-Typically, only the grammar specification is supplied by the programmer.

-Lemon comes with a default parser template which works fine for most

-applications. But the user is free to substitute a different parser

-template if desired.</p>

-<p>Depending on command-line options, Lemon will generate between

-one and three files of outputs.

-<ul>

-<li>C code to implement the parser.

-<li>A header file defining an integer ID for each terminal symbol.

-<li>An information file that describes the states of the generated parser

- automaton.

-</ul>

-By default, all three of these output files are generated.

-The header file is suppressed if the ``-m'' command-line option is

-used and the report file is omitted when ``-q'' is selected.</p>

-<p>The grammar specification file uses a ``.y'' suffix, by convention.

-In the examples used in this document, we'll assume the name of the

-grammar file is ``gram.y''. A typical use of Lemon would be the

-following command:

-<pre>

- lemon gram.y

-</pre>

-This command will generate three output files named ``gram.c'',

-``gram.h'' and ``gram.out''.

-The first is C code to implement the parser. The second

-is the header file that defines numerical values for all

-terminal symbols, and the last is the report that explains

-the states used by the parser automaton.</p>

-<h3>Command Line Options</h3>

-<p>The behavior of Lemon can be modified using command-line options.

-You can obtain a list of the available command-line options together

-with a brief explanation of what each does by typing

-<pre>

- lemon -?

-</pre>

-As of this writing, the following command-line options are supported:

-<ul>

-<li><tt>-b</tt>

-<li><tt>-c</tt>

-<li><tt>-g</tt>

-<li><tt>-m</tt>

-<li><tt>-q</tt>

-<li><tt>-s</tt>

-<li><tt>-x</tt>

-</ul>

-The ``-b'' option reduces the amount of text in the report file by

-printing only the basis of each parser state, rather than the full

-configuration.

-The ``-c'' option suppresses action table compression. Using -c

-will make the parser a little larger and slower but it will detect

-syntax errors sooner.

-The ``-g'' option causes no output files to be generated at all.

-Instead, the input grammar file is printed on standard output but

-with all comments, actions and other extraneous text deleted. This

-is a useful way to get a quick summary of a grammar.

-The ``-m'' option causes the output C source file to be compatible

-with the ``makeheaders'' program.

-Makeheaders is a program that automatically generates header files

-from C source code. When the ``-m'' option is used, the header

-file is not output since the makeheaders program will take care

-of generated all header files automatically.

-The ``-q'' option suppresses the report file.

-Using ``-s'' causes a brief summary of parser statistics to be

-printed. Like this:

-<pre>

- Parser statistics: 74 terminals, 70 nonterminals, 179 rules

- 340 states, 2026 parser table entries, 0 conflicts

-</pre>

-Finally, the ``-x'' option causes Lemon to print its version number

-and then stops without attempting to read the grammar or generate a parser.</p>

-<h3>The Parser Interface</h3>

-<p>Lemon doesn't generate a complete, working program. It only generates

-a few subroutines that implement a parser. This section describes

-the interface to those subroutines. It is up to the programmer to

-call these subroutines in an appropriate way in order to produce a

-complete system.</p>

-<p>Before a program begins using a Lemon-generated parser, the program

-must first create the parser.

-A new parser is created as follows:

-<pre>

- void *pParser = ParseAlloc( malloc );

-</pre>

-The ParseAlloc() routine allocates and initializes a new parser and

-returns a pointer to it.

-The actual data structure used to represent a parser is opaque --

-its internal structure is not visible or usable by the calling routine.

-For this reason, the ParseAlloc() routine returns a pointer to void

-rather than a pointer to some particular structure.

-The sole argument to the ParseAlloc() routine is a pointer to the

-subroutine used to allocate memory. Typically this means ``malloc()''.</p>

-<p>After a program is finished using a parser, it can reclaim all

-memory allocated by that parser by calling

-<pre>

- ParseFree(pParser, free);

-</pre>

-The first argument is the same pointer returned by ParseAlloc(). The

-second argument is a pointer to the function used to release bulk

-memory back to the system.</p>

-<p>After a parser has been allocated using ParseAlloc(), the programmer

-must supply the parser with a sequence of tokens (terminal symbols) to

-be parsed. This is accomplished by calling the following function

-once for each token:

-<pre>

- Parse(pParser, hTokenID, sTokenData, pArg);

-</pre>

-The first argument to the Parse() routine is the pointer returned by

-ParseAlloc().

-The second argument is a small positive integer that tells the parse the

-type of the next token in the data stream.

-There is one token type for each terminal symbol in the grammar.

-The gram.h file generated by Lemon contains #define statements that

-map symbolic terminal symbol names into appropriate integer values.

-(A value of 0 for the second argument is a special flag to the

-parser to indicate that the end of input has been reached.)

-The third argument is the value of the given token. By default,

-the type of the third argument is integer, but the grammar will

-usually redefine this type to be some kind of structure.

-Typically the second argument will be a broad category of tokens

-such as ``identifier'' or ``number'' and the third argument will

-be the name of the identifier or the value of the number.</p>

-<p>The Parse() function may have either three or four arguments,

-depending on the grammar. If the grammar specification file request

-it, the Parse() function will have a fourth parameter that can be

-of any type chosen by the programmer. The parser doesn't do anything

-with this argument except to pass it through to action routines.

-This is a convenient mechanism for passing state information down

-to the action routines without having to use global variables.</p>

-<p>A typical use of a Lemon parser might look something like the

-following:

-<pre>

- 01 ParseTree *ParseFile(const char *zFilename){

- 02 Tokenizer *pTokenizer;

- 03 void *pParser;

- 04 Token sToken;

- 05 int hTokenId;

- 06 ParserState sState;

- 07

- 08 pTokenizer = TokenizerCreate(zFilename);

- 09 pParser = ParseAlloc( malloc );

- 10 InitParserState(&sState);

- 11 while( GetNextToken(pTokenizer, &hTokenId, &sToken) ){

- 12 Parse(pParser, hTokenId, sToken, &sState);

- 13 }

- 14 Parse(pParser, 0, sToken, &sState);

- 15 ParseFree(pParser, free );

- 16 TokenizerFree(pTokenizer);

- 17 return sState.treeRoot;

- 18 }

-</pre>

-This example shows a user-written routine that parses a file of

-text and returns a pointer to the parse tree.

-(We've omitted all error-handling from this example to keep it

-simple.)

-We assume the existence of some kind of tokenizer which is created

-using TokenizerCreate() on line 8 and deleted by TokenizerFree()

-on line 16. The GetNextToken() function on line 11 retrieves the

-next token from the input file and puts its type in the

-integer variable hTokenId. The sToken variable is assumed to be

-some kind of structure that contains details about each token,

-such as its complete text, what line it occurs on, etc. </p>

-<p>This example also assumes the existence of structure of type

-ParserState that holds state information about a particular parse.

-An instance of such a structure is created on line 6 and initialized

-on line 10. A pointer to this structure is passed into the Parse()

-routine as the optional 4th argument.

-The action routine specified by the grammar for the parser can use

-the ParserState structure to hold whatever information is useful and

-appropriate. In the example, we note that the treeRoot field of

-the ParserState structure is left pointing to the root of the parse

-tree.</p>

-<p>The core of this example as it relates to Lemon is as follows:

-<pre>

- ParseFile(){

- pParser = ParseAlloc( malloc );

- while( GetNextToken(pTokenizer,&hTokenId, &sToken) ){

- Parse(pParser, hTokenId, sToken);

- }

- Parse(pParser, 0, sToken);

- ParseFree(pParser, free );

- }

-</pre>

-Basically, what a program has to do to use a Lemon-generated parser

-is first create the parser, then send it lots of tokens obtained by

-tokenizing an input source. When the end of input is reached, the

-Parse() routine should be called one last time with a token type

-of 0. This step is necessary to inform the parser that the end of

-input has been reached. Finally, we reclaim memory used by the

-parser by calling ParseFree().</p>

-<p>There is one other interface routine that should be mentioned

-before we move on.

-The ParseTrace() function can be used to generate debugging output

-from the parser. A prototype for this routine is as follows:

-<pre>

- ParseTrace(FILE *stream, char *zPrefix);

-</pre>

-After this routine is called, a short (one-line) message is written

-to the designated output stream every time the parser changes states

-or calls an action routine. Each such message is prefaced using

-the text given by zPrefix. This debugging output can be turned off

-by calling ParseTrace() again with a first argument of NULL (0).</p>

-<h3>Differences With YACC and BISON</h3>

-<p>Programmers who have previously used the yacc or bison parser

-generator will notice several important differences between yacc and/or

-bison and Lemon.

-<ul>

-<li>In yacc and bison, the parser calls the tokenizer. In Lemon,

- the tokenizer calls the parser.

-<li>Lemon uses no global variables. Yacc and bison use global variables

- to pass information between the tokenizer and parser.

-<li>Lemon allows multiple parsers to be running simultaneously. Yacc

- and bison do not.

-</ul>

-These differences may cause some initial confusion for programmers

-with prior yacc and bison experience.

-But after years of experience using Lemon, I firmly

-believe that the Lemon way of doing things is better.</p>

-<h2>Input File Syntax</h2>

-<p>The main purpose of the grammar specification file for Lemon is

-to define the grammar for the parser. But the input file also

-specifies additional information Lemon requires to do its job.

-Most of the work in using Lemon is in writing an appropriate

-grammar file.</p>

-<p>The grammar file for lemon is, for the most part, free format.

-It does not have sections or divisions like yacc or bison. Any

-declaration can occur at any point in the file.

-Lemon ignores whitespace (except where it is needed to separate

-tokens) and it honors the same commenting conventions as C and C++.</p>

-<h3>Terminals and Nonterminals</h3>

-<p>A terminal symbol (token) is any string of alphanumeric

-and underscore characters

-that begins with an upper case letter.

-A terminal can contain lowercase letters after the first character,

-but the usual convention is to make terminals all upper case.

-A nonterminal, on the other hand, is any string of alphanumeric

-and underscore characters than begins with a lower case letter.

-Again, the usual convention is to make nonterminals use all lower

-case letters.</p>

-<p>In Lemon, terminal and nonterminal symbols do not need to

-be declared or identified in a separate section of the grammar file.

-Lemon is able to generate a list of all terminals and nonterminals

-by examining the grammar rules, and it can always distinguish a

-terminal from a nonterminal by checking the case of the first

-character of the name.</p>

-<p>Yacc and bison allow terminal symbols to have either alphanumeric

-names or to be individual characters included in single quotes, like

-this: ')' or '$'. Lemon does not allow this alternative form for

-terminal symbols. With Lemon, all symbols, terminals and nonterminals,

-must have alphanumeric names.</p>

-<h3>Grammar Rules</h3>

-<p>The main component of a Lemon grammar file is a sequence of grammar

-rules.

-Each grammar rule consists of a nonterminal symbol followed by

-the special symbol ``::='' and then a list of terminals and/or nonterminals.

-The rule is terminated by a period.

-The list of terminals and nonterminals on the right-hand side of the

-rule can be empty.

-Rules can occur in any order, except that the left-hand side of the

-first rule is assumed to be the start symbol for the grammar (unless

-specified otherwise using the <tt>%start</tt> directive described below.)

-A typical sequence of grammar rules might look something like this:

-<pre>

- expr ::= expr PLUS expr.

- expr ::= expr TIMES expr.

- expr ::= LPAREN expr RPAREN.

- expr ::= VALUE.

-</pre>

-</p>

-<p>There is one non-terminal in this example, ``expr'', and five

-terminal symbols or tokens: ``PLUS'', ``TIMES'', ``LPAREN'',

-``RPAREN'' and ``VALUE''.</p>

-<p>Like yacc and bison, Lemon allows the grammar to specify a block

-of C code that will be executed whenever a grammar rule is reduced

-by the parser.

-In Lemon, this action is specified by putting the C code (contained

-within curly braces <tt>{...}</tt>) immediately after the

-period that closes the rule.

-For example:

-<pre>

- expr ::= expr PLUS expr. { printf("Doing an addition...\n"); }

-</pre>

-</p>

-<p>In order to be useful, grammar actions must normally be linked to

-their associated grammar rules.

-In yacc and bison, this is accomplished by embedding a ``$$'' in the

-action to stand for the value of the left-hand side of the rule and

-symbols ``$1'', ``$2'', and so forth to stand for the value of

-the terminal or nonterminal at position 1, 2 and so forth on the

-right-hand side of the rule.

-This idea is very powerful, but it is also very error-prone. The

-single most common source of errors in a yacc or bison grammar is

-to miscount the number of symbols on the right-hand side of a grammar

-rule and say ``$7'' when you really mean ``$8''.</p>

-<p>Lemon avoids the need to count grammar symbols by assigning symbolic

-names to each symbol in a grammar rule and then using those symbolic

-names in the action.

-In yacc or bison, one would write this:

-<pre>

- expr -> expr PLUS expr { $$ = $1 + $3; };

-</pre>

-But in Lemon, the same rule becomes the following:

-<pre>

- expr(A) ::= expr(B) PLUS expr(C). { A = B+C; }

-</pre>

-In the Lemon rule, any symbol in parentheses after a grammar rule

-symbol becomes a place holder for that symbol in the grammar rule.

-This place holder can then be used in the associated C action to

-stand for the value of that symbol.<p>

-<p>The Lemon notation for linking a grammar rule with its reduce

-action is superior to yacc/bison on several counts.

-First, as mentioned above, the Lemon method avoids the need to

-count grammar symbols.

-Secondly, if a terminal or nonterminal in a Lemon grammar rule

-includes a linking symbol in parentheses but that linking symbol

-is not actually used in the reduce action, then an error message

-is generated.

-For example, the rule

-<pre>

- expr(A) ::= expr(B) PLUS expr(C). { A = B; }

-</pre>

-will generate an error because the linking symbol ``C'' is used

-in the grammar rule but not in the reduce action.</p>

-<p>The Lemon notation for linking grammar rules to reduce actions

-also facilitates the use of destructors for reclaiming memory

-allocated by the values of terminals and nonterminals on the

-right-hand side of a rule.</p>

-<h3>Precedence Rules</h3>

-<p>Lemon resolves parsing ambiguities in exactly the same way as

-yacc and bison. A shift-reduce conflict is resolved in favor

-of the shift, and a reduce-reduce conflict is resolved by reducing

-whichever rule comes first in the grammar file.</p>

-<p>Just like in

-yacc and bison, Lemon allows a measure of control

-over the resolution of paring conflicts using precedence rules.

-A precedence value can be assigned to any terminal symbol

-using the %left, %right or %nonassoc directives. Terminal symbols

-mentioned in earlier directives have a lower precedence that

-terminal symbols mentioned in later directives. For example:</p>

-<p><pre>

- %left AND.

- %left OR.

- %nonassoc EQ NE GT GE LT LE.

- %left PLUS MINUS.

- %left TIMES DIVIDE MOD.

- %right EXP NOT.

-</pre></p>

-<p>In the preceding sequence of directives, the AND operator is

-defined to have the lowest precedence. The OR operator is one

-precedence level higher. And so forth. Hence, the grammar would

-attempt to group the ambiguous expression

-<pre>

- a AND b OR c

-</pre>

-like this

-<pre>

- a AND (b OR c).

-</pre>

-The associativity (left, right or nonassoc) is used to determine

-the grouping when the precedence is the same. AND is left-associative

-in our example, so

-<pre>

- a AND b AND c

-</pre>

-is parsed like this

-<pre>

- (a AND b) AND c.

-</pre>

-The EXP operator is right-associative, though, so

-<pre>

- a EXP b EXP c

-</pre>

-is parsed like this

-<pre>

- a EXP (b EXP c).

-</pre>

-The nonassoc precedence is used for non-associative operators.

-So

-<pre>

- a EQ b EQ c

-</pre>

-is an error.</p>

-<p>The precedence of non-terminals is transferred to rules as follows:

-The precedence of a grammar rule is equal to the precedence of the

-left-most terminal symbol in the rule for which a precedence is

-defined. This is normally what you want, but in those cases where

-you want to precedence of a grammar rule to be something different,

-you can specify an alternative precedence symbol by putting the

-symbol in square braces after the period at the end of the rule and

-before any C-code. For example:</p>

-<p><pre>

- expr = MINUS expr. [NOT]

-</pre></p>

-<p>This rule has a precedence equal to that of the NOT symbol, not the

-MINUS symbol as would have been the case by default.</p>

-<p>With the knowledge of how precedence is assigned to terminal

-symbols and individual

-grammar rules, we can now explain precisely how parsing conflicts

-are resolved in Lemon. Shift-reduce conflicts are resolved

-as follows:

-<ul>

-<li> If either the token to be shifted or the rule to be reduced

- lacks precedence information, then resolve in favor of the

- shift, but report a parsing conflict.

-<li> If the precedence of the token to be shifted is greater than

- the precedence of the rule to reduce, then resolve in favor

- of the shift. No parsing conflict is reported.

-<li> If the precedence of the token it be shifted is less than the

- precedence of the rule to reduce, then resolve in favor of the

- reduce action. No parsing conflict is reported.

-<li> If the precedences are the same and the shift token is

- right-associative, then resolve in favor of the shift.

- No parsing conflict is reported.

-<li> If the precedences are the same the shift token is

- left-associative, then resolve in favor of the reduce.

- No parsing conflict is reported.

-<li> Otherwise, resolve the conflict by doing the shift and

- report the parsing conflict.

-</ul>

-Reduce-reduce conflicts are resolved this way:

-<ul>

-<li> If either reduce rule

- lacks precedence information, then resolve in favor of the

- rule that appears first in the grammar and report a parsing

- conflict.

-<li> If both rules have precedence and the precedence is different

- then resolve the dispute in favor of the rule with the highest

- precedence and do not report a conflict.

-<li> Otherwise, resolve the conflict by reducing by the rule that

- appears first in the grammar and report a parsing conflict.

-</ul>

-<h3>Special Directives</h3>

-<p>The input grammar to Lemon consists of grammar rules and special

-directives. We've described all the grammar rules, so now we'll

-talk about the special directives.</p>

-<p>Directives in lemon can occur in any order. You can put them before

-the grammar rules, or after the grammar rules, or in the mist of the

-grammar rules. It doesn't matter. The relative order of

-directives used to assign precedence to terminals is important, but

-other than that, the order of directives in Lemon is arbitrary.</p>

-<p>Lemon supports the following special directives:

-<ul>

-<li><tt>%code</tt>

-<li><tt>%default_destructor</tt>

-<li><tt>%default_type</tt>

-<li><tt>%destructor</tt>

-<li><tt>%extra_argument</tt>

-<li><tt>%include</tt>

-<li><tt>%left</tt>

-<li><tt>%name</tt>

-<li><tt>%nonassoc</tt>

-<li><tt>%parse_accept</tt>

-<li><tt>%parse_failure </tt>

-<li><tt>%right</tt>

-<li><tt>%stack_overflow</tt>

-<li><tt>%stack_size</tt>

-<li><tt>%start_symbol</tt>

-<li><tt>%syntax_error</tt>

-<li><tt>%token_destructor</tt>

-<li><tt>%token_prefix</tt>

-<li><tt>%token_type</tt>

-<li><tt>%type</tt>

-</ul>

-Each of these directives will be described separately in the

-following sections:</p>

-<h4>The <tt>%code</tt> directive</h4>

-<p>The %code directive is used to specify addition C/C++ code that

-is added to the end of the main output file. This is similar to

-the %include directive except that %include is inserted at the

-beginning of the main output file.</p>

-<p>%code is typically used to include some action routines or perhaps

-a tokenizer as part of the output file.</p>

-<h4>The <tt>%default_destructor</tt> directive</h4>

-<p>The %default_destructor directive specifies a destructor to

-use for non-terminals that do not have their own destructor

-specified by a separate %destructor directive. See the documentation

-on the %destructor directive below for additional information.</p>

-<p>In some grammers, many different non-terminal symbols have the

-same datatype and hence the same destructor. This directive is

-a convenience way to specify the same destructor for all those

-non-terminals using a single statement.</p>

-<h4>The <tt>%default_type</tt> directive</h4>

-<p>The %default_type directive specifies the datatype of non-terminal

-symbols that do no have their own datatype defined using a separate

-%type directive. See the documentation on %type below for addition

-information.</p>

-<h4>The <tt>%destructor</tt> directive</h4>

-<p>The %destructor directive is used to specify a destructor for

-a non-terminal symbol.

-(See also the %token_destructor directive which is used to

-specify a destructor for terminal symbols.)</p>

-<p>A non-terminal's destructor is called to dispose of the

-non-terminal's value whenever the non-terminal is popped from

-the stack. This includes all of the following circumstances:

-<ul>

-<li> When a rule reduces and the value of a non-terminal on

- the right-hand side is not linked to C code.

-<li> When the stack is popped during error processing.

-<li> When the ParseFree() function runs.

-</ul>

-The destructor can do whatever it wants with the value of

-the non-terminal, but its design is to deallocate memory

-or other resources held by that non-terminal.</p>

-<p>Consider an example:

-<pre>

- %type nt {void*}

- %destructor nt { free($$); }

- nt(A) ::= ID NUM. { A = malloc( 100 ); }

-</pre>

-This example is a bit contrived but it serves to illustrate how

-destructors work. The example shows a non-terminal named

-``nt'' that holds values of type ``void*''. When the rule for

-an ``nt'' reduces, it sets the value of the non-terminal to

-space obtained from malloc(). Later, when the nt non-terminal

-is popped from the stack, the destructor will fire and call

-free() on this malloced space, thus avoiding a memory leak.

-(Note that the symbol ``$$'' in the destructor code is replaced

-by the value of the non-terminal.)</p>

-<p>It is important to note that the value of a non-terminal is passed

-to the destructor whenever the non-terminal is removed from the

-stack, unless the non-terminal is used in a C-code action. If

-the non-terminal is used by C-code, then it is assumed that the

-C-code will take care of destroying it if it should really

-be destroyed. More commonly, the value is used to build some

-larger structure and we don't want to destroy it, which is why

-the destructor is not called in this circumstance.</p>

-<p>By appropriate use of destructors, it is possible to

-build a parser using Lemon that can be used within a long-running

-program, such as a GUI, that will not leak memory or other resources.

-To do the same using yacc or bison is much more difficult.</p>

-<h4>The <tt>%extra_argument</tt> directive</h4>

-The %extra_argument directive instructs Lemon to add a 4th parameter

-to the parameter list of the Parse() function it generates. Lemon

-doesn't do anything itself with this extra argument, but it does

-make the argument available to C-code action routines, destructors,

-and so forth. For example, if the grammar file contains:</p>

-<p><pre>

- %extra_argument { MyStruct *pAbc }

-</pre></p>

-<p>Then the Parse() function generated will have an 4th parameter

-of type ``MyStruct*'' and all action routines will have access to

-a variable named ``pAbc'' that is the value of the 4th parameter

-in the most recent call to Parse().</p>

-<h4>The <tt>%include</tt> directive</h4>

-<p>The %include directive specifies C code that is included at the

-top of the generated parser. You can include any text you want --

-the Lemon parser generator copies it blindly. If you have multiple

-%include directives in your grammar file the value of the last

-%include directive overwrites all the others.</p.

-<p>The %include directive is very handy for getting some extra #include

-preprocessor statements at the beginning of the generated parser.

-For example:</p>

-<p><pre>

- %include {#include <unistd.h>}

-</pre></p>

-<p>This might be needed, for example, if some of the C actions in the

-grammar call functions that are prototyed in unistd.h.</p>

-<h4>The <tt>%left</tt> directive</h4>

-The %left directive is used (along with the %right and

-%nonassoc directives) to declare precedences of terminal

-symbols. Every terminal symbol whose name appears after

-a %left directive but before the next period (``.'') is

-given the same left-associative precedence value. Subsequent

-%left directives have higher precedence. For example:</p>

-<p><pre>

- %left AND.

- %left OR.

- %nonassoc EQ NE GT GE LT LE.

- %left PLUS MINUS.

- %left TIMES DIVIDE MOD.

- %right EXP NOT.

-</pre></p>

-<p>Note the period that terminates each %left, %right or %nonassoc

-directive.</p>

-<p>LALR(1) grammars can get into a situation where they require

-a large amount of stack space if you make heavy use or right-associative

-operators. For this reason, it is recommended that you use %left

-rather than %right whenever possible.</p>

-<h4>The <tt>%name</tt> directive</h4>

-<p>By default, the functions generated by Lemon all begin with the

-five-character string ``Parse''. You can change this string to something

-different using the %name directive. For instance:</p>

-<p><pre>

- %name Abcde

-</pre></p>

-<p>Putting this directive in the grammar file will cause Lemon to generate

-functions named

-<ul>

-<li> AbcdeAlloc(),

-<li> AbcdeFree(),

-<li> AbcdeTrace(), and

-<li> Abcde().

-</ul>

-The %name directive allows you to generator two or more different

-parsers and link them all into the same executable.

-</p>

-<h4>The <tt>%nonassoc</tt> directive</h4>

-<p>This directive is used to assign non-associative precedence to

-one or more terminal symbols. See the section on precedence rules

-or on the %left directive for additional information.</p>

-<h4>The <tt>%parse_accept</tt> directive</h4>

-<p>The %parse_accept directive specifies a block of C code that is

-executed whenever the parser accepts its input string. To ``accept''

-an input string means that the parser was able to process all tokens

-without error.</p>

-<p>For example:</p>

-<p><pre>

- %parse_accept {

- printf("parsing complete!\n");

- }

-</pre></p>

-<h4>The <tt>%parse_failure</tt> directive</h4>

-<p>The %parse_failure directive specifies a block of C code that

-is executed whenever the parser fails complete. This code is not

-executed until the parser has tried and failed to resolve an input

-error using is usual error recovery strategy. The routine is

-only invoked when parsing is unable to continue.</p>

-<p><pre>

- %parse_failure {

- fprintf(stderr,"Giving up. Parser is hopelessly lost...\n");

- }

-</pre></p>

-<h4>The <tt>%right</tt> directive</h4>

-<p>This directive is used to assign right-associative precedence to

-one or more terminal symbols. See the section on precedence rules

-or on the %left directive for additional information.</p>

-<h4>The <tt>%stack_overflow</tt> directive</h4>

-<p>The %stack_overflow directive specifies a block of C code that

-is executed if the parser's internal stack ever overflows. Typically

-this just prints an error message. After a stack overflow, the parser

-will be unable to continue and must be reset.</p>

-<p><pre>

- %stack_overflow {

- fprintf(stderr,"Giving up. Parser stack overflow\n");

- }

-</pre></p>

-<p>You can help prevent parser stack overflows by avoiding the use

-of right recursion and right-precedence operators in your grammar.

-Use left recursion and and left-precedence operators instead, to

-encourage rules to reduce sooner and keep the stack size down.

-For example, do rules like this:

-<pre>

- list ::= list element. // left-recursion. Good!

- list ::= .

-</pre>

-Not like this:

-<pre>

- list ::= element list. // right-recursion. Bad!

- list ::= .

-</pre>

-<h4>The <tt>%stack_size</tt> directive</h4>

-<p>If stack overflow is a problem and you can't resolve the trouble

-by using left-recursion, then you might want to increase the size

-of the parser's stack using this directive. Put an positive integer

-after the %stack_size directive and Lemon will generate a parse

-with a stack of the requested size. The default value is 100.</p>

-<p><pre>

- %stack_size 2000

-</pre></p>

-<h4>The <tt>%start_symbol</tt> directive</h4>

-<p>By default, the start-symbol for the grammar that Lemon generates

-is the first non-terminal that appears in the grammar file. But you

-can choose a different start-symbol using the %start_symbol directive.</p>

-<p><pre>

- %start_symbol prog

-</pre></p>

-<h4>The <tt>%token_destructor</tt> directive</h4>

-<p>The %destructor directive assigns a destructor to a non-terminal

-symbol. (See the description of the %destructor directive above.)

-This directive does the same thing for all terminal symbols.</p>

-<p>Unlike non-terminal symbols which may each have a different data type

-for their values, terminals all use the same data type (defined by

-the %token_type directive) and so they use a common destructor. Other

-than that, the token destructor works just like the non-terminal

-destructors.</p>

-<h4>The <tt>%token_prefix</tt> directive</h4>

-<p>Lemon generates #defines that assign small integer constants

-to each terminal symbol in the grammar. If desired, Lemon will

-add a prefix specified by this directive

-to each of the #defines it generates.

-So if the default output of Lemon looked like this:

-<pre>

- #define AND 1

- #define MINUS 2

- #define OR 3

- #define PLUS 4

-</pre>

-You can insert a statement into the grammar like this:

-<pre>

- %token_prefix TOKEN_

-</pre>

-to cause Lemon to produce these symbols instead:

-<pre>

- #define TOKEN_AND 1

- #define TOKEN_MINUS 2

- #define TOKEN_OR 3

- #define TOKEN_PLUS 4

-</pre>

-<h4>The <tt>%token_type</tt> and <tt>%type</tt> directives</h4>

-<p>These directives are used to specify the data types for values

-on the parser's stack associated with terminal and non-terminal

-symbols. The values of all terminal symbols must be of the same

-type. This turns out to be the same data type as the 3rd parameter

-to the Parse() function generated by Lemon. Typically, you will

-make the value of a terminal symbol by a pointer to some kind of

-token structure. Like this:</p>

-<p><pre>

- %token_type {Token*}

-</pre></p>

-<p>If the data type of terminals is not specified, the default value

-is ``int''.</p>

-<p>Non-terminal symbols can each have their own data types. Typically

-the data type of a non-terminal is a pointer to the root of a parse-tree

-structure that contains all information about that non-terminal.

-For example:</p>

-<p><pre>

- %type expr {Expr*}

-</pre></p>

-<p>Each entry on the parser's stack is actually a union containing

-instances of all data types for every non-terminal and terminal symbol.

-Lemon will automatically use the correct element of this union depending

-on what the corresponding non-terminal or terminal symbol is. But

-the grammar designer should keep in mind that the size of the union

-will be the size of its largest element. So if you have a single

-non-terminal whose data type requires 1K of storage, then your 100

-entry parser stack will require 100K of heap space. If you are willing

-and able to pay that price, fine. You just need to know.</p>

-<h3>Error Processing</h3>

-<p>After extensive experimentation over several years, it has been

-discovered that the error recovery strategy used by yacc is about

-as good as it gets. And so that is what Lemon uses.</p>

-<p>When a Lemon-generated parser encounters a syntax error, it

-first invokes the code specified by the %syntax_error directive, if

-any. It then enters its error recovery strategy. The error recovery

-strategy is to begin popping the parsers stack until it enters a

-state where it is permitted to shift a special non-terminal symbol

-named ``error''. It then shifts this non-terminal and continues

-parsing. But the %syntax_error routine will not be called again

-until at least three new tokens have been successfully shifted.</p>

-<p>If the parser pops its stack until the stack is empty, and it still

-is unable to shift the error symbol, then the %parse_failed routine

-is invoked and the parser resets itself to its start state, ready

-to begin parsing a new file. This is what will happen at the very

-first syntax error, of course, if there are no instances of the

-``error'' non-terminal in your grammar.</p>

-</body>

-</html>