<title>The Lemon Parser Generator</title>

<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:

<li>The grammar specification.

<li>A parser template file.

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.

<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.

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:

lemon gram.y

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

lemon -?

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

<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>

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:

Parser statistics: 74 terminals, 70 nonterminals, 179 rules

340 states, 2026 parser table entries, 0 conflicts

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:

void *pParser = ParseAlloc( malloc );

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

ParseFree(pParser, free);

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:

Parse(pParser, hTokenID, sTokenData, pArg);

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:

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 }

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:

ParseFile(){

pParser = ParseAlloc( malloc );

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

Parse(pParser, hTokenId, sToken);

}

Parse(pParser, 0, sToken);

ParseFree(pParser, free );

}

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:

ParseTrace(FILE *stream, char *zPrefix);

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.

<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.

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:

expr ::= expr PLUS expr.

expr ::= expr TIMES expr.

expr ::= LPAREN expr RPAREN.

expr ::= VALUE.

<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:

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

<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:

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

But in Lemon, the same rule becomes the following:

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

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

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

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>

%left AND.

%left OR.

%nonassoc EQ NE GT GE LT LE.

%left PLUS MINUS.

%left TIMES DIVIDE MOD.

%right EXP NOT.

<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

a AND b OR c

like this

a AND (b OR c).

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

a AND b AND c

is parsed like this

(a AND b) AND c.

The EXP operator is right-associative, though, so

a EXP b EXP c

is parsed like this

a EXP (b EXP c).

The nonassoc precedence is used for non-associative operators.

So

a EQ b EQ c

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>

expr = MINUS expr. [NOT]

<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:

<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.

Reduce-reduce conflicts are resolved this way:

<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.

<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:

<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>

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:

<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.

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:

%type nt {void*}

%destructor nt { free($$); }

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

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>

%extra_argument { MyStruct *pAbc }

<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>

%include {#include &lt;unistd.h&gt;}

<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>

%left AND.

%left OR.

%nonassoc EQ NE GT GE LT LE.

%left PLUS MINUS.

%left TIMES DIVIDE MOD.

%right EXP NOT.

<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>

%name Abcde

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

functions named

<li> AbcdeAlloc(),

<li> AbcdeFree(),

<li> AbcdeTrace(), and

<li> Abcde().

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

parsers and link them all into the same executable.

<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>

%parse_accept {

printf("parsing complete!\n");

}

<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>

%parse_failure {

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

}

<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>

%stack_overflow {

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

}

<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:

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

list ::= .

Not like this:

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

list ::= .

<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>

%stack_size 2000

<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>

%start_symbol prog

<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:

#define AND 1

#define MINUS 2

#define OR 3

#define PLUS 4

You can insert a statement into the grammar like this:

%token_prefix TOKEN_

to cause Lemon to produce these symbols instead:

#define TOKEN_AND 1

#define TOKEN_MINUS 2

#define TOKEN_OR 3

#define TOKEN_PLUS 4

<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>

%token_type {Token*}

<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>

%type expr {Expr*}

<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>