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Comment:Updates to the Lemon documentation.
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2016-03-19
18:11
Fix exclusive.test so that it works with -DSQLITE_TEMP_STORE=3. check-in: d7852c63 user: dan tags: trunk
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Updates to the Lemon documentation. check-in: f0953414 user: drh tags: trunk
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Add the sqlite3rbu_bp_progress() API to the RBU extension. Used to obtain the percentage progress of an RBU update. check-in: 209e31c7 user: dan tags: trunk
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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>

................................................................................
<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>

................................................................................
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
................................................................................
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
................................................................................
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 requests
it (via the <a href='#extraarg'><tt>extra_argument</tt> directive</a>),
the Parse() function will have a fourth parameter that can be
of any type chosen by the programmer.  The parser doesn't do anything
................................................................................
   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,
................................................................................
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>
................................................................................
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.
</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.
................................................................................
<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; };
................................................................................
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.
................................................................................

<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.
................................................................................
<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>

<a name="extraarg"></a>
<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
................................................................................
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 &lt;unistd.h&gt;}
</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.
................................................................................
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
................................................................................
<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>

................................................................................
</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 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
................................................................................
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>
................................................................................
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>
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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.
It does the same job as "bison" and "yacc".
But lemon is not a bison or yacc clone.  Lemon
uses a different grammar syntax which is designed to
reduce the number of coding errors.  Lemon also uses a
parsing engine that is faster than yacc and
bison and which is both reentrant and threadsafe.
(Update: Since the previous sentence was written, bison
has also been updated so that it too can generate a
reentrant and threadsafe parser.)
Lemon also 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>

................................................................................
<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>

................................................................................
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><b>-b</b>
Show only the basis for each parser state in the report file.
<li><b>-c</b>
Do not compress the generated action tables.
<li><b>-D<i>name</i></b>
Define C preprocessor macro <i>name</i>.  This macro is useable by
"%ifdef" lines in the grammar file.
<li><b>-g</b>
Do not generate a parser.  Instead write the input grammar to standard
output with all comments, actions, and other extraneous text removed.
<li><b>-l</b>
Omit "#line" directives int the generated parser C code.
<li><b>-m</b>
Cause the output C source code to be compatible with the "makeheaders"
program. 
<li><b>-p</b>
Display all conflicts that are resolved by 
<a href='#precrules'>precedence rules</a>.
<li><b>-q</b>
Suppress generation of the report file.
<li><b>-r</b>
Do not sort or renumber the parser states as part of optimization.
<li><b>-s</b>
Show parser statistics before existing.
<li><b>-T<i>file</i></b>
Use <i>file</i> as the template for the generated C-code parser implementation.
<li><b>-x</b>
Print the Lemon version number.
</ul>





<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
................................................................................
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 &mdash;
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
................................................................................
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 requests
it (via the <a href='#extraarg'><tt>extra_argument</tt> directive</a>),
the Parse() function will have a fourth parameter that can be
of any type chosen by the programmer.  The parser doesn't do anything
................................................................................
   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.
(All error-handling code is omitted 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,
................................................................................
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/or 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>
................................................................................
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.
</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.
................................................................................
<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; };
................................................................................
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>

<a name='precrules'></a>
<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 
<a href='#pleft'>%left</a>,
<a href='#pright'>%right</a> or
<a href='#pnonassoc'>%nonassoc</a> 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.
................................................................................

<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>%endif</tt>
<li><tt>%extra_argument</tt>
<li><tt>%fallback</tt>
<li><tt>%ifdef</tt>
<li><tt>%ifndef</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_class</tt>
<li><tt>%token_destructor</tt>
<li><tt>%token_prefix</tt>
<li><tt>%token_type</tt>
<li><tt>%type</tt>
<li><tt>%wildcard</tt>
</ul>
Each of these directives will be described separately in the
following sections:</p>

<a name='pcode'></a>
<h4>The <tt>%code</tt> directive</h4>

<p>The %code directive is used to specify addition C code that
is added to the end of the main output file.  This is similar to

the <a href='#pinclude'>%include</a> 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 or even the "main()" function 
as part of the output file.</p>

<a name='default_destructor'></a>
<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 <a name='#destructor'>%destructor</a> 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>

<a name='default_type'></a>
<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
<a href='#ptype'>%type</a> directive.  
</p>

<a name='destructor'></a>
<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 <a href='#token_destructor'>%token_destructor</a>
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.
................................................................................
<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.
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>Destructors help avoid memory leaks by automatically freeing
allocated objects when they go out of scope.

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

<a name="extraarg"></a>
<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
................................................................................
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>

<a name='pfallback'></a>
<h4>The <tt>%fallback</tt> directive</h4>

<p>The %fallback directive specifies an alternative meaning for one
or more tokens.  The alternative meaning is tried if the original token
would have generated a syntax error.

<p>The %fallback directive was added to support robust parsing of SQL
syntax in <a href="https://www.sqlite.org/">SQLite</a>.
The SQL language contains a large assortment of keywords, each of which
appears as a different token to the language parser.  SQL contains so
many keywords, that it can be difficult for programmers to keep up with
them all.  Programmers will, therefore, sometimes mistakenly use an
obscure language keyword for an identifier.  The %fallback directive
provides a mechanism to tell the parser:  "If you are unable to parse
this keyword, try treating it as an identifier instead."

<p>The syntax of %fallback is as follows:

<blockquote>
<tt>%fallback</tt>  <i>ID</i> <i>TOKEN...</i> <b>.</b>
</blockquote>

<p>In words, the %fallback directive is followed by a list of token names
terminated by a period.  The first token name is the fallback token - the
token to which all the other tokens fall back to.  The second and subsequent
arguments are tokens which fall back to the token identified by the first
argument.

<a name='pifdef'></a>
<h4>The <tt>%ifdef</tt>, <tt>%ifndef</tt>, and <tt>%endif</tt> directives.</h4>

<p>The %ifdef, %ifndef, and %endif directives are similar to
#ifdef, #ifndef, and #endif in the C-preprocessor, just not as general.
Each of these directives must begin at the left margin.  No whitespace
is allowed between the "%" and the directive name.

<p>Grammar text in between "%ifdef MACRO" and the next nested "%endif" is
ignored unless the "-DMACRO" command-line option is used.  Grammar text
betwen "%ifndef MACRO" and the next nested "%endif" is included except when
the "-DMACRO" command-line option is used.

<p>Note that the argument to %ifdef and %ifndef must be a single 
preprocessor symbol name, not a general expression.  There is no "%else"
directive.


<a name='pinclude'></a>
<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, their values are concatenated

so that all %include code ultimately appears near the top of the
generated parser, in the same order as it appeared in the grammer.</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 &lt;unistd.h&gt;}
</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>

<a name='pleft'></a>
<h4>The <tt>%left</tt> directive</h4>

The %left directive is used (along with the <a href='#pright'>%right</a> and
<a href='#pnonassoc'>%nonassoc</a> 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.
................................................................................
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>

<a name='pname'></a>
<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
................................................................................
<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>

<a name='pnonassoc'></a>
<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 
<a href='#precrules'>precedence rules</a>
or on the <a href='#pleft'>%left</a> directive for additional information.</p>

<a name='parse_accept'></a>
<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>

<a name='parse_failure'></a>
<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>

<a name='pright'></a>
<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 
<a href='#precrules'>precedence rules</a>
or on the <a href='#pleft'>%left</a> directive for additional information.</p>

<a name='stack_overflow'></a>
<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>

................................................................................
</pre>
Not like this:
<pre>
   list ::= element list.      // right-recursion.  Bad!
   list ::= .
</pre>

<a name='stack_size'></a>
<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>

<a name='start_symbol'></a>
<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>

<a name='token_destructor'></a>
<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>

<a name='token_prefix'></a>
<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 TOKEN_AND        1
    #define TOKEN_MINUS      2
    #define TOKEN_OR         3
    #define TOKEN_PLUS       4
</pre>

<a name='token_type'></a><a name='ptype'></a>
<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
................................................................................
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>
................................................................................
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>

<a name='pwildcard'></a>
<h4>The <tt>%wildcard</tt> directive</h4>

<p>The %wildcard directive is followed by a single token name and a
period.  This directive specifies that the identified token should 
match any input token.

<p>When the generated parser has the choice of matching an input against
the wildcard token and some other token, the other token is always used.
The wildcard token is only matched if there are no other alternatives.

<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>

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