https://sqlite.org/src/doc/trunk/doc/lemon.html
The Lemon Parser Generator
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 it suitable for use in
long-running programs such as graphical user interfaces or embedded
controllers.
This document is an introduction to the Lemon parser generator.
1.0 Table of Contents
* Introduction
* 1.0 Table of Contents
* 2.0 Security Notes
* 3.0 Theory of Operation
+ 3.1 Command Line Options
+ 3.2 The Parser Interface
o 3.2.1 Allocating The Parse Object On Stack
o 3.2.2 Interface Summary
+ 3.3 Differences With YACC and BISON
+ 3.4 Building The "lemon" Or "lemon.exe" Executable
* 4.0 Input File Syntax
+ 4.1 Terminals and Nonterminals
+ 4.2 Grammar Rules
+ 4.3 Precedence Rules
+ 4.4 Special Directives
* 5.0 Error Processing
* 6.0 History of Lemon
* 7.0 Copyright
2.0 Security Note
The language parser code created by Lemon is very robust and is
well-suited for use in internet-facing applications that need to
safely process maliciously crafted inputs.
The "lemon.exe" command-line tool itself works great when given a
valid input grammar file and almost always gives helpful error
messages for malformed inputs. However, it is possible for a
malicious user to craft a grammar file that will cause lemon.exe to
crash. We do not see this as a problem, as lemon.exe is not intended
to be used with hostile inputs. To summarize:
* Parser code generated by lemon - Robust and secure
* The "lemon.exe" command line tool itself - Not so much
3.0 Theory of Operation
Lemon is computer program that translates a context free grammar
(CFG) for a particular language into C code that implements a parser
for that language. The Lemon program has two inputs:
* The grammar specification.
* A parser template file.
Typically, only the grammar specification is supplied by the
programmer. Lemon comes with a default parser template ("lempar.c")
that works fine for most applications. But the user is free to
substitute a different parser template if desired.
Depending on command-line options, Lemon will generate up to three
output files.
* C code to implement a parser for the input grammar.
* A header file defining an integer ID for each terminal symbol (or
"token").
* 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.
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.
3.1 Command Line Options
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:
* -b Show only the basis for each parser state in the report file.
* -c Do not compress the generated action tables. The parser will
be a little larger and slower, but it will detect syntax errors
sooner.
* -ddirectory Write all output files into directory. Normally,
output files are written into the directory that contains the
input grammar file.
* -Dname Define C preprocessor macro name. This macro is usable by
"%ifdef", "%ifndef", and "%if lines in the grammar file.
* -E Run the "%if" preprocessor step only and print the revised
grammar file.
* -g Do not generate a parser. Instead write the input grammar to
standard output with all comments, actions, and other extraneous
text removed.
* -l Omit "#line" directives in the generated parser C code.
* -m Cause the output C source code to be compatible with the
"makeheaders" program.
* -p Display all conflicts that are resolved by precedence rules.
* -q Suppress generation of the report file.
* -r Do not sort or renumber the parser states as part of
optimization.
* -s Show parser statistics before exiting.
* -Tfile Use file as the template for the generated C-code parser
implementation.
* -x Print the Lemon version number.
3.2 The Parser Interface
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.
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().
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.
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 parser 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 "void*", 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.
The Parse() function may have either three or four arguments,
depending on the grammar. If the grammar specification file requests
it (via the %extra_argument directive), 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.
A typical use of a Lemon parser might look something like the
following:
1 ParseTree *ParseFile(const char *zFilename){
2 Tokenizer *pTokenizer;
3 void *pParser;
4 Token sToken;
5 int hTokenId;
6 ParserState sState;
7
8 pTokenizer = TokenizerCreate(zFilename);
9 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. (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, such as its complete text, what line it occurs on, etc.
This example also assumes the existence of a 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.
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().
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).
3.2.1 Allocating The Parse Object On Stack
If all calls to the Parse() interface are made from within %code
directives, then the parse object can be allocated from the stack
rather than from the heap. These are the steps:
* Declare a local variable of type "yyParser"
* Initialize the variable using ParseInit()
* Pass a pointer to the variable in calls ot Parse()
* Deallocate substructure in the parse variable using ParseFinalize
().
The following code illustrates how this is done:
ParseFile(){
yyParser x;
ParseInit( &x );
while( GetNextToken(pTokenizer,&hTokenId, &sToken) ){
Parse(&x, hTokenId, sToken);
}
Parse(&x, 0, sToken);
ParseFinalize( &x );
}
3.2.2 Interface Summary
Here is a quick overview of the C-language interface to a
Lemon-generated parser:
void *ParseAlloc( (void*(*malloc)(size_t) );
void ParseFree(void *pParser, (void(*free)(void*) );
void Parse(void *pParser, int tokenCode, ParseTOKENTYPE token, ...);
void ParseTrace(FILE *stream, char *zPrefix);
Notes:
* Use the %name directive to change the "Parse" prefix names of the
procedures in the interface.
* Use the %token_type directive to define the "ParseTOKENTYPE"
type.
* Use the %extra_argument directive to specify the type and name of
the 4th parameter to the Parse() function.
3.3 Differences With YACC and BISON
Programmers who have previously used the yacc or bison parser
generator will notice several important differences between yacc and/
or bison and Lemon.
* In yacc and bison, the parser calls the tokenizer. In Lemon, the
tokenizer calls the parser.
* Lemon uses no global variables. Yacc and bison use global
variables to pass information between the tokenizer and parser.
* 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.
Updated as of 2016-02-16: The text above was written in the 1990s. We
are told that Bison has lately been enhanced to support the
tokenizer-calls-parser paradigm used by Lemon, eliminating the need
for global variables.
3.4 Building The "lemon" or "lemon.exe" Executable
The "lemon" or "lemon.exe" program is built from a single file of
C-code named "lemon.c". The Lemon source code is generic C89 code
that uses no unusual or non-standard libraries. Any reasonable C
compiler should suffice to compile the lemon program. A command-line
like the following will usually work:
cc -o lemon lemon.c
On Windows machines with Visual C++ installed, bring up a "VS20NN x64
Native Tools Command Prompt" window and enter:
cl lemon.c
Compiling Lemon really is that simple. Additional compiler options
such as "-O2" or "-g" or "-Wall" can be added if desired, but they
are not necessary.
4.0 Input File Syntax
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.
The grammar file for Lemon is, for the most part, a 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++.
4.1 Terminals and Nonterminals
A terminal symbol (token) is any string of alphanumeric and/or
underscore characters that begins with an uppercase letter. A
terminal can contain lowercase letters after the first character, but
the usual convention is to make terminals all uppercase. A
nonterminal, on the other hand, is any string of alphanumeric and
underscore characters than begins with a lowercase letter. Again, the
usual convention is to make nonterminals use all lowercase letters.
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.
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.
4.2 Grammar Rules
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 %start_symbol 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.
There is one non-terminal in this example, "expr", and five terminal
symbols or tokens: "PLUS", "TIMES", "LPAREN", "RPAREN" and "VALUE".
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 {...}) immediately after the period
that closes the rule. For example:
expr ::= expr PLUS expr. { printf("Doing an addition...\n"); }
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 "$$" in the action to stand for the value
of the left-hand side of the rule and symbols "$1", "$2", and so
forth to stand for the value of the terminal or nonterminal at
position 1, 2 and so forth on the right-hand side of the rule. This
idea is very powerful, but it is also very error-prone. The single
most common source of errors in a yacc or bison grammar is to
miscount the number of symbols on the right-hand side of a grammar
rule and say "$7" when you really mean "$8".
Lemon avoids the need to count grammar symbols by assigning symbolic
names to each symbol in a grammar rule and then using those symbolic
names in the action. In yacc or bison, one would write this:
expr -> expr PLUS expr { $$ = $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.
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.
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.
4.3 Precedence Rules
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.
Just like in yacc and bison, Lemon allows a measure of control over
the resolution of parsing 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 than terminal symbols
mentioned in later directives. For example:
%left AND.
%left OR.
%nonassoc EQ NE GT GE LT LE.
%left PLUS MINUS.
%left TIMES DIVIDE MOD.
%right EXP NOT.
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.
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 the 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:
expr = MINUS expr. [NOT]
This rule has a precedence equal to that of the NOT symbol, not the
MINUS symbol as would have been the case by default.
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:
* 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.
* 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.
* If the precedence of the token to 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.
* 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.
* If the precedences are the same and the shift token is
left-associative, then resolve in favor of the reduce. No parsing
conflict is reported.
* Otherwise, resolve the conflict by doing the shift, and report a
parsing conflict.
Reduce-reduce conflicts are resolved this way:
* 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.
* 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.
* Otherwise, resolve the conflict by reducing by the rule that
appears first in the grammar, and report a parsing conflict.
4.4 Special Directives
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.
Directives in Lemon can occur in any order. You can put them before
the grammar rules, or after the grammar rules, or in the midst 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.
Lemon supports the following special directives:
* %code
* %default_destructor
* %default_type
* %destructor
* %else
* %endif
* %extra_argument
* %fallback
* %if
* %ifdef
* %ifndef
* %include
* %left
* %name
* %nonassoc
* %parse_accept
* %parse_failure
* %right
* %stack_overflow
* %stack_size
* %start_symbol
* %syntax_error
* %token
* %token_class
* %token_destructor
* %token_prefix
* %token_type
* %type
* %wildcard
Each of these directives will be described separately in the
following sections:
4.4.1 The %code directive
The %code directive is used to specify additional 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.
%code is typically used to include some action routines or perhaps a
tokenizer or even the "main()" function as part of the output file.
There can be multiple %code directives. The arguments of all %code
directives are concatenated.
4.4.2 The %default_destructor directive
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.
In some grammars, many different non-terminal symbols have the same
data type and hence the same destructor. This directive is a
convenient way to specify the same destructor for all those
non-terminals using a single statement.
4.4.3 The %default_type directive
The %default_type directive specifies the data type of non-terminal
symbols that do not have their own data type defined using a separate
%type directive.
4.4.4 The %destructor directive
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.)
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:
* When a rule reduces and the value of a non-terminal on the
right-hand side is not linked to C code.
* When the stack is popped during error processing.
* 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.
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.)
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.
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.
4.4.5 The %extra_argument directive
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:
%extra_argument { MyStruct *pAbc }
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().
The %extra_context directive works the same except that it is passed
in on the ParseAlloc() or ParseInit() routines instead of on Parse().
4.4.6 The %extra_context directive
The %extra_context directive instructs Lemon to add a 2nd parameter
to the parameter list of the ParseAlloc() and ParseInit() functions.
Lemon doesn't do anything itself with these extra argument, but it
does store the value make it available to C-code action routines,
destructors, and so forth. For example, if the grammar file contains:
%extra_context { MyStruct *pAbc }
Then the ParseAlloc() and ParseInit() functions will have an 2nd
parameter of type "MyStruct*" and all action routines will have
access to a variable named "pAbc" that is the value of that 2nd
parameter.
The %extra_argument directive works the same except that it is passed
in on the Parse() routine instead of on ParseAlloc()/ParseInit().
4.4.7 The %fallback directive
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.
The %fallback directive was added to support robust parsing of SQL
syntax in SQLite. 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."
The syntax of %fallback is as follows:
%fallback ID TOKEN... .
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.
4.4.8 The %if directive and its friends
The %if, %ifdef, %ifndef, %else, and %endif directives are similar to
#if, #ifdef, #ifndef, #else, 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.
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.
The text in between "%if CONDITIONAL" and its corresponding %endif is
included only if CONDITIONAL is true. The CONDITION is one or more
macro names, optionally connected using the "||" and "&&" binary
operators, the "!" unary operator, and grouped using balanced
parentheses. Each term is true if the corresponding macro exists, and
false if it does not exist.
An optional "%else" directive can occur anywhere in between a %ifdef,
%ifndef, or %if directive and its corresponding %endif.
Note that the argument to %ifdef and %ifndef is intended to be a
single preprocessor symbol name, not a general expression. Use the
"%if" directive for general expressions.
4.4.9 The %include directive
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
grammar.
The %include directive is very handy for getting some extra #include
preprocessor statements at the beginning of the generated parser. For
example:
%include {#include }
This might be needed, for example, if some of the C actions in the
grammar call functions that are prototyped in unistd.h.
Use the %code directive to add code to the end of the generated
parser.
4.4.10 The %left directive
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:
%left AND.
%left OR.
%nonassoc EQ NE GT GE LT LE.
%left PLUS MINUS.
%left TIMES DIVIDE MOD.
%right EXP NOT.
Note the period that terminates each %left, %right or %nonassoc
directive.
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.
4.4.11 The %name directive
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:
%name Abcde
Putting this directive in the grammar file will cause Lemon to
generate functions named
* AbcdeAlloc(),
* AbcdeFree(),
* AbcdeTrace(), and
* Abcde().
The %name directive allows you to generate two or more different
parsers and link them all into the same executable.
4.4.12 The %nonassoc directive
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.
4.4.13 The %parse_accept directive
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.
For example:
%parse_accept {
printf("parsing complete!\n");
}
4.4.14 The %parse_failure directive
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.
%parse_failure {
fprintf(stderr,"Giving up. Parser is hopelessly lost...\n");
}
4.4.15 The %right directive
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.
4.4.16 The %stack_overflow directive
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.
%stack_overflow {
fprintf(stderr,"Giving up. Parser stack overflow\n");
}
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 ::= .
4.4.17 The %stack_size directive
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.
%stack_size 2000
4.4.18 The %start_symbol directive
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.
%start_symbol prog
4.4.19 The %syntax_error directive
See Error Processing.
4.4.20 The %token directive
Tokens are normally created automatically, the first time they are
used. Any identifier that begins with an upper-case letter is a
token.
Sometimes it is useful to declare tokens in advance, however. The
integer values assigned to each token determined by the order in
which the tokens are seen. So by declaring tokens in advance, it is
possible to cause some tokens to have low-numbered values, which
might be desirable in some grammers, or to have sequential values
assigned to a sequence of related tokens. For this reason, the %token
directive is provided to declare tokens in advance. The syntax is as
follows:
%token TOKEN TOKEN... .
The %token directive is followed by zero or more token symbols and
terminated by a single ".". Each token named is created if it does
not already exist. Tokens are created in order.
4.4.21 The %token_class directive
Undocumented. Appears to be related to the MULTITERMINAL concept.
Implementation.
4.4.22 The %token_destructor directive
The %destructor directive assigns a destructor to a non-terminal
symbol. (See the description of the %destructor directive above.) The
%token_destructor directive does the same thing for all terminal
symbols.
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.
4.4.23 The %token_prefix directive
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
4.4.24 The %token_type and %type directives
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 be a pointer to some kind of token structure. Like
this:
%token_type {Token*}
If the data type of terminals is not specified, the default value is
"void*".
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:
%type expr {Expr*}
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.
4.4.25 The %wildcard directive
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.
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
alternatives.
5.0 Error Processing
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.
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. The
%syntax_error routine will not be called again until at least three
new tokens have been successfully shifted.
If the parser pops its stack until the stack is empty, and it still
is unable to shift the error symbol, then the %parse_failure 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.
6.0 History of Lemon
Lemon was originally written by Richard Hipp sometime in the late
1980s on a Sun4 Workstation using K&R C. There was a companion LL(1)
parser generator program named "Lime", the source code to which as
been lost.
The lemon.c source file was originally many separate files that were
compiled together to generate the "lemon" executable. Sometime in the
1990s, the individual source code files were combined together into
the current single large "lemon.c" source file. You can still see
traces of original filenames in the code.
Since 2001, Lemon has been part of the SQLite project and the source
code to Lemon has been managed as a part of the SQLite source tree in
the following files:
* tool/lemon.c
* tool/lempar.c
* doc/lemon.html
7.0 Copyright
All of the source code to Lemon, including the template parser file
"lempar.c" and this documentation file ("lemon.html") are in the
public domain. You can use the code for any purpose and without
attribution.
The code comes with no warranty. If it breaks, you get to keep both
pieces.