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
* 3.2.1 Allocating The Parse Object On Stack
* 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_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 <unistd.h>}
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_class directive
Undocumented. Appears to be related to the MULTITERMINAL concept.
Implementation.
4.4.21 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.
4.4.22 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
4.4.23 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.
4.4.24 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.
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