| <html> |
| <head> |
| <title>The Lemon Parser Generator</title> |
| </head> |
| <body> |
| <a id="main"></a> |
| <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 it 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> |
| |
| <a id="toc"></a> |
| <h2>1.0 Table of Contents</h2> |
| <ul> |
| <li><a href="#main">Introduction</a> |
| <li><a href="#toc">1.0 Table of Contents</a> |
| <li><a href="#secnot">2.0 Security Notes</a><br> |
| <li><a href="#optheory">3.0 Theory of Operation</a> |
| <ul> |
| <li><a href="#options">3.1 Command Line Options</a> |
| <li><a href="#interface">3.2 The Parser Interface</a> |
| <ul> |
| <li><a href="#onstack">3.2.1 Allocating The Parse Object On Stack</a> |
| <li><a href="#ifsum">3.2.2 Interface Summary</a> |
| </ul> |
| <li><a href="#yaccdiff">3.3 Differences With YACC and BISON</a> |
| <li><a href="#build">3.4 Building The "lemon" Or "lemon.exe" Executable</a> |
| </ul> |
| <li><a href="#syntax">4.0 Input File Syntax</a> |
| <ul> |
| <li><a href="#tnt">4.1 Terminals and Nonterminals</a> |
| <li><a href="#rules">4.2 Grammar Rules</a> |
| <li><a href="#precrules">4.3 Precedence Rules</a> |
| <li><a href="#special">4.4 Special Directives</a> |
| </ul> |
| <li><a href="#errors">5.0 Error Processing</a> |
| <li><a href="#history">6.0 History of Lemon</a> |
| <li><a href="#copyright">7.0 Copyright</a> |
| </ul> |
| |
| <a id="secnot"></a> |
| <h2>2.0 Security Note</h2> |
| |
| <p>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.</p> |
| |
| <p>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:</p> |
| |
| <ul> |
| <li>Parser code generated by lemon → Robust and secure |
| <li>The "lemon.exe" command line tool itself → Not so much |
| </ul> |
| |
| <a id="optheory"></a> |
| <h2>3.0 Theory of Operation</h2> |
| |
| <p>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:</p> |
| <ul> |
| <li>The grammar specification. |
| <li>A parser template file. |
| </ul> |
| <p>Typically, only the grammar specification is supplied by the programmer. |
| Lemon comes with a default parser template |
| ("<a href="https://sqlite.org/src/file/tool/lempar.c">lempar.c</a>") |
| that works fine for most applications. But the user is free to substitute |
| a different parser template if desired.</p> |
| |
| <p>Depending on command-line options, Lemon will generate up to |
| three output files.</p> |
| <ul> |
| <li>C code to implement a parser for the input grammar. |
| <li>A header file defining an integer ID for each terminal symbol |
| (or "token"). |
| <li>An information file that describes the states of the generated parser |
| automaton. |
| </ul> |
| <p>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:</p> |
| <pre> |
| lemon gram.y |
| </pre> |
| <p>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> |
| |
| <a id="options"></a> |
| <h3>3.1 Command Line Options</h3> |
| |
| <p>The behavior of Lemon can be modified using command-line options. |
| You can obtain a list of the available command-line options together |
| with a brief explanation of what each does by typing</p> |
| <pre> |
| lemon "-?" |
| </pre> |
| <p>As of this writing, the following command-line options are supported:</p> |
| <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. The parser will be a |
| little larger and slower, but it will detect syntax errors sooner. |
| <li><b>-d</b><i>directory</i> |
| Write all output files into <i>directory</i>. Normally, output files |
| are written into the directory that contains the input grammar file. |
| <li><b>-D<i>name</i></b> |
| Define C preprocessor macro <i>name</i>. This macro is usable by |
| "<tt><a href='#pifdef'>%ifdef</a></tt>", |
| "<tt><a href='#pifdef'>%ifndef</a></tt>", and |
| "<tt><a href="#pifdef">%if</a></tt> lines |
| in the grammar file. |
| <li><b>-E</b> |
| Run the "%if" preprocessor step only and print the revised 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 in 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 exiting. |
| <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> |
| |
| <a id="interface"></a> |
| <h3>3.2 The Parser Interface</h3> |
| |
| <p>Lemon doesn't generate a complete, working program. It only generates |
| a few subroutines that implement a parser. This section describes |
| the interface to those subroutines. It is up to the programmer to |
| call these subroutines in an appropriate way in order to produce a |
| complete system.</p> |
| |
| <p>Before a program begins using a Lemon-generated parser, the program |
| must first create the parser. |
| A new parser is created as follows:</p> |
| <pre> |
| void *pParser = ParseAlloc( malloc ); |
| </pre> |
| <p>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</p> |
| <pre> |
| ParseFree(pParser, free); |
| </pre> |
| <p>The first argument is the same pointer returned by ParseAlloc(). The |
| second argument is a pointer to the function used to release bulk |
| memory back to the system.</p> |
| |
| <p>After a parser has been allocated using ParseAlloc(), the programmer |
| must supply the parser with a sequence of tokens (terminal symbols) to |
| be parsed. This is accomplished by calling the following function |
| once for each token:<p> |
| <pre> |
| Parse(pParser, hTokenID, sTokenData, pArg); |
| </pre> |
| <p>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.</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 <tt><a href='#extraarg'>%extra_argument</a></tt> 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.</p> |
| |
| <p>A typical use of a Lemon parser might look something like the |
| following:</p> |
| <pre> |
| 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 } |
| </pre> |
| <p>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.</p> |
| |
| <p>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.</p> |
| |
| <p>The core of this example as it relates to Lemon is as follows:</p> |
| <pre> |
| ParseFile(){ |
| pParser = ParseAlloc( malloc ); |
| while( GetNextToken(pTokenizer,&hTokenId, &sToken) ){ |
| Parse(pParser, hTokenId, sToken); |
| } |
| Parse(pParser, 0, sToken); |
| ParseFree(pParser, free ); |
| } |
| </pre> |
| <p>Basically, what a program has to do to use a Lemon-generated parser |
| is first create the parser, then send it lots of tokens obtained by |
| tokenizing an input source. When the end of input is reached, the |
| Parse() routine should be called one last time with a token type |
| of 0. This step is necessary to inform the parser that the end of |
| input has been reached. Finally, we reclaim memory used by the |
| parser by calling ParseFree().</p> |
| |
| <p>There is one other interface routine that should be mentioned |
| before we move on. |
| The ParseTrace() function can be used to generate debugging output |
| from the parser. A prototype for this routine is as follows:</p> |
| <pre> |
| ParseTrace(FILE *stream, char *zPrefix); |
| </pre> |
| <p>After this routine is called, a short (one-line) message is written |
| to the designated output stream every time the parser changes states |
| or calls an action routine. Each such message is prefaced using |
| the text given by zPrefix. This debugging output can be turned off |
| by calling ParseTrace() again with a first argument of NULL (0).</p> |
| |
| <a id="onstack"></a> |
| <h4>3.2.1 Allocating The Parse Object On Stack</h4> |
| |
| <p>If all calls to the Parse() interface are made from within |
| <a href="#pcode"><tt>%code</tt> directives</a>, then the parse |
| object can be allocated from the stack rather than from the heap. |
| These are the steps: |
| |
| <ul> |
| <li> Declare a local variable of type "yyParser" |
| <li> Initialize the variable using ParseInit() |
| <li> Pass a pointer to the variable in calls to Parse() |
| <li> Deallocate substructure in the parse variable using ParseFinalize(). |
| </ul> |
| |
| <p>The following code illustrates how this is done: |
| |
| <pre> |
| ParseFile(){ |
| yyParser x; |
| ParseInit( &x ); |
| while( GetNextToken(pTokenizer,&hTokenId, &sToken) ){ |
| Parse(&x, hTokenId, sToken); |
| } |
| Parse(&x, 0, sToken); |
| ParseFinalize( &x ); |
| } |
| </pre> |
| |
| <a id="ifsum"></a> |
| <h4>3.2.2 Interface Summary</h4> |
| |
| <p>Here is a quick overview of the C-language interface to a |
| Lemon-generated parser:</p> |
| |
| <blockquote><pre> |
| 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); |
| </pre></blockquote> |
| |
| <p>Notes:</p> |
| <ul> |
| <li> Use the <a href="#pname"><tt>%name</tt> directive</a> to change |
| the "Parse" prefix names of the procedures in the interface. |
| <li> Use the <a href="#token_type"><tt>%token_type</tt> directive</a> |
| to define the "ParseTOKENTYPE" type. |
| <li> Use the <a href="#extraarg"><tt>%extra_argument</tt> directive</a> |
| to specify the type and name of the 4th parameter to the |
| Parse() function. |
| </ul> |
| |
| <a id="yaccdiff"></a> |
| <h3>3.3 Differences With YACC and BISON</h3> |
| |
| <p>Programmers who have previously used the yacc or bison parser |
| generator will notice several important differences between yacc and/or |
| bison and Lemon.</p> |
| <ul> |
| <li>In yacc and bison, the parser calls the tokenizer. In Lemon, |
| the tokenizer calls the parser. |
| <li>Lemon uses no global variables. Yacc and bison use global variables |
| to pass information between the tokenizer and parser. |
| <li>Lemon allows multiple parsers to be running simultaneously. Yacc |
| and bison do not. |
| </ul> |
| <p>These differences may cause some initial confusion for programmers |
| with prior yacc and bison experience. |
| But after years of experience using Lemon, I firmly |
| believe that the Lemon way of doing things is better.</p> |
| |
| <p><i>Updated as of 2016-02-16:</i> |
| 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.</p> |
| |
| <a id="build"><a> |
| <h3>3.4 Building The "lemon" or "lemon.exe" Executable</h3> |
| |
| <p>The "lemon" or "lemon.exe" program is built from a single file |
| of C-code named |
| "<a href="https://sqlite.org/src/tool/lemon.c">lemon.c</a>". |
| 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:</p> |
| |
| <blockquote><pre> |
| cc -o lemon lemon.c |
| </pre></blockquote |
| |
| <p>On Windows machines with Visual C++ installed, bring up a |
| "VS20<i>NN</i> x64 Native Tools Command Prompt" window and enter: |
| |
| <blockquote><pre> |
| cl lemon.c |
| </pre></blockquote> |
| |
| <p>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.</p> |
| |
| |
| <a id="syntax"></a> |
| <h2>4.0 Input File Syntax</h2> |
| |
| <p>The main purpose of the grammar specification file for Lemon is |
| to define the grammar for the parser. But the input file also |
| specifies additional information Lemon requires to do its job. |
| Most of the work in using Lemon is in writing an appropriate |
| grammar file.</p> |
| |
| <p>The grammar file for Lemon is, for the most part, 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++.</p> |
| |
| <a id="tnt"></a> |
| <h3>4.1 Terminals and Nonterminals</h3> |
| |
| <p>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.</p> |
| |
| <p>In Lemon, terminal and nonterminal symbols do not need to |
| be declared or identified in a separate section of the grammar file. |
| Lemon is able to generate a list of all terminals and nonterminals |
| by examining the grammar rules, and it can always distinguish a |
| terminal from a nonterminal by checking the case of the first |
| character of the name.</p> |
| |
| <p>Yacc and bison allow terminal symbols to have either alphanumeric |
| names or to be individual characters included in single quotes, like |
| this: ')' or '$'. Lemon does not allow this alternative form for |
| terminal symbols. With Lemon, all symbols, terminals and nonterminals, |
| must have alphanumeric names.</p> |
| |
| <a id="rules"></a> |
| <h3>4.2 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><a href='#start_symbol'>%start_symbol</a></tt> |
| directive described below.) |
| A typical sequence of grammar rules might look something like this:</p> |
| <pre> |
| expr ::= expr PLUS expr. |
| expr ::= expr TIMES expr. |
| expr ::= LPAREN expr RPAREN. |
| expr ::= VALUE. |
| </pre> |
| |
| <p>There is one non-terminal in this example, "expr", and five |
| terminal symbols or tokens: "PLUS", "TIMES", "LPAREN", |
| "RPAREN" and "VALUE".</p> |
| |
| <p>Like yacc and bison, Lemon allows the grammar to specify a block |
| of C code that will be executed whenever a grammar rule is reduced |
| by the parser. |
| In Lemon, this action is specified by putting the C code (contained |
| within curly braces <tt>{...}</tt>) immediately after the |
| period that closes the rule. |
| For example:</p> |
| <pre> |
| expr ::= expr PLUS expr. { printf("Doing an addition...\n"); } |
| </pre> |
| |
| <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 "$$" in the |
| action to stand for the value of the left-hand side of the rule and |
| symbols "$1", "$2", and so forth to stand for the value of |
| the terminal or nonterminal at position 1, 2 and so forth on the |
| right-hand side of the rule. |
| This idea is very powerful, but it is also very error-prone. The |
| single most common source of errors in a yacc or bison grammar is |
| to miscount the number of symbols on the right-hand side of a grammar |
| rule and say "$7" when you really mean "$8".</p> |
| |
| <p>Lemon avoids the need to count grammar symbols by assigning symbolic |
| names to each symbol in a grammar rule and then using those symbolic |
| names in the action. |
| In yacc or bison, one would write this:</p> |
| <pre> |
| expr -> expr PLUS expr { $$ = $1 + $3; }; |
| </pre> |
| <p>But in Lemon, the same rule becomes the following:</p> |
| <pre> |
| expr(A) ::= expr(B) PLUS expr(C). { A = B+C; } |
| </pre> |
| <p>In the Lemon rule, any symbol in parentheses after a grammar rule |
| symbol becomes a place holder for that symbol in the grammar rule. |
| This place holder can then be used in the associated C action to |
| stand for the value of that symbol.</p> |
| |
| <p>The Lemon notation for linking a grammar rule with its reduce |
| action is superior to yacc/bison on several counts. |
| First, as mentioned above, the Lemon method avoids the need to |
| count grammar symbols. |
| Secondly, if a terminal or nonterminal in a Lemon grammar rule |
| includes a linking symbol in parentheses but that linking symbol |
| is not actually used in the reduce action, then an error message |
| is generated. |
| For example, the rule</p> |
| <pre> |
| expr(A) ::= expr(B) PLUS expr(C). { A = B; } |
| </pre> |
| <p>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 id='precrules'></a> |
| <h3>4.3 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 parsing conflicts using precedence rules. |
| A precedence value can be assigned to any terminal symbol |
| using the |
| <tt><a href='#pleft'>%left</a></tt>, |
| <tt><a href='#pright'>%right</a></tt> or |
| <tt><a href='#pnonassoc'>%nonassoc</a></tt> directives. Terminal symbols |
| mentioned in earlier directives have a lower precedence than |
| terminal symbols mentioned in later directives. For example:</p> |
| |
| <pre> |
| %left AND. |
| %left OR. |
| %nonassoc EQ NE GT GE LT LE. |
| %left PLUS MINUS. |
| %left TIMES DIVIDE MOD. |
| %right EXP NOT. |
| </pre> |
| |
| <p>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</p> |
| <pre> |
| a AND b OR c |
| </pre> |
| <p>like this</p> |
| <pre> |
| a AND (b OR c). |
| </pre> |
| <p>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</p> |
| <pre> |
| a AND b AND c |
| </pre> |
| <p>is parsed like this</p> |
| <pre> |
| (a AND b) AND c. |
| </pre> |
| <p>The EXP operator is right-associative, though, so</p> |
| <pre> |
| a EXP b EXP c |
| </pre> |
| <p>is parsed like this</p> |
| <pre> |
| a EXP (b EXP c). |
| </pre> |
| <p>The nonassoc precedence is used for non-associative operators. |
| So</p> |
| <pre> |
| a EQ b EQ c |
| </pre> |
| <p>is an error.</p> |
| |
| <p>The precedence of non-terminals is transferred to rules as follows: |
| The precedence of a grammar rule is equal to the precedence of the |
| left-most terminal symbol in the rule for which a precedence is |
| defined. This is normally what you want, but in those cases where |
| you want 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:</p> |
| |
| <pre> |
| expr = MINUS expr. [NOT] |
| </pre> |
| |
| <p>This rule has a precedence equal to that of the NOT symbol, not the |
| MINUS symbol as would have been the case by default.</p> |
| |
| <p>With the knowledge of how precedence is assigned to terminal |
| symbols and individual |
| grammar rules, we can now explain precisely how parsing conflicts |
| are resolved in Lemon. Shift-reduce conflicts are resolved |
| as follows:</p> |
| <ul> |
| <li> If either the token to be shifted or the rule to be reduced |
| lacks precedence information, then resolve in favor of the |
| shift, but report a parsing conflict. |
| <li> If the precedence of the token to be shifted is greater than |
| the precedence of the rule to reduce, then resolve in favor |
| of the shift. No parsing conflict is reported. |
| <li> If the precedence of the token 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. |
| <li> If the precedences are the same and the shift token is |
| right-associative, then resolve in favor of the shift. |
| No parsing conflict is reported. |
| <li> If the precedences are the same and the shift token is |
| left-associative, then resolve in favor of the reduce. |
| No parsing conflict is reported. |
| <li> Otherwise, resolve the conflict by doing the shift, and |
| report a parsing conflict. |
| </ul> |
| <p>Reduce-reduce conflicts are resolved this way:</p> |
| <ul> |
| <li> If either reduce rule |
| lacks precedence information, then resolve in favor of the |
| rule that appears first in the grammar, and report a parsing |
| conflict. |
| <li> If both rules have precedence and the precedence is different, |
| then resolve the dispute in favor of the rule with the highest |
| precedence, and do not report a conflict. |
| <li> Otherwise, resolve the conflict by reducing by the rule that |
| appears first in the grammar, and report a parsing conflict. |
| </ul> |
| |
| <a id="special"></a> |
| <h3>4.4 Special Directives</h3> |
| |
| <p>The input grammar to Lemon consists of grammar rules and special |
| directives. We've described all the grammar rules, so now we'll |
| talk about the special directives.</p> |
| |
| <p>Directives in Lemon can occur in any order. You can put them before |
| the grammar rules, or after the grammar rules, or in the 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.</p> |
| |
| <p>Lemon supports the following special directives:</p> |
| <ul> |
| <li><tt><a href='#pcode'>%code</a></tt> |
| <li><tt><a href='#default_destructor'>%default_destructor</a></tt> |
| <li><tt><a href='#default_type'>%default_type</a></tt> |
| <li><tt><a href='#destructor'>%destructor</a></tt> |
| <li><tt><a href='#pifdef'>%else</a></tt> |
| <li><tt><a href='#pifdef'>%endif</a></tt> |
| <li><tt><a href='#extraarg'>%extra_argument</a></tt> |
| <li><tt><a href='#pfallback'>%fallback</a></tt> |
| <li><tt><a href='#reallc'>%free</a></tt> |
| <li><tt><a href='#pifdef'>%if</a></tt> |
| <li><tt><a href='#pifdef'>%ifdef</a></tt> |
| <li><tt><a href='#pifdef'>%ifndef</a></tt> |
| <li><tt><a href='#pinclude'>%include</a></tt> |
| <li><tt><a href='#pleft'>%left</a></tt> |
| <li><tt><a href='#pname'>%name</a></tt> |
| <li><tt><a href='#pnonassoc'>%nonassoc</a></tt> |
| <li><tt><a href='#parse_accept'>%parse_accept</a></tt> |
| <li><tt><a href='#parse_failure'>%parse_failure</a></tt> |
| <li><tt><a href='#pright'>%right</a></tt> |
| <li><tt><a href='#reallc'>%realloc</a></tt> |
| <li><tt><a href='#stack_overflow'>%stack_overflow</a></tt> |
| <li><tt><a href='#stack_size'>%stack_size</a></tt> |
| <li><tt><a href='#start_symbol'>%start_symbol</a></tt> |
| <li><tt><a href='#syntax_error'>%syntax_error</a></tt> |
| <li><tt><a href='#token'>%token</a></tt> |
| <li><tt><a href='#token_class'>%token_class</a></tt> |
| <li><tt><a href='#token_destructor'>%token_destructor</a></tt> |
| <li><tt><a href='#token_prefix'>%token_prefix</a></tt> |
| <li><tt><a href='#token_type'>%token_type</a></tt> |
| <li><tt><a href='#ptype'>%type</a></tt> |
| <li><tt><a href='#pwildcard'>%wildcard</a></tt> |
| </ul> |
| <p>Each of these directives will be described separately in the |
| following sections:</p> |
| |
| <a id='pcode'></a> |
| <h4>4.4.1 The <tt>%code</tt> directive</h4> |
| |
| <p>The <tt>%code</tt> directive is used to specify additional C code that |
| is added to the end of the main output file. This is similar to |
| the <tt><a href='#pinclude'>%include</a></tt> directive except that |
| <tt>%include</tt> is inserted at the beginning of the main output file.</p> |
| |
| <p><tt>%code</tt> is typically used to include some action routines or perhaps |
| a tokenizer or even the "main()" function |
| as part of the output file.</p> |
| |
| <p>There can be multiple <tt>%code</tt> directives. The arguments of |
| all <tt>%code</tt> directives are concatenated.</p> |
| |
| <a id='default_destructor'></a> |
| <h4>4.4.2 The <tt>%default_destructor</tt> directive</h4> |
| |
| <p>The <tt>%default_destructor</tt> directive specifies a destructor to |
| use for non-terminals that do not have their own destructor |
| specified by a separate <tt>%destructor</tt> directive. See the documentation |
| on the <tt><a href='#destructor'>%destructor</a></tt> directive below for |
| additional information.</p> |
| |
| <p>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.</p> |
| |
| <a id='default_type'></a> |
| <h4>4.4.3 The <tt>%default_type</tt> directive</h4> |
| |
| <p>The <tt>%default_type</tt> directive specifies the data type of non-terminal |
| symbols that do not have their own data type defined using a separate |
| <tt><a href='#ptype'>%type</a></tt> directive.</p> |
| |
| <a id='destructor'></a> |
| <h4>4.4.4 The <tt>%destructor</tt> directive</h4> |
| |
| <p>The <tt>%destructor</tt> directive is used to specify a destructor for |
| a non-terminal symbol. |
| (See also the <tt><a href='#token_destructor'>%token_destructor</a></tt> |
| 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:</p> |
| <ul> |
| <li> When a rule reduces and the value of a non-terminal on |
| the right-hand side is not linked to C code. |
| <li> When the stack is popped during error processing. |
| <li> When the ParseFree() function runs. |
| </ul> |
| <p>The destructor can do whatever it wants with the value of |
| the non-terminal, but its design is to deallocate memory |
| or other resources held by that non-terminal.</p> |
| |
| <p>Consider an example:</p> |
| <pre> |
| %type nt {void*} |
| %destructor nt { free($$); } |
| nt(A) ::= ID NUM. { A = malloc( 100 ); } |
| </pre> |
| <p>This example is a bit contrived, but it serves to illustrate how |
| destructors work. The example shows a non-terminal named |
| "nt" that holds values of type "void*". When the rule for |
| an "nt" reduces, it sets the value of the non-terminal to |
| space obtained from malloc(). Later, when the nt non-terminal |
| is popped from the stack, the destructor will fire and call |
| free() on this malloced space, thus avoiding a memory leak. |
| (Note that the symbol "$$" 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 id='extraarg'></a> |
| <h4>4.4.5 The <tt>%extra_argument</tt> directive</h4> |
| |
| <p>The <tt>%extra_argument</tt> directive instructs Lemon to add a 4th parameter |
| to the parameter list of the Parse() function it generates. Lemon |
| doesn't do anything itself with this extra argument, but it does |
| make the argument available to C-code action routines, destructors, |
| and so forth. For example, if the grammar file contains:</p> |
| |
| <pre> |
| %extra_argument { MyStruct *pAbc } |
| </pre> |
| |
| <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> |
| |
| <p>The <tt>%extra_context</tt> directive works the same except that it |
| is passed in on the ParseAlloc() or ParseInit() routines instead of |
| on Parse().</p> |
| |
| <a id='extractx'></a> |
| <h4>4.4.6 The <tt>%extra_context</tt> directive</h4> |
| |
| <p>The <tt>%extra_context</tt> 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:</p> |
| |
| <pre> |
| %extra_context { MyStruct *pAbc } |
| </pre> |
| |
| <p>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.</p> |
| |
| <p>The <tt>%extra_argument</tt> directive works the same except that it |
| is passed in on the Parse() routine instead of on ParseAlloc()/ParseInit().</p> |
| |
| <a id='pfallback'></a> |
| <h4>4.4.7 The <tt>%fallback</tt> directive</h4> |
| |
| <p>The <tt>%fallback</tt> 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> |
| |
| <p>The <tt>%fallback</tt> 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 <tt>%fallback</tt> directive |
| provides a mechanism to tell the parser: "If you are unable to parse |
| this keyword, try treating it as an identifier instead."</p> |
| |
| <p>The syntax of <tt>%fallback</tt> is as follows:</p> |
| |
| <blockquote> |
| <tt>%fallback</tt> <i>ID</i> <i>TOKEN...</i> <b>.</b> |
| </blockquote></p> |
| |
| <p>In words, the <tt>%fallback</tt> 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.</p> |
| |
| <a id='pifdef'></a> |
| <h4>4.4.8 The <tt>%if</tt> directive and its friends</h4> |
| |
| <p>The <tt>%if</tt>, <tt>%ifdef</tt>, <tt>%ifndef</tt>, <tt>%else</tt>, |
| and <tt>%endif</tt> 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.</p> |
| |
| <p>Grammar text in between "<tt>%ifdef MACRO</tt>" and the next nested |
| "<tt>%endif</tt>" is |
| ignored unless the "-DMACRO" command-line option is used. Grammar text |
| betwen "<tt>%ifndef MACRO</tt>" and the next nested "<tt>%endif</tt>" is |
| included except when the "-DMACRO" command-line option is used.<p> |
| |
| <p>The text in between "<tt>%if</tt> <i>CONDITIONAL</i>" and its |
| corresponding <tt>%endif</tt> is included only if <i>CONDITIONAL</i> |
| 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.</p> |
| |
| <p>An optional "<tt>%else</tt>" directive can occur anywhere in between a |
| <tt>%ifdef</tt>, <tt>%ifndef</tt>, or <tt>%if</tt> directive and |
| its corresponding <tt>%endif</tt>.</p> |
| |
| <p>Note that the argument to <tt>%ifdef</tt> and <tt>%ifndef</tt> is |
| intended to be a single preprocessor symbol name, not a general expression. |
| Use the "<tt>%if</tt>" directive for general expressions.</p> |
| |
| <a id='pinclude'></a> |
| <h4>4.4.9 The <tt>%include</tt> directive</h4> |
| |
| <p>The <tt>%include</tt> 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 |
| <tt>%include</tt> directives in your grammar file, their values are concatenated |
| so that all <tt>%include</tt> code ultimately appears near the top of the |
| generated parser, in the same order as it appeared in the grammar.</p> |
| |
| <p>The <tt>%include</tt> directive is very handy for getting some extra #include |
| preprocessor statements at the beginning of the generated parser. |
| For example:</p> |
| |
| <pre> |
| %include {#include <unistd.h>} |
| </pre> |
| |
| <p>This might be needed, for example, if some of the C actions in the |
| grammar call functions that are prototyped in unistd.h.</p> |
| |
| <p>Use the <tt><a href="#pcode">%code</a></tt> directive to add code to |
| the end of the generated parser.</p> |
| |
| <a id='pleft'></a> |
| <h4>4.4.10 The <tt>%left</tt> directive</h4> |
| |
| The <tt>%left</tt> directive is used (along with the |
| <tt><a href='#pright'>%right</a></tt> and |
| <tt><a href='#pnonassoc'>%nonassoc</a></tt> directives) to declare |
| precedences of terminal symbols. |
| Every terminal symbol whose name appears after |
| a <tt>%left</tt> directive but before the next period (".") is |
| given the same left-associative precedence value. Subsequent |
| <tt>%left</tt> directives have higher precedence. For example:</p> |
| |
| <pre> |
| %left AND. |
| %left OR. |
| %nonassoc EQ NE GT GE LT LE. |
| %left PLUS MINUS. |
| %left TIMES DIVIDE MOD. |
| %right EXP NOT. |
| </pre> |
| |
| <p>Note the period that terminates each <tt>%left</tt>, |
| <tt>%right</tt> or <tt>%nonassoc</tt> |
| 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 <tt>%left</tt> |
| rather than <tt>%right</tt> whenever possible.</p> |
| |
| <a id='pname'></a> |
| <h4>4.4.11 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 <tt>%name</tt> directive. For instance:</p> |
| |
| <pre> |
| %name Abcde |
| </pre> |
| |
| <p>Putting this directive in the grammar file will cause Lemon to generate |
| functions named</p> |
| <ul> |
| <li> AbcdeAlloc(), |
| <li> AbcdeFree(), |
| <li> AbcdeTrace(), and |
| <li> Abcde(). |
| </ul> |
| </p>The <tt>%name</tt> directive allows you to generate two or more different |
| parsers and link them all into the same executable.</p> |
| |
| <a id='pnonassoc'></a> |
| <h4>4.4.12 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 <tt><a href='#pleft'>%left</a></tt> directive |
| for additional information.</p> |
| |
| <a id='parse_accept'></a> |
| <h4>4.4.13 The <tt>%parse_accept</tt> directive</h4> |
| |
| <p>The <tt>%parse_accept</tt> 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> |
| |
| <pre> |
| %parse_accept { |
| printf("parsing complete!\n"); |
| } |
| </pre> |
| |
| <a id='parse_failure'></a> |
| <h4>4.4.14 The <tt>%parse_failure</tt> directive</h4> |
| |
| <p>The <tt>%parse_failure</tt> 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> |
| |
| <pre> |
| %parse_failure { |
| fprintf(stderr,"Giving up. Parser is hopelessly lost...\n"); |
| } |
| </pre> |
| |
| <a id='pright'></a> |
| <h4>4.4.15 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 id='stack_overflow'></a> |
| <h4>4.4.16 The <tt>%stack_overflow</tt> directive</h4> |
| |
| <p>The <tt>%stack_overflow</tt> 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> |
| %stack_overflow { |
| fprintf(stderr,"Giving up. Parser stack overflow\n"); |
| } |
| </pre> |
| |
| <p>You can help prevent parser stack overflows by avoiding the use |
| of right recursion and right-precedence operators in your grammar. |
| Use left recursion and and left-precedence operators instead to |
| encourage rules to reduce sooner and keep the stack size down. |
| For example, do rules like this:</p> |
| <pre> |
| list ::= list element. // left-recursion. Good! |
| list ::= . |
| </pre> |
| <p>Not like this:</p> |
| <pre> |
| list ::= element list. // right-recursion. Bad! |
| list ::= . |
| </pre> |
| |
| <a id='stack_size'></a> |
| <h4>4.4.17 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 <tt>%stack_size</tt> directive and Lemon will generate a parse |
| with a stack of the requested size. The default value is 100.</p> |
| |
| <pre> |
| %stack_size 2000 |
| </pre> |
| |
| <a id='start_symbol'></a> |
| <h4>4.4.18 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 |
| <tt>%start_symbol</tt> directive.</p> |
| |
| <pre> |
| %start_symbol prog |
| </pre> |
| |
| <a id='syntax_error'></a> |
| <h4>4.4.19 The <tt>%syntax_error</tt> directive</h4> |
| |
| <p>See <a href='#errors'>Error Processing</a>.</p> |
| |
| <a id='token'></a> |
| <h4>4.4.20 The <tt>%token</tt> directive</h4> |
| |
| <p>Tokens are normally created automatically, the first time they are used. |
| Any identifier that begins with an upper-case letter is a token. |
| |
| <p>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: |
| |
| <blockquote> |
| <tt>%token</tt> <i>TOKEN</i> <i>TOKEN...</i> <b>.</b> |
| </blockquote></p> |
| |
| <p>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. |
| |
| |
| <a id='token_class'></a> |
| <h4>4.4.21 The <tt>%token_class</tt> directive</h4> |
| |
| <p>Undocumented. Appears to be related to the MULTITERMINAL concept. |
| <a href='http://sqlite.org/src/fdiff?v1=796930d5fc2036c7&v2=624b24c5dc048e09&sbs=0'>Implementation</a>.</p> |
| |
| <a id='token_destructor'></a> |
| <h4>4.4.22 The <tt>%token_destructor</tt> directive</h4> |
| |
| <p>The <tt>%destructor</tt> directive assigns a destructor to a non-terminal |
| symbol. (See the description of the |
| <tt><a href='%destructor'>%destructor</a></tt> directive above.) |
| The <tt>%token_destructor</tt> 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 <tt><a href='#token_type'>%token_type</a></tt> directive) |
| and so they use a common destructor. |
| Other than that, the token destructor works just like the non-terminal |
| destructors.</p> |
| |
| <a id='token_prefix'></a> |
| <h4>4.4.23 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.</p> |
| |
| <p>So if the default output of Lemon looked like this:</p> |
| <pre> |
| #define AND 1 |
| #define MINUS 2 |
| #define OR 3 |
| #define PLUS 4 |
| </pre> |
| <p>You can insert a statement into the grammar like this:</p> |
| <pre> |
| %token_prefix TOKEN_ |
| </pre> |
| <p>to cause Lemon to produce these symbols instead:</p> |
| <pre> |
| #define TOKEN_AND 1 |
| #define TOKEN_MINUS 2 |
| #define TOKEN_OR 3 |
| #define TOKEN_PLUS 4 |
| </pre> |
| |
| <a id='token_type'></a><a id='ptype'></a> |
| <h4>4.4.24 The <tt>%token_type</tt> and <tt>%type</tt> directives</h4> |
| |
| <p>These directives are used to specify the data types for values |
| on the parser's stack associated with terminal and non-terminal |
| symbols. The values of all terminal symbols must be of the same |
| type. This turns out to be the same data type as the 3rd parameter |
| to the Parse() function generated by Lemon. Typically, you will |
| make the value of a terminal symbol be a pointer to some kind of |
| token structure. Like this:</p> |
| |
| <pre> |
| %token_type {Token*} |
| </pre> |
| |
| <p>If the data type of terminals is not specified, the default value |
| is "void*".</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> |
| |
| <pre> |
| %type expr {Expr*} |
| </pre> |
| |
| <p>Each entry on the parser's stack is actually a union containing |
| instances of all data types for every non-terminal and terminal symbol. |
| Lemon will automatically use the correct element of this union depending |
| on what the corresponding non-terminal or terminal symbol is. But |
| the grammar designer should keep in mind that the size of the union |
| will be the size of its largest element. So if you have a single |
| non-terminal whose data type requires 1K of storage, then your 100 |
| entry parser stack will require 100K of heap space. If you are willing |
| and able to pay that price, fine. You just need to know.</p> |
| |
| <a id='pwildcard'></a> |
| <h4>4.4.25 The <tt>%wildcard</tt> directive</h4> |
| |
| <p>The <tt>%wildcard</tt> directive is followed by a single token name and a |
| period. This directive specifies that the identified token should |
| match any input token.</p> |
| |
| <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 alternatives.</p> |
| |
| <a id='reallc'></a> |
| <h4>4.4.26 The <tt>%realloc</tt> and <tt>%free</tt> directives</h4> |
| |
| <p>The <tt>%realloc</tt> and <tt>%free</tt> directives defines function |
| that allocate and free heap memory. The signatures of these functions |
| should be the same as the realloc() and free() functions from the standard |
| C library. |
| |
| <p>If both of these functions are defined |
| then these functions are used to allocate and free |
| memory for supplemental parser stack space, if the initial |
| parse stack space is exceeded. The initial parser stack size |
| is specified by either <tt>%stack_size</tt> or the |
| -DYYSTACKDEPTH compile-time flag. |
| |
| <a id='errors'></a> |
| <h2>5.0 Error Processing</h2> |
| |
| <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 <tt>%syntax_error</tt> 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 <tt>%syntax_error</tt> 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 |
| <tt><a href='#parse_failure'>%parse_failure</a></tt> 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> |
| |
| |
| <a id='history'></a> |
| <h2>6.0 History of Lemon</h2> |
| |
| <p>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 Lime source code has been lost.</p> |
| |
| <p>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.</p> |
| |
| <p>Since 2001, Lemon has been part of the |
| <a href="https://sqlite.org/">SQLite project</a> and the source code |
| to Lemon has been managed as a part of the |
| <a href="https://sqlite.org/src">SQLite source tree</a> in the following |
| files:</p> |
| |
| <ul> |
| <li> <a href="https://sqlite.org/src/file/tool/lemon.c">tool/lemon.c</a> |
| <li> <a href="https://sqlite.org/src/file/tool/lempar.c">tool/lempar.c</a> |
| <li> <a href="https://sqlite.org/src/file/doc/lemon.html">doc/lemon.html</a> |
| </ul> |
| |
| <a id="copyright"></a> |
| <h2>7.0 Copyright</h2> |
| |
| <p>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.</p> |
| |
| <p>The code comes with no warranty. If it breaks, you get to keep both |
| pieces.</p> |
| |
| </body> |
| </html> |