| <html> |
| <head> |
| <title>The Lemon Parser Generator</title> |
| </head> |
| <body bgcolor=white> |
| <h1 align=center>The Lemon Parser Generator</h1> |
| |
| <p>Lemon is an LALR(1) parser generator for C or C++. |
| It does the same job as ``bison'' and ``yacc''. |
| But lemon is not another bison or yacc clone. It |
| uses a different grammar syntax which is designed to |
| reduce the number of coding errors. Lemon also uses a more |
| sophisticated parsing engine that is faster than yacc and |
| bison and which is both reentrant and thread-safe. |
| Furthermore, Lemon implements features that can be used |
| to eliminate resource leaks, making is suitable for use |
| in long-running programs such as graphical user interfaces |
| or embedded controllers.</p> |
| |
| <p>This document is an introduction to the Lemon |
| parser generator.</p> |
| |
| <h2>Theory of Operation</h2> |
| |
| <p>The main goal of Lemon is to translate a context free grammar (CFG) |
| for a particular language into C code that implements a parser for |
| that language. |
| The program has two inputs: |
| <ul> |
| <li>The grammar specification. |
| <li>A parser template file. |
| </ul> |
| Typically, only the grammar specification is supplied by the programmer. |
| Lemon comes with a default parser template which works fine for most |
| applications. But the user is free to substitute a different parser |
| template if desired.</p> |
| |
| <p>Depending on command-line options, Lemon will generate between |
| one and three files of outputs. |
| <ul> |
| <li>C code to implement the parser. |
| <li>A header file defining an integer ID for each terminal symbol. |
| <li>An information file that describes the states of the generated parser |
| automaton. |
| </ul> |
| By default, all three of these output files are generated. |
| The header file is suppressed if the ``-m'' command-line option is |
| used and the report file is omitted when ``-q'' is selected.</p> |
| |
| <p>The grammar specification file uses a ``.y'' suffix, by convention. |
| In the examples used in this document, we'll assume the name of the |
| grammar file is ``gram.y''. A typical use of Lemon would be the |
| following command: |
| <pre> |
| lemon gram.y |
| </pre> |
| This command will generate three output files named ``gram.c'', |
| ``gram.h'' and ``gram.out''. |
| The first is C code to implement the parser. The second |
| is the header file that defines numerical values for all |
| terminal symbols, and the last is the report that explains |
| the states used by the parser automaton.</p> |
| |
| <h3>Command Line Options</h3> |
| |
| <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 |
| <pre> |
| lemon -? |
| </pre> |
| As of this writing, the following command-line options are supported: |
| <ul> |
| <li><tt>-b</tt> |
| <li><tt>-c</tt> |
| <li><tt>-g</tt> |
| <li><tt>-m</tt> |
| <li><tt>-q</tt> |
| <li><tt>-s</tt> |
| <li><tt>-x</tt> |
| </ul> |
| The ``-b'' option reduces the amount of text in the report file by |
| printing only the basis of each parser state, rather than the full |
| configuration. |
| The ``-c'' option suppresses action table compression. Using -c |
| will make the parser a little larger and slower but it will detect |
| syntax errors sooner. |
| The ``-g'' option causes no output files to be generated at all. |
| Instead, the input grammar file is printed on standard output but |
| with all comments, actions and other extraneous text deleted. This |
| is a useful way to get a quick summary of a grammar. |
| The ``-m'' option causes the output C source file to be compatible |
| with the ``makeheaders'' program. |
| Makeheaders is a program that automatically generates header files |
| from C source code. When the ``-m'' option is used, the header |
| file is not output since the makeheaders program will take care |
| of generated all header files automatically. |
| The ``-q'' option suppresses the report file. |
| Using ``-s'' causes a brief summary of parser statistics to be |
| printed. Like this: |
| <pre> |
| Parser statistics: 74 terminals, 70 nonterminals, 179 rules |
| 340 states, 2026 parser table entries, 0 conflicts |
| </pre> |
| Finally, the ``-x'' option causes Lemon to print its version number |
| and then stops without attempting to read the grammar or generate a parser.</p> |
| |
| <h3>The Parser Interface</h3> |
| |
| <p>Lemon doesn't generate a complete, working program. It only generates |
| a few subroutines that implement a parser. This section describes |
| the interface to those subroutines. It is up to the programmer to |
| call these subroutines in an appropriate way in order to produce a |
| 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: |
| <pre> |
| void *pParser = ParseAlloc( malloc ); |
| </pre> |
| The ParseAlloc() routine allocates and initializes a new parser and |
| returns a pointer to it. |
| The actual data structure used to represent a parser is opaque -- |
| its internal structure is not visible or usable by the calling routine. |
| For this reason, the ParseAlloc() routine returns a pointer to void |
| rather than a pointer to some particular structure. |
| The sole argument to the ParseAlloc() routine is a pointer to the |
| subroutine used to allocate memory. Typically this means ``malloc()''.</p> |
| |
| <p>After a program is finished using a parser, it can reclaim all |
| memory allocated by that parser by calling |
| <pre> |
| ParseFree(pParser, free); |
| </pre> |
| The first argument is the same pointer returned by ParseAlloc(). The |
| 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: |
| <pre> |
| Parse(pParser, hTokenID, sTokenData, pArg); |
| </pre> |
| The first argument to the Parse() routine is the pointer returned by |
| ParseAlloc(). |
| The second argument is a small positive integer that tells the parse the |
| type of the next token in the data stream. |
| There is one token type for each terminal symbol in the grammar. |
| The gram.h file generated by Lemon contains #define statements that |
| map symbolic terminal symbol names into appropriate integer values. |
| (A value of 0 for the second argument is a special flag to the |
| parser to indicate that the end of input has been reached.) |
| The third argument is the value of the given token. By default, |
| the type of the third argument is integer, but the grammar will |
| usually redefine this type to be some kind of structure. |
| Typically the second argument will be a broad category of tokens |
| such as ``identifier'' or ``number'' and the third argument will |
| be the name of the identifier or the value of the number.</p> |
| |
| <p>The Parse() function may have either three or four arguments, |
| depending on the grammar. If the grammar specification file request |
| it, the Parse() function will have a fourth parameter that can be |
| of any type chosen by the programmer. The parser doesn't do anything |
| with this argument except to pass it through to action routines. |
| This is a convenient mechanism for passing state information down |
| to the action routines without having to use global variables.</p> |
| |
| <p>A typical use of a Lemon parser might look something like the |
| following: |
| <pre> |
| 01 ParseTree *ParseFile(const char *zFilename){ |
| 02 Tokenizer *pTokenizer; |
| 03 void *pParser; |
| 04 Token sToken; |
| 05 int hTokenId; |
| 06 ParserState sState; |
| 07 |
| 08 pTokenizer = TokenizerCreate(zFilename); |
| 09 pParser = ParseAlloc( malloc ); |
| 10 InitParserState(&sState); |
| 11 while( GetNextToken(pTokenizer, &hTokenId, &sToken) ){ |
| 12 Parse(pParser, hTokenId, sToken, &sState); |
| 13 } |
| 14 Parse(pParser, 0, sToken, &sState); |
| 15 ParseFree(pParser, free ); |
| 16 TokenizerFree(pTokenizer); |
| 17 return sState.treeRoot; |
| 18 } |
| </pre> |
| This example shows a user-written routine that parses a file of |
| text and returns a pointer to the parse tree. |
| (We've omitted all error-handling from this example to keep it |
| simple.) |
| We assume the existence of some kind of tokenizer which is created |
| using TokenizerCreate() on line 8 and deleted by TokenizerFree() |
| on line 16. The GetNextToken() function on line 11 retrieves the |
| next token from the input file and puts its type in the |
| integer variable hTokenId. The sToken variable is assumed to be |
| some kind of structure that contains details about each token, |
| such as its complete text, what line it occurs on, etc. </p> |
| |
| <p>This example also assumes the existence of structure of type |
| ParserState that holds state information about a particular parse. |
| An instance of such a structure is created on line 6 and initialized |
| on line 10. A pointer to this structure is passed into the Parse() |
| routine as the optional 4th argument. |
| The action routine specified by the grammar for the parser can use |
| the ParserState structure to hold whatever information is useful and |
| appropriate. In the example, we note that the treeRoot field of |
| the ParserState structure is left pointing to the root of the parse |
| tree.</p> |
| |
| <p>The core of this example as it relates to Lemon is as follows: |
| <pre> |
| ParseFile(){ |
| pParser = ParseAlloc( malloc ); |
| while( GetNextToken(pTokenizer,&hTokenId, &sToken) ){ |
| Parse(pParser, hTokenId, sToken); |
| } |
| Parse(pParser, 0, sToken); |
| ParseFree(pParser, free ); |
| } |
| </pre> |
| 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: |
| <pre> |
| ParseTrace(FILE *stream, char *zPrefix); |
| </pre> |
| After this routine is called, a short (one-line) message is written |
| to the designated output stream every time the parser changes states |
| or calls an action routine. Each such message is prefaced using |
| the text given by zPrefix. This debugging output can be turned off |
| by calling ParseTrace() again with a first argument of NULL (0).</p> |
| |
| <h3>Differences With YACC and BISON</h3> |
| |
| <p>Programmers who have previously used the yacc or bison parser |
| generator will notice several important differences between yacc and/or |
| bison and Lemon. |
| <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> |
| These differences may cause some initial confusion for programmers |
| with prior yacc and bison experience. |
| But after years of experience using Lemon, I firmly |
| believe that the Lemon way of doing things is better.</p> |
| |
| <h2>Input File Syntax</h2> |
| |
| <p>The main purpose of the grammar specification file for Lemon is |
| to define the grammar for the parser. But the input file also |
| specifies additional information Lemon requires to do its job. |
| Most of the work in using Lemon is in writing an appropriate |
| grammar file.</p> |
| |
| <p>The grammar file for lemon is, for the most part, free format. |
| It does not have sections or divisions like yacc or bison. Any |
| declaration can occur at any point in the file. |
| Lemon ignores whitespace (except where it is needed to separate |
| tokens) and it honors the same commenting conventions as C and C++.</p> |
| |
| <h3>Terminals and Nonterminals</h3> |
| |
| <p>A terminal symbol (token) is any string of alphanumeric |
| and underscore characters |
| that begins with an upper case letter. |
| A terminal can contain lowercase letters after the first character, |
| but the usual convention is to make terminals all upper case. |
| A nonterminal, on the other hand, is any string of alphanumeric |
| and underscore characters than begins with a lower case letter. |
| Again, the usual convention is to make nonterminals use all lower |
| case letters.</p> |
| |
| <p>In Lemon, terminal and nonterminal symbols do not need to |
| be declared or identified in a separate section of the grammar file. |
| Lemon is able to generate a list of all terminals and nonterminals |
| by examining the grammar rules, and it can always distinguish a |
| terminal from a nonterminal by checking the case of the first |
| character of the name.</p> |
| |
| <p>Yacc and bison allow terminal symbols to have either alphanumeric |
| names or to be individual characters included in single quotes, like |
| this: ')' or '$'. Lemon does not allow this alternative form for |
| terminal symbols. With Lemon, all symbols, terminals and nonterminals, |
| must have alphanumeric names.</p> |
| |
| <h3>Grammar Rules</h3> |
| |
| <p>The main component of a Lemon grammar file is a sequence of grammar |
| rules. |
| Each grammar rule consists of a nonterminal symbol followed by |
| the special symbol ``::='' and then a list of terminals and/or nonterminals. |
| The rule is terminated by a period. |
| The list of terminals and nonterminals on the right-hand side of the |
| rule can be empty. |
| Rules can occur in any order, except that the left-hand side of the |
| first rule is assumed to be the start symbol for the grammar (unless |
| specified otherwise using the <tt>%start</tt> directive described below.) |
| A typical sequence of grammar rules might look something like this: |
| <pre> |
| expr ::= expr PLUS expr. |
| expr ::= expr TIMES expr. |
| expr ::= LPAREN expr RPAREN. |
| expr ::= VALUE. |
| </pre> |
| </p> |
| |
| <p>There is one non-terminal in this example, ``expr'', and five |
| terminal symbols or tokens: ``PLUS'', ``TIMES'', ``LPAREN'', |
| ``RPAREN'' and ``VALUE''.</p> |
| |
| <p>Like yacc and bison, Lemon allows the grammar to specify a block |
| of C code that will be executed whenever a grammar rule is reduced |
| by the parser. |
| In Lemon, this action is specified by putting the C code (contained |
| within curly braces <tt>{...}</tt>) immediately after the |
| period that closes the rule. |
| For example: |
| <pre> |
| expr ::= expr PLUS expr. { printf("Doing an addition...\n"); } |
| </pre> |
| </p> |
| |
| <p>In order to be useful, grammar actions must normally be linked to |
| their associated grammar rules. |
| In yacc and bison, this is accomplished by embedding a ``$$'' in the |
| action to stand for the value of the left-hand side of the rule and |
| symbols ``$1'', ``$2'', and so forth to stand for the value of |
| the terminal or nonterminal at position 1, 2 and so forth on the |
| right-hand side of the rule. |
| This idea is very powerful, but it is also very error-prone. The |
| single most common source of errors in a yacc or bison grammar is |
| to miscount the number of symbols on the right-hand side of a grammar |
| rule and say ``$7'' when you really mean ``$8''.</p> |
| |
| <p>Lemon avoids the need to count grammar symbols by assigning symbolic |
| names to each symbol in a grammar rule and then using those symbolic |
| names in the action. |
| In yacc or bison, one would write this: |
| <pre> |
| expr -> expr PLUS expr { $$ = $1 + $3; }; |
| </pre> |
| But in Lemon, the same rule becomes the following: |
| <pre> |
| expr(A) ::= expr(B) PLUS expr(C). { A = B+C; } |
| </pre> |
| 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 |
| <pre> |
| expr(A) ::= expr(B) PLUS expr(C). { A = B; } |
| </pre> |
| will generate an error because the linking symbol ``C'' is used |
| in the grammar rule but not in the reduce action.</p> |
| |
| <p>The Lemon notation for linking grammar rules to reduce actions |
| also facilitates the use of destructors for reclaiming memory |
| allocated by the values of terminals and nonterminals on the |
| right-hand side of a rule.</p> |
| |
| <h3>Precedence Rules</h3> |
| |
| <p>Lemon resolves parsing ambiguities in exactly the same way as |
| yacc and bison. A shift-reduce conflict is resolved in favor |
| of the shift, and a reduce-reduce conflict is resolved by reducing |
| whichever rule comes first in the grammar file.</p> |
| |
| <p>Just like in |
| yacc and bison, Lemon allows a measure of control |
| over the resolution of paring conflicts using precedence rules. |
| A precedence value can be assigned to any terminal symbol |
| using the %left, %right or %nonassoc directives. Terminal symbols |
| mentioned in earlier directives have a lower precedence that |
| terminal symbols mentioned in later directives. For example:</p> |
| |
| <p><pre> |
| %left AND. |
| %left OR. |
| %nonassoc EQ NE GT GE LT LE. |
| %left PLUS MINUS. |
| %left TIMES DIVIDE MOD. |
| %right EXP NOT. |
| </pre></p> |
| |
| <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 |
| <pre> |
| a AND b OR c |
| </pre> |
| like this |
| <pre> |
| a AND (b OR c). |
| </pre> |
| 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 |
| <pre> |
| a AND b AND c |
| </pre> |
| is parsed like this |
| <pre> |
| (a AND b) AND c. |
| </pre> |
| The EXP operator is right-associative, though, so |
| <pre> |
| a EXP b EXP c |
| </pre> |
| is parsed like this |
| <pre> |
| a EXP (b EXP c). |
| </pre> |
| The nonassoc precedence is used for non-associative operators. |
| So |
| <pre> |
| a EQ b EQ c |
| </pre> |
| is an error.</p> |
| |
| <p>The precedence of non-terminals is transferred to rules as follows: |
| The precedence of a grammar rule is equal to the precedence of the |
| left-most terminal symbol in the rule for which a precedence is |
| defined. This is normally what you want, but in those cases where |
| you want to precedence of a grammar rule to be something different, |
| you can specify an alternative precedence symbol by putting the |
| symbol in square braces after the period at the end of the rule and |
| before any C-code. For example:</p> |
| |
| <p><pre> |
| expr = MINUS expr. [NOT] |
| </pre></p> |
| |
| <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: |
| <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 it be shifted is less than the |
| precedence of the rule to reduce, then resolve in favor of the |
| reduce action. No parsing conflict is reported. |
| <li> If the precedences are the same and the shift token is |
| right-associative, then resolve in favor of the shift. |
| No parsing conflict is reported. |
| <li> If the precedences are the same the shift token is |
| left-associative, then resolve in favor of the reduce. |
| No parsing conflict is reported. |
| <li> Otherwise, resolve the conflict by doing the shift and |
| report the parsing conflict. |
| </ul> |
| Reduce-reduce conflicts are resolved this way: |
| <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> |
| |
| <h3>Special Directives</h3> |
| |
| <p>The input grammar to Lemon consists of grammar rules and special |
| directives. We've described all the grammar rules, so now we'll |
| talk about the special directives.</p> |
| |
| <p>Directives in lemon can occur in any order. You can put them before |
| the grammar rules, or after the grammar rules, or in the mist of the |
| grammar rules. It doesn't matter. The relative order of |
| directives used to assign precedence to terminals is important, but |
| other than that, the order of directives in Lemon is arbitrary.</p> |
| |
| <p>Lemon supports the following special directives: |
| <ul> |
| <li><tt>%code</tt> |
| <li><tt>%default_destructor</tt> |
| <li><tt>%default_type</tt> |
| <li><tt>%destructor</tt> |
| <li><tt>%extra_argument</tt> |
| <li><tt>%include</tt> |
| <li><tt>%left</tt> |
| <li><tt>%name</tt> |
| <li><tt>%nonassoc</tt> |
| <li><tt>%parse_accept</tt> |
| <li><tt>%parse_failure </tt> |
| <li><tt>%right</tt> |
| <li><tt>%stack_overflow</tt> |
| <li><tt>%stack_size</tt> |
| <li><tt>%start_symbol</tt> |
| <li><tt>%syntax_error</tt> |
| <li><tt>%token_destructor</tt> |
| <li><tt>%token_prefix</tt> |
| <li><tt>%token_type</tt> |
| <li><tt>%type</tt> |
| </ul> |
| Each of these directives will be described separately in the |
| following sections:</p> |
| |
| <h4>The <tt>%code</tt> directive</h4> |
| |
| <p>The %code directive is used to specify addition C/C++ code that |
| is added to the end of the main output file. This is similar to |
| the %include directive except that %include is inserted at the |
| beginning of the main output file.</p> |
| |
| <p>%code is typically used to include some action routines or perhaps |
| a tokenizer as part of the output file.</p> |
| |
| <h4>The <tt>%default_destructor</tt> directive</h4> |
| |
| <p>The %default_destructor directive specifies a destructor to |
| use for non-terminals that do not have their own destructor |
| specified by a separate %destructor directive. See the documentation |
| on the %destructor directive below for additional information.</p> |
| |
| <p>In some grammers, many different non-terminal symbols have the |
| same datatype and hence the same destructor. This directive is |
| a convenience way to specify the same destructor for all those |
| non-terminals using a single statement.</p> |
| |
| <h4>The <tt>%default_type</tt> directive</h4> |
| |
| <p>The %default_type directive specifies the datatype of non-terminal |
| symbols that do no have their own datatype defined using a separate |
| %type directive. See the documentation on %type below for addition |
| information.</p> |
| |
| <h4>The <tt>%destructor</tt> directive</h4> |
| |
| <p>The %destructor directive is used to specify a destructor for |
| a non-terminal symbol. |
| (See also the %token_destructor directive which is used to |
| specify a destructor for terminal symbols.)</p> |
| |
| <p>A non-terminal's destructor is called to dispose of the |
| non-terminal's value whenever the non-terminal is popped from |
| the stack. This includes all of the following circumstances: |
| <ul> |
| <li> When a rule reduces and the value of a non-terminal on |
| the right-hand side is not linked to C code. |
| <li> When the stack is popped during error processing. |
| <li> When the ParseFree() function runs. |
| </ul> |
| 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: |
| <pre> |
| %type nt {void*} |
| %destructor nt { free($$); } |
| nt(A) ::= ID NUM. { A = malloc( 100 ); } |
| </pre> |
| This example is a bit contrived but it serves to illustrate how |
| destructors work. The example shows a non-terminal named |
| ``nt'' that holds values of type ``void*''. When the rule for |
| an ``nt'' reduces, it sets the value of the non-terminal to |
| space obtained from malloc(). Later, when the nt non-terminal |
| is popped from the stack, the destructor will fire and call |
| free() on this malloced space, thus avoiding a memory leak. |
| (Note that the symbol ``$$'' in the destructor code is replaced |
| by the value of the non-terminal.)</p> |
| |
| <p>It is important to note that the value of a non-terminal is passed |
| to the destructor whenever the non-terminal is removed from the |
| stack, unless the non-terminal is used in a C-code action. If |
| the non-terminal is used by C-code, then it is assumed that the |
| C-code will take care of destroying it if it should really |
| be destroyed. More commonly, the value is used to build some |
| larger structure and we don't want to destroy it, which is why |
| the destructor is not called in this circumstance.</p> |
| |
| <p>By appropriate use of destructors, it is possible to |
| build a parser using Lemon that can be used within a long-running |
| program, such as a GUI, that will not leak memory or other resources. |
| To do the same using yacc or bison is much more difficult.</p> |
| |
| <h4>The <tt>%extra_argument</tt> directive</h4> |
| |
| The %extra_argument directive instructs Lemon to add a 4th parameter |
| to the parameter list of the Parse() function it generates. Lemon |
| doesn't do anything itself with this extra argument, but it does |
| make the argument available to C-code action routines, destructors, |
| and so forth. For example, if the grammar file contains:</p> |
| |
| <p><pre> |
| %extra_argument { MyStruct *pAbc } |
| </pre></p> |
| |
| <p>Then the Parse() function generated will have an 4th parameter |
| of type ``MyStruct*'' and all action routines will have access to |
| a variable named ``pAbc'' that is the value of the 4th parameter |
| in the most recent call to Parse().</p> |
| |
| <h4>The <tt>%include</tt> directive</h4> |
| |
| <p>The %include directive specifies C code that is included at the |
| top of the generated parser. You can include any text you want -- |
| the Lemon parser generator copies it blindly. If you have multiple |
| %include directives in your grammar file the value of the last |
| %include directive overwrites all the others.</p. |
| |
| <p>The %include directive is very handy for getting some extra #include |
| preprocessor statements at the beginning of the generated parser. |
| For example:</p> |
| |
| <p><pre> |
| %include {#include <unistd.h>} |
| </pre></p> |
| |
| <p>This might be needed, for example, if some of the C actions in the |
| grammar call functions that are prototyed in unistd.h.</p> |
| |
| <h4>The <tt>%left</tt> directive</h4> |
| |
| The %left directive is used (along with the %right and |
| %nonassoc directives) to declare precedences of terminal |
| symbols. Every terminal symbol whose name appears after |
| a %left directive but before the next period (``.'') is |
| given the same left-associative precedence value. Subsequent |
| %left directives have higher precedence. For example:</p> |
| |
| <p><pre> |
| %left AND. |
| %left OR. |
| %nonassoc EQ NE GT GE LT LE. |
| %left PLUS MINUS. |
| %left TIMES DIVIDE MOD. |
| %right EXP NOT. |
| </pre></p> |
| |
| <p>Note the period that terminates each %left, %right or %nonassoc |
| directive.</p> |
| |
| <p>LALR(1) grammars can get into a situation where they require |
| a large amount of stack space if you make heavy use or right-associative |
| operators. For this reason, it is recommended that you use %left |
| rather than %right whenever possible.</p> |
| |
| <h4>The <tt>%name</tt> directive</h4> |
| |
| <p>By default, the functions generated by Lemon all begin with the |
| five-character string ``Parse''. You can change this string to something |
| different using the %name directive. For instance:</p> |
| |
| <p><pre> |
| %name Abcde |
| </pre></p> |
| |
| <p>Putting this directive in the grammar file will cause Lemon to generate |
| functions named |
| <ul> |
| <li> AbcdeAlloc(), |
| <li> AbcdeFree(), |
| <li> AbcdeTrace(), and |
| <li> Abcde(). |
| </ul> |
| The %name directive allows you to generator two or more different |
| parsers and link them all into the same executable. |
| </p> |
| |
| <h4>The <tt>%nonassoc</tt> directive</h4> |
| |
| <p>This directive is used to assign non-associative precedence to |
| one or more terminal symbols. See the section on precedence rules |
| or on the %left directive for additional information.</p> |
| |
| <h4>The <tt>%parse_accept</tt> directive</h4> |
| |
| <p>The %parse_accept directive specifies a block of C code that is |
| executed whenever the parser accepts its input string. To ``accept'' |
| an input string means that the parser was able to process all tokens |
| without error.</p> |
| |
| <p>For example:</p> |
| |
| <p><pre> |
| %parse_accept { |
| printf("parsing complete!\n"); |
| } |
| </pre></p> |
| |
| |
| <h4>The <tt>%parse_failure</tt> directive</h4> |
| |
| <p>The %parse_failure directive specifies a block of C code that |
| is executed whenever the parser fails complete. This code is not |
| executed until the parser has tried and failed to resolve an input |
| error using is usual error recovery strategy. The routine is |
| only invoked when parsing is unable to continue.</p> |
| |
| <p><pre> |
| %parse_failure { |
| fprintf(stderr,"Giving up. Parser is hopelessly lost...\n"); |
| } |
| </pre></p> |
| |
| <h4>The <tt>%right</tt> directive</h4> |
| |
| <p>This directive is used to assign right-associative precedence to |
| one or more terminal symbols. See the section on precedence rules |
| or on the %left directive for additional information.</p> |
| |
| <h4>The <tt>%stack_overflow</tt> directive</h4> |
| |
| <p>The %stack_overflow directive specifies a block of C code that |
| is executed if the parser's internal stack ever overflows. Typically |
| this just prints an error message. After a stack overflow, the parser |
| will be unable to continue and must be reset.</p> |
| |
| <p><pre> |
| %stack_overflow { |
| fprintf(stderr,"Giving up. Parser stack overflow\n"); |
| } |
| </pre></p> |
| |
| <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: |
| <pre> |
| list ::= list element. // left-recursion. Good! |
| list ::= . |
| </pre> |
| Not like this: |
| <pre> |
| list ::= element list. // right-recursion. Bad! |
| list ::= . |
| </pre> |
| |
| <h4>The <tt>%stack_size</tt> directive</h4> |
| |
| <p>If stack overflow is a problem and you can't resolve the trouble |
| by using left-recursion, then you might want to increase the size |
| of the parser's stack using this directive. Put an positive integer |
| after the %stack_size directive and Lemon will generate a parse |
| with a stack of the requested size. The default value is 100.</p> |
| |
| <p><pre> |
| %stack_size 2000 |
| </pre></p> |
| |
| <h4>The <tt>%start_symbol</tt> directive</h4> |
| |
| <p>By default, the start-symbol for the grammar that Lemon generates |
| is the first non-terminal that appears in the grammar file. But you |
| can choose a different start-symbol using the %start_symbol directive.</p> |
| |
| <p><pre> |
| %start_symbol prog |
| </pre></p> |
| |
| <h4>The <tt>%token_destructor</tt> directive</h4> |
| |
| <p>The %destructor directive assigns a destructor to a non-terminal |
| symbol. (See the description of the %destructor directive above.) |
| This directive does the same thing for all terminal symbols.</p> |
| |
| <p>Unlike non-terminal symbols which may each have a different data type |
| for their values, terminals all use the same data type (defined by |
| the %token_type directive) and so they use a common destructor. Other |
| than that, the token destructor works just like the non-terminal |
| destructors.</p> |
| |
| <h4>The <tt>%token_prefix</tt> directive</h4> |
| |
| <p>Lemon generates #defines that assign small integer constants |
| to each terminal symbol in the grammar. If desired, Lemon will |
| add a prefix specified by this directive |
| to each of the #defines it generates. |
| So if the default output of Lemon looked like this: |
| <pre> |
| #define AND 1 |
| #define MINUS 2 |
| #define OR 3 |
| #define PLUS 4 |
| </pre> |
| You can insert a statement into the grammar like this: |
| <pre> |
| %token_prefix TOKEN_ |
| </pre> |
| to cause Lemon to produce these symbols instead: |
| <pre> |
| #define TOKEN_AND 1 |
| #define TOKEN_MINUS 2 |
| #define TOKEN_OR 3 |
| #define TOKEN_PLUS 4 |
| </pre> |
| |
| <h4>The <tt>%token_type</tt> and <tt>%type</tt> directives</h4> |
| |
| <p>These directives are used to specify the data types for values |
| on the parser's stack associated with terminal and non-terminal |
| symbols. The values of all terminal symbols must be of the same |
| type. This turns out to be the same data type as the 3rd parameter |
| to the Parse() function generated by Lemon. Typically, you will |
| make the value of a terminal symbol by a pointer to some kind of |
| token structure. Like this:</p> |
| |
| <p><pre> |
| %token_type {Token*} |
| </pre></p> |
| |
| <p>If the data type of terminals is not specified, the default value |
| is ``int''.</p> |
| |
| <p>Non-terminal symbols can each have their own data types. Typically |
| the data type of a non-terminal is a pointer to the root of a parse-tree |
| structure that contains all information about that non-terminal. |
| For example:</p> |
| |
| <p><pre> |
| %type expr {Expr*} |
| </pre></p> |
| |
| <p>Each entry on the parser's stack is actually a union containing |
| instances of all data types for every non-terminal and terminal symbol. |
| Lemon will automatically use the correct element of this union depending |
| on what the corresponding non-terminal or terminal symbol is. But |
| the grammar designer should keep in mind that the size of the union |
| will be the size of its largest element. So if you have a single |
| non-terminal whose data type requires 1K of storage, then your 100 |
| entry parser stack will require 100K of heap space. If you are willing |
| and able to pay that price, fine. You just need to know.</p> |
| |
| <h3>Error Processing</h3> |
| |
| <p>After extensive experimentation over several years, it has been |
| discovered that the error recovery strategy used by yacc is about |
| as good as it gets. And so that is what Lemon uses.</p> |
| |
| <p>When a Lemon-generated parser encounters a syntax error, it |
| first invokes the code specified by the %syntax_error directive, if |
| any. It then enters its error recovery strategy. The error recovery |
| strategy is to begin popping the parsers stack until it enters a |
| state where it is permitted to shift a special non-terminal symbol |
| named ``error''. It then shifts this non-terminal and continues |
| parsing. But the %syntax_error routine will not be called again |
| until at least three new tokens have been successfully shifted.</p> |
| |
| <p>If the parser pops its stack until the stack is empty, and it still |
| is unable to shift the error symbol, then the %parse_failed routine |
| is invoked and the parser resets itself to its start state, ready |
| to begin parsing a new file. This is what will happen at the very |
| first syntax error, of course, if there are no instances of the |
| ``error'' non-terminal in your grammar.</p> |
| |
| </body> |
| </html> |
