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.
+ +This document is an introduction to the Lemon +parser generator.
+ +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: +
Depending on command-line options, Lemon will generate between +one and three files of outputs. +
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. + +
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: +
+ Parser statistics: 74 terminals, 70 nonterminals, 179 rules + 340 states, 2026 parser table entries, 0 conflicts ++Finally, the ``-x'' option causes Lemon to print its version number +and then stops without attempting to read the grammar or generate a parser. + +
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 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. + +
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.
+ +A typical use of a Lemon parser might look something like the +following: +
+ 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 }
+
+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.
+
+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.
+ +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). + +
Programmers who have previously used the yacc or bison parser +generator will notice several important differences between yacc and/or +bison and Lemon. +
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, 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++.
+ +A terminal symbol (token) is any string of alphanumeric +and underscore characters +that begins with an upper case letter. +A terminal can contain lower class 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.
+ +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.
+ +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 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.
+ +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 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:
+ ++ %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 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:
+ ++ 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: +
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 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.
+ +Lemon supports the following special directives: +
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.
+ +%code is typically used to include some action routines or perhaps +a tokenizer as part of the output file.
+ +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 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.
+ +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.
+ +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: +
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 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.
+ +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.
+ +
+ %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 %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.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 prototyed in unistd.h.
+ ++ %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.
+ +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 +
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.
+ +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");
+ }
+
+
+
+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");
+ }
+
+
+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.
+ +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 ::= . ++ +
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 ++ +
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 ++ +
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.
+ +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.
+ +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 ++ +
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:
+ +
+ %token_type {Token*}
+
+
+If the data type of terminals is not specified, the default value +is ``int''.
+ +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.
+ +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. But 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_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.
+ + + -- cgit v1.1