893 lines
34 KiB
HTML
893 lines
34 KiB
HTML
<html>
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<head>
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<title>The Lemon Parser Generator</title>
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</head>
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<body bgcolor=white>
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<h1 align=center>The Lemon Parser Generator</h1>
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<p>Lemon is an LALR(1) parser generator for C or C++.
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It does the same job as ``bison'' and ``yacc''.
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But lemon is not another bison or yacc clone. It
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uses a different grammar syntax which is designed to
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reduce the number of coding errors. Lemon also uses a more
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sophisticated parsing engine that is faster than yacc and
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bison and which is both reentrant and thread-safe.
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Furthermore, Lemon implements features that can be used
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to eliminate resource leaks, making is suitable for use
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in long-running programs such as graphical user interfaces
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or embedded controllers.</p>
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<p>This document is an introduction to the Lemon
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parser generator.</p>
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<h2>Theory of Operation</h2>
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<p>The main goal of Lemon is to translate a context free grammar (CFG)
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for a particular language into C code that implements a parser for
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that language.
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The program has two inputs:
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<ul>
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<li>The grammar specification.
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<li>A parser template file.
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</ul>
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Typically, only the grammar specification is supplied by the programmer.
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Lemon comes with a default parser template which works fine for most
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applications. But the user is free to substitute a different parser
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template if desired.</p>
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<p>Depending on command-line options, Lemon will generate between
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one and three files of outputs.
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<ul>
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<li>C code to implement the parser.
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<li>A header file defining an integer ID for each terminal symbol.
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<li>An information file that describes the states of the generated parser
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automaton.
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</ul>
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By default, all three of these output files are generated.
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The header file is suppressed if the ``-m'' command-line option is
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used and the report file is omitted when ``-q'' is selected.</p>
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<p>The grammar specification file uses a ``.y'' suffix, by convention.
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In the examples used in this document, we'll assume the name of the
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grammar file is ``gram.y''. A typical use of Lemon would be the
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following command:
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<pre>
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lemon gram.y
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</pre>
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This command will generate three output files named ``gram.c'',
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``gram.h'' and ``gram.out''.
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The first is C code to implement the parser. The second
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is the header file that defines numerical values for all
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terminal symbols, and the last is the report that explains
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the states used by the parser automaton.</p>
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<h3>Command Line Options</h3>
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<p>The behavior of Lemon can be modified using command-line options.
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You can obtain a list of the available command-line options together
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with a brief explanation of what each does by typing
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<pre>
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lemon -?
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</pre>
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As of this writing, the following command-line options are supported:
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<ul>
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<li><tt>-b</tt>
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<li><tt>-c</tt>
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<li><tt>-g</tt>
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<li><tt>-m</tt>
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<li><tt>-q</tt>
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<li><tt>-s</tt>
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<li><tt>-x</tt>
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</ul>
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The ``-b'' option reduces the amount of text in the report file by
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printing only the basis of each parser state, rather than the full
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configuration.
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The ``-c'' option suppresses action table compression. Using -c
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will make the parser a little larger and slower but it will detect
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syntax errors sooner.
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The ``-g'' option causes no output files to be generated at all.
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Instead, the input grammar file is printed on standard output but
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with all comments, actions and other extraneous text deleted. This
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is a useful way to get a quick summary of a grammar.
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The ``-m'' option causes the output C source file to be compatible
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with the ``makeheaders'' program.
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Makeheaders is a program that automatically generates header files
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from C source code. When the ``-m'' option is used, the header
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file is not output since the makeheaders program will take care
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of generated all header files automatically.
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The ``-q'' option suppresses the report file.
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Using ``-s'' causes a brief summary of parser statistics to be
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printed. Like this:
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<pre>
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Parser statistics: 74 terminals, 70 nonterminals, 179 rules
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340 states, 2026 parser table entries, 0 conflicts
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</pre>
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Finally, the ``-x'' option causes Lemon to print its version number
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and then stops without attempting to read the grammar or generate a parser.</p>
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<h3>The Parser Interface</h3>
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<p>Lemon doesn't generate a complete, working program. It only generates
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a few subroutines that implement a parser. This section describes
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the interface to those subroutines. It is up to the programmer to
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call these subroutines in an appropriate way in order to produce a
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complete system.</p>
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<p>Before a program begins using a Lemon-generated parser, the program
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must first create the parser.
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A new parser is created as follows:
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<pre>
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void *pParser = ParseAlloc( malloc );
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</pre>
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The ParseAlloc() routine allocates and initializes a new parser and
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returns a pointer to it.
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The actual data structure used to represent a parser is opaque --
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its internal structure is not visible or usable by the calling routine.
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For this reason, the ParseAlloc() routine returns a pointer to void
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rather than a pointer to some particular structure.
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The sole argument to the ParseAlloc() routine is a pointer to the
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subroutine used to allocate memory. Typically this means ``malloc()''.</p>
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<p>After a program is finished using a parser, it can reclaim all
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memory allocated by that parser by calling
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<pre>
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ParseFree(pParser, free);
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</pre>
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The first argument is the same pointer returned by ParseAlloc(). The
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second argument is a pointer to the function used to release bulk
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memory back to the system.</p>
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<p>After a parser has been allocated using ParseAlloc(), the programmer
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must supply the parser with a sequence of tokens (terminal symbols) to
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be parsed. This is accomplished by calling the following function
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once for each token:
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<pre>
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Parse(pParser, hTokenID, sTokenData, pArg);
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</pre>
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The first argument to the Parse() routine is the pointer returned by
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ParseAlloc().
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The second argument is a small positive integer that tells the parse the
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type of the next token in the data stream.
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There is one token type for each terminal symbol in the grammar.
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The gram.h file generated by Lemon contains #define statements that
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map symbolic terminal symbol names into appropriate integer values.
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(A value of 0 for the second argument is a special flag to the
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parser to indicate that the end of input has been reached.)
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The third argument is the value of the given token. By default,
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the type of the third argument is integer, but the grammar will
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usually redefine this type to be some kind of structure.
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Typically the second argument will be a broad category of tokens
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such as ``identifier'' or ``number'' and the third argument will
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be the name of the identifier or the value of the number.</p>
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<p>The Parse() function may have either three or four arguments,
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depending on the grammar. If the grammar specification file request
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it, the Parse() function will have a fourth parameter that can be
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of any type chosen by the programmer. The parser doesn't do anything
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with this argument except to pass it through to action routines.
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This is a convenient mechanism for passing state information down
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to the action routines without having to use global variables.</p>
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<p>A typical use of a Lemon parser might look something like the
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following:
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<pre>
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01 ParseTree *ParseFile(const char *zFilename){
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02 Tokenizer *pTokenizer;
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03 void *pParser;
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04 Token sToken;
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05 int hTokenId;
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06 ParserState sState;
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07
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08 pTokenizer = TokenizerCreate(zFilename);
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09 pParser = ParseAlloc( malloc );
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10 InitParserState(&sState);
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11 while( GetNextToken(pTokenizer, &hTokenId, &sToken) ){
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12 Parse(pParser, hTokenId, sToken, &sState);
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13 }
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14 Parse(pParser, 0, sToken, &sState);
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15 ParseFree(pParser, free );
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16 TokenizerFree(pTokenizer);
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17 return sState.treeRoot;
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18 }
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</pre>
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This example shows a user-written routine that parses a file of
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text and returns a pointer to the parse tree.
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(We've omitted all error-handling from this example to keep it
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simple.)
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We assume the existence of some kind of tokenizer which is created
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using TokenizerCreate() on line 8 and deleted by TokenizerFree()
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on line 16. The GetNextToken() function on line 11 retrieves the
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next token from the input file and puts its type in the
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integer variable hTokenId. The sToken variable is assumed to be
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some kind of structure that contains details about each token,
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such as its complete text, what line it occurs on, etc. </p>
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<p>This example also assumes the existence of structure of type
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ParserState that holds state information about a particular parse.
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An instance of such a structure is created on line 6 and initialized
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on line 10. A pointer to this structure is passed into the Parse()
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routine as the optional 4th argument.
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The action routine specified by the grammar for the parser can use
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the ParserState structure to hold whatever information is useful and
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appropriate. In the example, we note that the treeRoot field of
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the ParserState structure is left pointing to the root of the parse
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tree.</p>
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<p>The core of this example as it relates to Lemon is as follows:
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<pre>
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ParseFile(){
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pParser = ParseAlloc( malloc );
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while( GetNextToken(pTokenizer,&hTokenId, &sToken) ){
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Parse(pParser, hTokenId, sToken);
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}
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Parse(pParser, 0, sToken);
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ParseFree(pParser, free );
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}
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</pre>
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Basically, what a program has to do to use a Lemon-generated parser
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is first create the parser, then send it lots of tokens obtained by
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tokenizing an input source. When the end of input is reached, the
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Parse() routine should be called one last time with a token type
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of 0. This step is necessary to inform the parser that the end of
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input has been reached. Finally, we reclaim memory used by the
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parser by calling ParseFree().</p>
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<p>There is one other interface routine that should be mentioned
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before we move on.
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The ParseTrace() function can be used to generate debugging output
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from the parser. A prototype for this routine is as follows:
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<pre>
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ParseTrace(FILE *stream, char *zPrefix);
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</pre>
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After this routine is called, a short (one-line) message is written
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to the designated output stream every time the parser changes states
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or calls an action routine. Each such message is prefaced using
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the text given by zPrefix. This debugging output can be turned off
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by calling ParseTrace() again with a first argument of NULL (0).</p>
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<h3>Differences With YACC and BISON</h3>
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<p>Programmers who have previously used the yacc or bison parser
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generator will notice several important differences between yacc and/or
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bison and Lemon.
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<ul>
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<li>In yacc and bison, the parser calls the tokenizer. In Lemon,
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the tokenizer calls the parser.
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<li>Lemon uses no global variables. Yacc and bison use global variables
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to pass information between the tokenizer and parser.
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<li>Lemon allows multiple parsers to be running simultaneously. Yacc
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and bison do not.
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</ul>
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These differences may cause some initial confusion for programmers
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with prior yacc and bison experience.
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But after years of experience using Lemon, I firmly
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believe that the Lemon way of doing things is better.</p>
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<h2>Input File Syntax</h2>
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<p>The main purpose of the grammar specification file for Lemon is
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to define the grammar for the parser. But the input file also
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specifies additional information Lemon requires to do its job.
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Most of the work in using Lemon is in writing an appropriate
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grammar file.</p>
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<p>The grammar file for lemon is, for the most part, free format.
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It does not have sections or divisions like yacc or bison. Any
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declaration can occur at any point in the file.
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Lemon ignores whitespace (except where it is needed to separate
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tokens) and it honors the same commenting conventions as C and C++.</p>
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<h3>Terminals and Nonterminals</h3>
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<p>A terminal symbol (token) is any string of alphanumeric
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and underscore characters
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that begins with an upper case letter.
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A terminal can contain lowercase letters after the first character,
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but the usual convention is to make terminals all upper case.
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A nonterminal, on the other hand, is any string of alphanumeric
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and underscore characters than begins with a lower case letter.
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Again, the usual convention is to make nonterminals use all lower
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case letters.</p>
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<p>In Lemon, terminal and nonterminal symbols do not need to
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be declared or identified in a separate section of the grammar file.
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Lemon is able to generate a list of all terminals and nonterminals
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by examining the grammar rules, and it can always distinguish a
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terminal from a nonterminal by checking the case of the first
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character of the name.</p>
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<p>Yacc and bison allow terminal symbols to have either alphanumeric
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names or to be individual characters included in single quotes, like
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this: ')' or '$'. Lemon does not allow this alternative form for
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terminal symbols. With Lemon, all symbols, terminals and nonterminals,
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must have alphanumeric names.</p>
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<h3>Grammar Rules</h3>
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<p>The main component of a Lemon grammar file is a sequence of grammar
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rules.
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Each grammar rule consists of a nonterminal symbol followed by
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the special symbol ``::='' and then a list of terminals and/or nonterminals.
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The rule is terminated by a period.
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The list of terminals and nonterminals on the right-hand side of the
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rule can be empty.
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Rules can occur in any order, except that the left-hand side of the
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first rule is assumed to be the start symbol for the grammar (unless
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specified otherwise using the <tt>%start</tt> directive described below.)
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A typical sequence of grammar rules might look something like this:
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<pre>
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expr ::= expr PLUS expr.
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expr ::= expr TIMES expr.
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expr ::= LPAREN expr RPAREN.
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expr ::= VALUE.
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</pre>
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</p>
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<p>There is one non-terminal in this example, ``expr'', and five
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terminal symbols or tokens: ``PLUS'', ``TIMES'', ``LPAREN'',
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``RPAREN'' and ``VALUE''.</p>
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<p>Like yacc and bison, Lemon allows the grammar to specify a block
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of C code that will be executed whenever a grammar rule is reduced
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by the parser.
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In Lemon, this action is specified by putting the C code (contained
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within curly braces <tt>{...}</tt>) immediately after the
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period that closes the rule.
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For example:
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<pre>
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expr ::= expr PLUS expr. { printf("Doing an addition...\n"); }
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</pre>
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</p>
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<p>In order to be useful, grammar actions must normally be linked to
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their associated grammar rules.
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In yacc and bison, this is accomplished by embedding a ``$$'' in the
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action to stand for the value of the left-hand side of the rule and
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symbols ``$1'', ``$2'', and so forth to stand for the value of
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the terminal or nonterminal at position 1, 2 and so forth on the
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right-hand side of the rule.
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This idea is very powerful, but it is also very error-prone. The
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single most common source of errors in a yacc or bison grammar is
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to miscount the number of symbols on the right-hand side of a grammar
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rule and say ``$7'' when you really mean ``$8''.</p>
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<p>Lemon avoids the need to count grammar symbols by assigning symbolic
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names to each symbol in a grammar rule and then using those symbolic
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names in the action.
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In yacc or bison, one would write this:
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<pre>
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expr -> expr PLUS expr { $$ = $1 + $3; };
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</pre>
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But in Lemon, the same rule becomes the following:
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<pre>
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expr(A) ::= expr(B) PLUS expr(C). { A = B+C; }
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</pre>
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In the Lemon rule, any symbol in parentheses after a grammar rule
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symbol becomes a place holder for that symbol in the grammar rule.
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This place holder can then be used in the associated C action to
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stand for the value of that symbol.<p>
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<p>The Lemon notation for linking a grammar rule with its reduce
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action is superior to yacc/bison on several counts.
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First, as mentioned above, the Lemon method avoids the need to
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count grammar symbols.
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Secondly, if a terminal or nonterminal in a Lemon grammar rule
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includes a linking symbol in parentheses but that linking symbol
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is not actually used in the reduce action, then an error message
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is generated.
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For example, the rule
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<pre>
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expr(A) ::= expr(B) PLUS expr(C). { A = B; }
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</pre>
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will generate an error because the linking symbol ``C'' is used
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in the grammar rule but not in the reduce action.</p>
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<p>The Lemon notation for linking grammar rules to reduce actions
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also facilitates the use of destructors for reclaiming memory
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allocated by the values of terminals and nonterminals on the
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right-hand side of a rule.</p>
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<h3>Precedence Rules</h3>
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<p>Lemon resolves parsing ambiguities in exactly the same way as
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yacc and bison. A shift-reduce conflict is resolved in favor
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of the shift, and a reduce-reduce conflict is resolved by reducing
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whichever rule comes first in the grammar file.</p>
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<p>Just like in
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yacc and bison, Lemon allows a measure of control
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over the resolution of paring conflicts using precedence rules.
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A precedence value can be assigned to any terminal symbol
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using the %left, %right or %nonassoc directives. Terminal symbols
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mentioned in earlier directives have a lower precedence that
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terminal symbols mentioned in later directives. For example:</p>
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<p><pre>
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%left AND.
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%left OR.
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%nonassoc EQ NE GT GE LT LE.
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%left PLUS MINUS.
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%left TIMES DIVIDE MOD.
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%right EXP NOT.
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</pre></p>
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<p>In the preceding sequence of directives, the AND operator is
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defined to have the lowest precedence. The OR operator is one
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precedence level higher. And so forth. Hence, the grammar would
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attempt to group the ambiguous expression
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<pre>
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a AND b OR c
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</pre>
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like this
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<pre>
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a AND (b OR c).
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</pre>
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The associativity (left, right or nonassoc) is used to determine
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the grouping when the precedence is the same. AND is left-associative
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in our example, so
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<pre>
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a AND b AND c
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</pre>
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is parsed like this
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<pre>
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(a AND b) AND c.
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</pre>
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The EXP operator is right-associative, though, so
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<pre>
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a EXP b EXP c
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</pre>
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is parsed like this
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<pre>
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a EXP (b EXP c).
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</pre>
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The nonassoc precedence is used for non-associative operators.
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So
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<pre>
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a EQ b EQ c
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</pre>
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is an error.</p>
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<p>The precedence of non-terminals is transferred to rules as follows:
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The precedence of a grammar rule is equal to the precedence of the
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left-most terminal symbol in the rule for which a precedence is
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defined. This is normally what you want, but in those cases where
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you want to precedence of a grammar rule to be something different,
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you can specify an alternative precedence symbol by putting the
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symbol in square braces after the period at the end of the rule and
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before any C-code. For example:</p>
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<p><pre>
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expr = MINUS expr. [NOT]
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</pre></p>
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<p>This rule has a precedence equal to that of the NOT symbol, not the
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MINUS symbol as would have been the case by default.</p>
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<p>With the knowledge of how precedence is assigned to terminal
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symbols and individual
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grammar rules, we can now explain precisely how parsing conflicts
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are resolved in Lemon. Shift-reduce conflicts are resolved
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as follows:
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<ul>
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<li> If either the token to be shifted or the rule to be reduced
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lacks precedence information, then resolve in favor of the
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shift, but report a parsing conflict.
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<li> If the precedence of the token to be shifted is greater than
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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>
|