Newsgroups: comp.compilers From: Chris DoddI've produced an enhanced version of yacc, which supports backtracking in the event of parse conflicts and inherited attributes. These are actually closely connected as inherited attributes in a bottom-up parse tend to introduce conflicts. The program is public domain and is available for ftp atSubject: BTYACC -- yacc with backtracking and inherited attributes. Keywords: yacc, attribute, available, FTP Organization: Compilers Central Date: 07 Mar 1995 02:57:28 MET
ftp://ftp.csl.sri.com/pub/dodd/btyacc.tar.gz
Chris Dodd
dodd@csl.sri.com
README file from btyacc: BTYACC -- backtracking yacc. BTYACC was created by Chris Dodd using ideas from many places and lots of code from the Berkeley Yacc distribution, which is a public domain yacc clone put together by the good folks at Berkeley. This code is distributed with NO WARRANTEE and is public domain. It is certain to contain bugs, which you should report to: dodd@csl.sri.com See the README.BYACC, NEW_FEATURES, and ACKNOWLEDGEMENTS files for more about Berkeley Yacc. BTYACC is a modified version of yacc that supports automatic backtracking and semantic disambiguation to parse ambiguous grammars, as well as syntactic sugar for inherited attributes (which tend to introduce conflicts). Whenever a btyacc generated parser runs into a shift-reduce or reduce-reduce error in the parse table, it remembers the current parse point (yacc stack and input stream state), and goes into trial parse mode. It then continues parsing, ignoring most rule actions. If it runs into an error (either through the parse table or through an action calling YYERROR), it backtracks to the most recent conflict point and tries a different alternative. If it finds a successful parse (reaches the end of the input or an action calls YYVALID), it backtracks to the point where it first entered trial parse mode, and continues with a full parse (executing all actions), following the path of the successful trial. Actions in btyacc come in two flavors -- {}-actions, which are only executed when not in trial mode, and []-actions which are executed regardless of mode. There are also inherited attributes, which look like arguments (they're enclosed in `()') and act like []-actions. What this buys you: * No more lexer feedback hack. In yacc grammars for C, a standard hack, known as the `lexer feedback hack' is used to find typedef names. The lexer uses semantic information to decide if any given identifier is a typedef-name or not and returns a special token. With btyacc, you no longer need to do this; the lexer should just always return an identifier. The btyacc grammar then needs a rule of the form: typename: ID [ if (!IsTypeName(LookupId($1))) YYERROR; ] While the hack works adequately well for parsing C, it becomes a nightmare when you try to parse something like C++, where treating an ID as a typedef becomes heavily dependent on context. * Easy disambiguation via simple ordering. Btyacc runs its trials via the rule ``try shifting first, then try reducing by the order that the conflicting rules appear in the input file''. This means you can deal with semantic a disambiguation rule like: [1] If it looks like a declaration it is, otherwise [2] If it looks like an expression it is, otherwise [3] it is a syntax error [Ellis & Stroustrup, Annotated C++ Reference Manual, p93] To deal with this, you need only put all the rules for declarations before the rules for expressions in the grammar file. * No extra cost if don't use it. Backtracking is only triggered when the parse hits a shift/reduce or reduce/reduce conflict in the table. If you have no conflicts in your grammar, there's no extra cost, other than some extra code which will never be invoked. * C++ and ANSI C compatible parsers. The parsers produced by btyacc can be compiled with C++ correctly. If you `#define' YYSTYPE to be some C++ type with constructor and destructor, everything will work fine. My favorite is `#define YYSTYPE SmartPointer', where SmartPointer is a smart pointer type that does garbage collection on the pointed to objects. BTYACC was originally written to make it easy to write a C++ parser (my goal was to be able to use the grammar out of the back of the ARM with as few modifications as possible). Anyone who has ever looked at Jim Roskind's public domain C++ yacc grammar, or the yacc-based grammar used in g++ knows how difficult this is. BTYACC is very useful for parsing any ambiguous grammar, particularly ones that come from trying to merge two (or more) complete grammars. Inherited attributes in btyacc: Inherited attributes look a lot like function arguments to non-terminals, which is what they end up being in a recursive descent parser, but NOT how they're implemented in btyacc. Basically they're just syntactic sugar for embedded semantic actions and $0, $-1, ... in normal yacc. btyacc gives you two big advantages besides just the syntax: 1. it does type checking on the inherited attributes, so you don't have to specify $0 and makes sure you give the correct` number of arguments (inherited attributes) to every use of a non-terminal. 2. It `collapses' identical actions. It actually does this for all actions, not just the ones from inherited attributes. This eliminates reduce-reduce conflicts due to identical actions, which actually happens quite a lot when you use inherited attributes. You use inherited attributes by declaring the types of the attributes in the preamble with a type declaration and declaring names of the attributes on the lhs of the yacc rule. You can of course have more than one rule with the same rhs, and you can even give them different names in each, but the type and number must be the same. Here's a small example: %type lhs( , ) /* lhs takes two inherited attributes */ stuff( , ) %% lhs($i1, $i2) : { $$ = $i1 } | lhs($i1, $i2) stuff($1,$i2) { $$ = $2; } This is roughly equivalent to the following yacc code: lhs : { $$ = $ -1; } | lhs [ $ $ = $1; ] [ $ $ = $ 0; ] stuff { $$ = $4; } ; See the file `test/t2.y' for a longer and more complete example. At the current time, the start symbol cannot have any arguments. -- Send compilers articles to compilers@iecc.com, meta-mail to compilers-request@iecc.com.