More LR Parsing and Bison CPSC 388 Ellen Walker Hiram College

More than SLR(1) SLR(k) Parsing –Multiple-token lookahead (for shifts) and multiple-token follow information (for reductons) General LR(1) parsing –Include lookaheads in DFA construction LALR(1) parsing –Simplified state diagram for GLR(1) –What YACC / Bison uses

LALR: LR(0) + Lookahead NFA states are [ LR(0) item, lookahead] –Examples: [S->.(S), $], [S->.a,)] –After comma is first token after the RHS Building DFA –[S->.(S),$] --(--> [S->(.S),$] same as SLR –[S->(.S),$] --  --> [S->.(S),)] propagate LA Rule for every S-rule, every first of what follows S in original rule

YACC / Bison “Yet another Compiler-Compiler” Given CFG, automatically creates LALR table Using bison: –Input file: grammar.y –Output file: grammar.tab.c –Generic main reads lines, executes rules

Structure of a Bison File Definitions, including direct code in %{ %} % Rules of the grammar, with actions % Additional code, e.g. main(){ return yyparse() }

Example: Expression Calculator Rules describe the usual grammar –S’ -> exp –exp -> exp + term | exp - term | term –term -> term * factor | factor –factor -> NUMBER | ( exp ) Associated actions execute the arithmetic

Bison Rule Syntax LHS followed by : Each alternative followed by action, then | ; after the last action Example factor : NUMBER {$$ = $1;} | '(' exp ')' {$$ = $2;} ;

Bison Actions Rules include actions in { } (code) Predefined variables: –$$ value of result of rule (YYSTYPE or int) –$1 value of first token, $2 value of second token, etc. Example –Exp: exp ‘+’ term {$$ = $1 + $3;}

Bison and Flex together Define tokens in definition section: –%token ID –Choose values > 256 Make sure lex.yy.c and yy.tab.c agree on token ID defs #define ID val Compile both together –g++ -o myparser yy.tab.c lex.yy.c -lfl

Flex for Bison Each rule should return a token type –E.g. return NUMBER; In addition, a token value can be saved in the global variable yylval –E.g. yylval = myAtoI(yytext);

Mixing Characters and Tokens Don’t assign token values < 256 Allow characters to be their own tokens (rule):.return(yytext[0]); Or be specific: [-+*()]return(yytext[0]);

A Few Gotcha’s Bison (and flex) like tabs, not spaces Beware of commenting out closing } with // C (++) requires functions to be defined before they are used –Copy signatures to top of file –“extern” for functions and variables defined in other files

Bison Individual Homework Use Bison to parse and interpret simple LISP-like commands –( cons a (cons b nil)) => (a b) –( cons (cons a nil) (cons b nil) => ((a) b) –(car (cons a (cons b nil))) => a –(cdr (cons a (cons b nil))) => (b) –(cdr (cdr (cons a (cons b nil)))) => nil See handout for details

Error Handling in BU parsing Error = blank entry in parsing table To give specific error messages –Many error entries (but bigger table!) –Detect error before reducing when possible LR(1) is better than SLR(1) here

Recovery Panic mode: –Pop states from the stack until the parse can be restarted –Advance input until a legal transition is available Error productions –Treat “error” as a pseudotoken –Rules indicate how much to throw away

Error Example command : exp {cout << $1 << endl;} | error {yyerror “bad cmd”;} Once a command is in error, parser will –Perform the error action –Delete tokens until a legal follow of command ($ here)