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30 Bison 2.3 ; %% | exp ’*’ exp { $$ = $1 * $3; } | exp ’/’ exp { $$ = $1 / $3; } | ’-’ exp %prec NEG { $$ = -$2; } | exp ’^’ exp { $$ = pow ($1, $3); } | ’(’ exp ’)’ { $$ = $2; } The functions yylex, yyerror and main can be the same as before. There are two important new features shown in this code. In the second section (Bison declarations), %left declares token types and says they are left-associative operators. The declarations %left and %right (right associativity) take the place of %token which is used to declare a token type name without associativity. (These tokens are single-character literals, which ordinarily don’t need to be declared. We declare them here to specify the associativity.) Operator precedence is determined by the line ordering of the declarations; the higher the line number of the declaration (lower on the page or screen), the higher the precedence. Hence, exponentiation has the highest precedence, unary minus (NEG) is next, followed by ‘*’ and ‘/’, and so on. See Section 5.3 [Operator Precedence], page 73. The other important new feature is the %prec in the grammar section for the unary minus operator. The %prec simply instructs Bison that the rule ‘| ’-’ exp’ has the same precedence as NEG—in this case the next-to-highest. See Section 5.4 [Context-Dependent Precedence], page 75. Here is a sample run of ‘calc.y’: $ calc 4 + 4.5 - (34/(8*3+-3)) 6.880952381 -56 + 2 -54 3 ^ 2 9 2.3 Simple Error Recovery Up to this point, this manual has not addressed the issue of error recovery—how to continue parsing after the parser detects a syntax error. All we have handled is error reporting with yyerror. Recall that by default yyparse returns after calling yyerror. This means that an erroneous input line causes the calculator program to exit. Now we show how to rectify this deficiency. The Bison language itself includes the reserved word error, which may be included in the grammar rules. In the example below it has been added to one of the alternatives for line: line: ; ’\n’ | exp ’\n’ { printf ("\t%.10g\n", $1); } | error ’\n’ { yyerrok; }
Chapter 2: Examples 31 This addition to the grammar allows for simple error recovery in the event of a syntax error. If an expression that cannot be evaluated is read, the error will be recognized by the third rule for line, and parsing will continue. (The yyerror function is still called upon to print its message as well.) The action executes the statement yyerrok, a macro defined automatically by Bison; its meaning is that error recovery is complete (see Chapter 6 [Error Recovery], page 83). Note the difference between yyerrok and yyerror; neither one is a misprint. This form of error recovery deals with syntax errors. There are other kinds of errors; for example, division by zero, which raises an exception signal that is normally fatal. A real calculator program must handle this signal and use longjmp to return to main and resume parsing input lines; it would also have to discard the rest of the current line of input. We won’t discuss this issue further because it is not specific to Bison programs. 2.4 Location Tracking Calculator: ltcalc This example extends the infix notation calculator with location tracking. This feature will be used to improve the error messages. For the sake of clarity, this example is a simple integer calculator, since most of the work needed to use locations will be done in the lexical analyzer. 2.4.1 Declarations for ltcalc The C and Bison declarations for the location tracking calculator are the same as the declarations for the infix notation calculator. /* Location tracking calculator. */ %{ #define YYSTYPE int #include int yylex (void); void yyerror (char const *); %} /* Bison declarations. */ %token NUM %left ’-’ ’+’ %left ’*’ ’/’ %left NEG %right ’^’ %% /* The grammar follows. */ Note there are no declarations specific to locations. Defining a data type for storing locations is not needed: we will use the type provided by default (see Section 3.6.1 [Data Types of Locations], page 51), which is a four member structure with the following integer fields: first_line, first_column, last_line and last_column.
Page 1 and 2: Bison The Yacc-compatible Parser Ge
Page 3 and 4: i Table of Contents Introduction .
Page 5 and 6: 4 Parser C-Language Interface . . .
Page 7 and 8: Introduction 1 Introduction Bison i
Page 9 and 10: Conditions for Using Bison 3 Condit
Page 11 and 12: GNU GENERAL PUBLIC LICENSE 5 GNU GE
Page 13 and 14: GNU GENERAL PUBLIC LICENSE 7 Thus,
Page 15 and 16: GNU GENERAL PUBLIC LICENSE 9 which
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Page 31 and 32: Chapter 2: Examples 25 ; This defin
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Chapter 6: Error Recovery 83 6 Erro
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Chapter 7: Handling Context Depende
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Chapter 8: Debugging Your Parser 89
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Chapter 9: Invoking Bison 97 9 Invo
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Chapter 9: Invoking Bison 99 ‘--d
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Chapter 10: C++ Language Interface
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Chapter 11: Frequently Asked Questi
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Appendix A: Bison Symbols 117 Appen
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Appendix A: Bison Symbols 119 two s
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Appendix A: Bison Symbols 121 or (i
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Appendix B: Glossary 123 Appendix B
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Appendix B: Glossary 125 Reverse po
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Appendix C: Copying This Manual 127
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Index 135 Index $ $$ . . . . . . .
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Index 137 LALR(1) grammars . . . .
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