Transcript Yacc Example: Calculator - College of Engineering and
Compiler Design
Yacc Example
"
Yet Another Compiler Compiler"
Kanat Bolazar
Lex and Yacc
• • • Two classical tools for compilers: – –
Lex
: A Lexical Analyzer Generator
Yacc
: “Yet Another Compiler Compiler” (Parser Generator)
Lex
creates programs that scan your tokens one by one.
Yacc
takes a grammar (sentence structure) and generates a parser.
Lexical Rules Grammar Rules Input
Lex
yylex()
Yacc
yyparse() Parsed Input 2
Lex and Yacc
•
Lex
and
Yacc
generate C code for your analyzer & parser.
Lexical Rules Grammar Rules C code
Lex
C code
Yacc
Input char stream yylex() C code Lexical Analyzer (Tokenizer) token stream yyparse() C code Parser Parsed Input 3
Flex, Yacc, Bison, Byacc
• Often, instead of the standard
Lex
and
Yacc
,
Flex
and
Bison
are used: –
Flex
: A fast lexical analyzer – (GNU)
Bison
: A drop-in replacement for (backwards compatible with) Yacc •
Byacc
is Berkeley implementation of • Resources:
Yacc
(so it is
Yacc
).
http://en.wikipedia.org/wiki/Flex_lexical_analyser http://en.wikipedia.org/wiki/GNU_Bison • The Lex & Yacc Page (manuals, links): http://dinosaur.compilertools.net/ 4
Yacc: A Standard Parser Generator
• Yacc is not a new tool, and yet, it is still used in many projects.
• Yacc syntax is similar to Lex/Flex at the top level.
• Lex/Flex rules were regular expression – action pairs.
• Yacc rules are grammar rule – action pairs.
declarations %% rules %% programs 5
Yacc Examples: Calculator
• • • • • A standard Yacc example is the int-valued calculator.
Appendix A of Yacc manual at Lex and Yacc Page shows such a calculator.
We'll examine this example in parts.
Let's start with four operations: E -> E + E | E – E | E * E | E / E Note that this grammar is ambiguous because 2 + 5 * 7 could be parsed 2 + 5 first or 5 * 7 first.
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Yacc Calculator Example: Declarations
%{ # include
%} %start list %token DIGIT LETTER DIGIT & LETTER are tokens; (other tokens use ASCII codes, as in '+', '=', etc) %left '+' '-' %left '*' '/' '%' Precedence and associativity (left) of %left UMINUS /* precedence for unary minus */ operators: +, - have lowest precedence *, / have higher precedence Unary minus has highest precedence 7
Yacc Calculator Example: Rules
%% /* begin rules section */ list : /* empty */ | list stat '\n' ; | list error '\n' { yyerrok; } list: a list of one-line statements / expressions.
Error handling allows a statement to be corrupt, but list continues with next statement.
statement: expression to calculate, or assignment stat : expr { printf( "%d\n", $1 ); } ; | LETTER '=' expr { regs[$1] = $3; } number: made up of digits (tokenizer should handle this, but this is a simple example).
number: DIGIT { $$ = $1; base = ($1==0) ? 8 : 10; } ; | number DIGIT { $$ = base * $1 + $2; } 8
Yacc Calculator Example: Rules, cont'd
expr : '(' expr ')' { $$ = $2; } | expr '+' expr { $$ = $1 + $3; } | expr '-' expr { $$ = $1 - $3; } | expr '*' expr { $$ = $1 * $3; } | expr '/' expr { $$ = $1 / $3; } | '-' expr %prec UMINUS { $$ = - $2; } | LETTER { $$ = regs[$1]; } ; | number Unary minus Letter: Register/var 9
Yacc Calculator Example: Programs (C Code)
%% /* start of programs */ yylex() { /* lexical analysis routine */ /* returns LETTER for a lower case letter, yylval = 0 through 25 */ /* return DIGIT for a digit, yylval = 0 through 9 */ /* all other characters are returned immediately */ int c; while( (c=getchar()) == ' ' ) {/* skip blanks */ } /* c is now nonblank */ if( islower( c ) ) { yylval = c - 'a'; } return ( LETTER ); if( isdigit( c ) ) { yylval = c - '0'; } return( DIGIT ); } return( c ); 10