2. Syntax Specification with YACC © Allan C. Milne Abertay University v14.6.16

3. Agenda. What is Yacc/Bison? Yacc/Lex Example. The Parser. Parsing Approaches.

4. What Is Yacc? Yet Another Compiler Compiler. A parser generator for LALR(1) grammars. The generated parser also includes associated semantic actions. Yacc generates C code as the implementation of the parser/semantic processor.

5. What Is Bison? An implementation of a YACC– compatible parser generator. Includes extensions and C++ support. Distributed as open software by Gnu.

6. Expressing A Grammar In Yacc. Yacc uses a variation on the BNF meta-language. For a full description of the Yacc grammar see chapter 3 of the Bison manual. Using this approach normally requires two specifications: –The BNF syntactic structure defined as a Yacc program; and –The lexical token structure defined as a Lex program.

7. The Yacc Specification. %token tANT tBAT tCAT tDOG % AnimalList : ‘(‘ ‘)’ | ‘(‘ Animal ‘)’ | '(' MoreAnimals ‘,’ Animal ')' ; MoreAnimals : Animal | MoreAnimals ',' Animal ; Animal : tANT | tBAT | tCAT | tDOG ; %

8. Terminal Tokens Represented By … A C-style character literal. A C-style string literal: –In Bison only, not available in Yacc; –May be aliased using %token LE “<=”. An identifier: –ERROR is reserved for error recovery; –Convention is to use uppercase; prefix with ‘t’ to clarify its meaning; –Must be defined in %token statement.

9. The Lex Specification. %{ #include "AnimalList.tab.h" void yyerror (char*); %} % ant return tANT; bat return tBAT; cat return tCAT; dog return tDOG; [,()] return *yytext; [ \t\n] ;. yyerror ("Invalid character found."); % int yywrap () { return 1; }

10. Note The Differences … Lex is used here as a scanner rather than a self- contained tool. The ‘#include’ exposes the ‘%token’ symbols defined as integers. No main() method; the yylex() method is called repeatedly by the Yacc parser. Return an integer representing the token found; ASCII codes reserved for corresponding characters.

11. The Parsing Phase. This phase of a translator is primarily concerned with determining the syntactic structure of an input program. It also often acts as the controlling phase of lexical analysis and semantic processing.

12. Parsing Approaches. Parsing a program is finding a derivation sequence for the program. There are two general approaches to this: –Top-down; start from the distinguished symbol and find substitutions that result in the target program. –Bottom-up; start with the input program and try to reduce this back to the distinguished symbol. This latter is the approach used by Yacc.