Yacc

Yacc: Yet Another Compiler-Compiler

1. COMPILER DESIGN YACC:Yet Another Compiler -Compiler

2. YACC: Yet Another Compiler-Compiler Yacc generates C code for syntax analyzer, of parser. Yacc uses grammar rules that allow it to analyze tokens from LEX and create a syntax tree. Yacc provides a general tool for describing the input to a computer program. The Yacc user specifies the structures of his input, together with code to be invoked as each such structure is recognized. Yacc is written in portable C . The class of specifications accepted is a very general one: LALR grammars with disambiguating rules. In addition to compilers for C, APL, Pascal, RATFOR , etc., Yacc has also been used for less conventional languages, including a phototypesetter language, several desk calculator languages, a document retrieval system, and a Fortran debugging system. 1/31/2017 ANKUR SRIVASTAVA ASSISTANT PROFESSOR JIT 2

3. Yacc contd……. Yacc  is a computer program for the Unix operating system. It is a  Look Ahead Left-to-Right (LALR) parser generator, generating a parser. The part of a compiler that tries to make syntactic sense of the source code, specifically a LALR parser, based on an analytic grammar written in a notation similar to Backus–Naur Form (BNF). Yacc was originally developed in the early 1970s by  Stephen C. Johnson  at AT&T Corporation and written in the B programming language, but soon rewritten in C. Yacc has also been rewritten for other languages, including  OCaml ,  Ratfor ,  ML ,  Ada ,  Pascal ,  Java ,  Python ,  Ruby ,  Go  and  Common Lisp . 1/31/2017 ANKUR SRIVASTAVA ASSISTANT PROFESSOR JIT 3

4. 4 yacc Generate a new parser code from grammar YACC source (*.y) y.tab.h y.tab.c C compiler/linker Compile a new parser y.tab.c a.out a.out Parse source code Token stream Abstract Syntax Tree y.output How Does YACC Work?

5. PLLab, NTHU,Cs2403 Programming Languages 5 Lex with Yacc Lex Yacc yylex() yyparse() Lex source (Lexical Rules) Yacc source (Grammar Rules) Input Parsed Input Lex Source (Lexical Rules) Yacc Source (Grammar Rules) y.tab.c lex.yy.c call Parsed Input Input Return token

6. LALR PARSER GENERATOR- yacc The tool yacc can be used to generate automatically an LALR parser. Input of yacc is divided into three sections: ………..definitions……….. %% …..rules….. %% …….subroutines……….. The def section consists of token declarations & C code bracketed by % { and % }. The grammar is placed in the rules section, & User subroutines are added in subroutines section . 1/31/2017 ANKUR SRIVASTAVA ASSISTANT PROFESSOR JIT 6

7. Example- small calculator The def section for the yacc input file: %token INTEGER This def declares an INTEGER token. When we run yacc, it generates a parser file y.tab.c & also creates an include file y.tab.h : #ifndef YYSTYPE #define YYSTYPE int #endif #define INTEGER 258 extern YYSTYPE yylval ; 1/31/2017 ANKUR SRIVASTAVA ASSISTANT PROFESSOR JIT 7

8. Contd…… Lex includes this file & utilizes the definitions for token values. To obtain tokens, yacc calls yylex. Function yylex has a return type of int & returns the token value. Attributes associated with the token are returned by lex in variable yylval . File Content calc.lex Specifies the  lex  command specification file that defines the lexical analysis rules. calc.yacc Specifies the  yacc  command grammar file that defines the parsing rules, and calls the  yylex  subroutine created by the  lex  command to provide input. 1/31/2017 ANKUR SRIVASTAVA ASSISTANT PROFESSOR JIT 8

9. Free APL Compilers and Interpreters ELI ELI is an array programming language that has most of the features of APL with additional ones not present in the language. NGN APL This is an APL written in JavaScript. (Actually, it is written in Coffee Script, which compiles to JavaScript.)  NARS2000 NARS2000 (where NARS stands for Nested Arrays Research System ) is an experimental APL interpreter used as a test bed for new ideas in the language. J J is a programming language in the APL language family. Unlike APL, it uses ASCII characters. 1/31/2017 ANKUR SRIVASTAVA ASSISTANT PROFESSOR JIT 9

10. Contd……. A+ According to its website, "A+ is a descendent of the language 'A'". Unlike APL, A+ is "geared" to business, supporting large capacity and high performance, with the "+" referring to the "electric graphical user interface". It is released under the GNU General Public License. Supported platforms include Linux, FreeBSD, AIX, IRIX, Mac OS X, NetBSD, Solaris, and Tru64. OpenAPL This package, for Linux, FreeBSD, OpenBSD, Mac OS X, and POSIX/Unix-like systems, includes the APL font as well as the interpreter for APL11. We will have to compile it from the sources provided. APL Special Edition APLSE is a freeware version of APL*Plus PC, a compiler for the APL programming language, a highly symbolic language ( ie , it uses pictorial symbols rather than the traditional English words) and compact language. 1/31/2017 ANKUR SRIVASTAVA ASSISTANT PROFESSOR JIT 10

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