Computer Science 332

Compiler Construction

Parser Generators(Supercedes 4.9)

Parser Generators A.k.a. parser compilers, compiler compilers

Uses LALR method to convert grammar to parser saves us a lot of work!

Basic idea:

LALRGrammar Parser GeneratorParsing Program(in C or Java)

Detect and reportconflicts(shift/reduce, reduce/reduce)

Build sets of LR(1)items

Build DFABuild parse table

Parser Generators Basic idea: Annotate RHS of rules with method calls typically, code that builds parse tree

Classic version is yacc (Yet Another Compiler Compiler), which outputs C.

Modern version is bison. We'll use (J)CUP: Java-Based Constructor of Useful Parsers:

http://www.cs.princeton.edu/~appel/modern/java/CUP

CUP : Grammar E.g, expression grammar

expr expr + expr | expr * expr | ( expr ) | number

In CUP format:import java_cup.runtime.*;

terminal PLUS, TIMES, LPAR, RPARterminal Integer NUMBER

non terminal Integer expr

expr ::= expr PLUS expr | expr TIMES expr | LPAR expr RPAR | NUMBER

CUP: Parse Actions

expr ::= expr:e1 PLUS expr:e2 {: RESULT = new Integer(e1.intValue()+ e2.intValue(); :}

| expr:e1 TIMES expr:e2 {: RESULT = new Integer(e1.intValue() * e2.intValue(); :}

| LPAR expr:e RPAR {: RESULT = e; :} | NUMBER:n {: RESULT = n; :}

CUP: Precedenceimport java_cup.runtime.*;

terminal PLUS, TIMES, LPAR, RPAR;terminal Integer NUMBER

non terminal Integer expr;

precedence left PLUS;precedence left TIMES;

expr ::= expr PLUS expr | expr TIMES expr | LPAR expr RPAR | NUMBER;

CUP: Working with Jflex, main()

.cup grammar JCup

parser.javasym.java

javac javac

sym.class parser.class

public static final int PLUS = 9;public static final int LPAR = 2;// etc.

public parser(Scanner s) { // ...}

CUP: Working with Jflex, main()

.flex file JFlex

Yylex.java

javac

Yylex.class

class Yylex implements Scanner { // ...}

"+" {return(sym.PLUS); }"(" {return(sym.LPAR); }

CUP: Working with Jflex, main()public class ExprParser {

public ExprParser(String filename) {

java.io.InputStream inp;

try {inp=new java.io.FileInputStream(filename);

} catch (java.io.FileNotFoundException e) { throw new Error("File not found: " + filename);

} parser parser_obj = new parser(new Yylex(inp));

try { parser_obj.parse(); } catch // blah blah blah }}