1 Assignment 3 Jianguo Lu

2 Task: check whether the a program is syntactically correct /** this is a comment line in the sample program **/ INT f2(INT x, INT y ) BEGIN INT z; z := x*x - y*y; RETURN z; END INT MAIN f1() BEGIN INT x; READ(x, "A41.input"); INT y; READ(y, "A42.input"); INT z; z := f2(x,y) + f2(y,x); WRITE (z, "A4.output"); END You only need to check the syntax You don’t need to check things such as –Whether the types are correct; –Whether the program makes sense. How to count methods: program ::= methodDecl {:RESULT= 1;:} | methodDecl program:e {: RESULT=e+1); :} ;

3 Modify the EBNF to javaCup Expression -> MultiplicativeExpr (( '+' | '-' ) MultiplicativeExpr)* MultiplicativeExpr -> PrimaryExpr (( '*' | '/' ) PrimaryExpr)* PrimaryExpr -> Num // Integer or Real numbers | Id | '(' Expression ')' | Id '(' ActualParams ')' BoolExpression -> Expression '==' Expression | Expression '!=' Expression ActualParams -> [Expression ( ',' Expression)*] You need to remove * and [ ]. P::=M | M P ; optionalMain::=MAIN | ; You don’t need to use the expression definition here –Use precedence rule would be more concise –Try to reduce file sizes.

4 If you have difficult in locating the errors, send s to GAs or to me, attaching your lex and cup files and detailed error message. Test extensively on your program. Those sample inputs are not enough. Write short programs. Remove anything that you are not sure of. A3 is based on A2. – Lex file should be enhanced {ID} { return new Symbol(…); } –A4 is based on A3. –Will be tested in exam –important

5 Common problems: JavaCup syntax –{: :} for actions –; after each grammar rule –types for terminals and non terminals program ::= methodDecl {:RESULT= 1;:} | methodDecl program:e {: RESULT=e+1); :} ; JavaCup semantics –shift/reduce conflicts –empty parameter list, empty statement list –if-then-else Connection between Jlex and javacup –Terminals not declared in JavaCup

6 Debugging methods turn on debugging mode in the parser, by changing parser.parse().value; to parser.debug_parse().value; Print out symbols in scanner "+" {System.out.println(yytext()); return new Symbol(CalcSymbol.PLUS); } "-" {System.out.println(yytext()); return new Symbol(CalcSymbol.MINUS); } {NUMBER} { System.out.println(yytext()); return new Symbol(CalcSymbol.NUMBER, new Integer(yytext()));} \r|\n|. {System.out.println(yytext()); }

7 Use debug_parse() luna:~/214>java A3User # Initializing parser # Current Symbol is #10 Syntax error # Attempting error recovery # Finding recovery state on stack # Pop stack by one, state was # 0 # No recovery state found on stack # Error recovery fails Couldn't repair and continue parse Exception in thread "main" java.lang.Exception: Can't recover from previous error(s) at java_cup.runtime.lr_parser.report_fatal_error(lr_parser.java:361) at java_cup.runtime.lr_parser.unrecovered_syntax_error(lr_parser.java:409) at java_cup.runtime.lr_parser.debug_parse(lr_parser.java:801) at A3User.main(A3User.java:6)

8 Check A3Symbol.java // // The following code was generated by CUP v0.10k // Fri Mar 16 11:28:02 EDT 2007 // /** CUP generated class containing symbol constants. */ public class A3Symbol { /* terminals */ public static final int TIMES = 21; public static final int READ = 16; public static final int ELSE = 4; public static final int PLUS = 19; public static final int RPAREN = 24; public static final int INT = 11; public static final int EQUAL = 5; public static final int THEN = 3; public static final int SEMI = 15; public static final int NOTEQUAL = 6; public static final int RETURN = 18; public static final int END = 14; public static final int IF = 2; … public static final int ID = 10; …

9 Error tokens What if the input is Scanner:. { return new Symbol(ERROR); } Parser: –Does not need to do anything; –You don’t need to specify rules such as ID ERROR + ID {: some error msg :} – You only need to specify the correct rules, such as ID+ID –When the parser expects to see “+”, but an ERROR token (or any other token) is encountered, the parser knows there is an error

10 Follow the first example in javacup slides. –the second the and third examples are for assignment 4. create script in a bat file, say a3.bat in windows: java JLex.Main A3.lex java java_cup.Main -parser A3Parser -symbols A3Symbol < A3Lu.cup javac -classpath..;. A3Lu.lex.java A3Parser.java A3Symbol.java A3User.java java A3User more A3.output > a3 in unix, change access mode of the file (say “a3”) to executable % chmod 700 a3 % a3