What's New in Version 0.91?

What is it?

 This is the standard YACC tool that is in use every day to produce C/C++ parsers. I have added a "-j" flag which will cause BYACC to generate Java source code, instead. So there finally is a YACC for Java now!

How does BYACC/Java compare to other parser generators?

• BYACC/Java can be executed from existing Makefiles and IDE's.
• BYACC/Java is coded in C, so the generation of Java code is extremely fast.
• The resulting byte code is small -- starting at about 11 kbytes.
• Only one or two classfiles are included. If you need only a single type or an Object class, then one class file is generated. If you need a simple generic type, a simple data class is generated for you, making another small file.
• No additional runtime libraries are required. The generated source code is the entire parser.
• It can parse existing YACC grammars, enabling the 'Javanizing' ;-) of a large installed base of YACC source code (of course, your 'actions' need to be in Java).
• Many developers are already very familiar with the workings of YACC.
• It is absolutely free; no license, no royalties, free!

How do I use it?

1. The first part of the file is the DECLARATIONS area, where you define tokens, precedences, etc.
2. The second part is the ACTIONS area, where the grammar and the user's C actions are parsed.
3. The third part is the CODE area, where user functions are added.

Portions of the file can be set off and ignored by YACC by surrounding them with %{ and %} . This ability is typically used in YACC C files to insert definitions and #include statements. BYACC/Java uses this area for the Java package and import statements. Everything after this will be wrapped in a Java class called parser. Thus all functions you write will become methods belonging to parser.
All of the user actions you write will be Java code inserted into a method called yyparse(). This means that only curly braces '{,}' are allowed. No classes or methods can be defined in the ACTIONS area. Of course, your code can instantiate classes and call methods.

Here is our example Java implementation of the classic YACC calculator demo:

%{ import java.lang.Math; import java.io.*; import java.util.StringTokenizer; %}       /* YACC Declarations */ %token NUM %left '-' '+' %left '*' '/' %left NEG     /* negation--unary minus */ %right '^'    /* exponentiation        */       /* Grammar follows */ %% input:    /* empty string */              | input line      ;       line:     '\n'           | exp '\n'  { System.out.println(" " + $1.dval + " "); }      ;       exp:      NUM                { $$ = $1;         }              | exp '+' exp        { $$.dval = $1.dval + $3.dval;    }              | exp '-' exp        { $$.dval = $1.dval - $3.dval;    }              | exp '*' exp        { $$.dval = $1.dval * $3.dval;    }              | exp '/' exp        { $$.dval = $1.dval / $3.dval;    }              | '-' exp  %prec NEG { $$.dval = -$2.dval;        }              | exp '^' exp        { $$.dval = Math.pow($1.dval, $3.dval); }              | '(' exp ')'        { $$.dval = $2.dval;         }      ; %% String ins; StringTokenizer st; void yyerror(String s) {   System.out.println("par:"+s); } boolean newline; int yylex() { String s; int tok; Double d;   //System.out.print("yylex ");   if (!st.hasMoreTokens())     if (!newline)       {       newline=true;       return '\n';  //So we look like classic YACC example       }     else       return 0;   s = st.nextToken();   //System.out.println("tok:"+s);   try     {     d = Double.valueOf(s);/*this may fail*/     yylval = new parserval(d.doubleValue()); //SEE BELOW     tok = NUM;     }   catch (Exception e)     {     tok = s.charAt(0);/*if not float, return char*/     }   return tok; } void dotest() { BufferedReader in = new BufferedReader(new InputStreamReader(System.in));   System.out.println("BYACC/Java Calculator Demo");   System.out.println("Note: Since this example uses the StringTokenizer");   System.out.println("for simplicity, you will need to separate the items");   System.out.println("with spaces, i.e.:  '( 3 + 5 ) * 2'");   while (true)     {     System.out.print("expression:");     try       {       ins = in.readLine();       }     catch (Exception e)       {       }     st = new StringTokenizer(ins);     newline=false;     yyparse();     } } public static void main(String args[]) {   parser par = new parser(false);   par.dotest(); }

If this code were in a file called calc.y, you would yacc-process it with the command:
yacc -j calc.y
This will generate the file parser.java, which can then be compiled by:
javac parser.java
to create the file parser.class which can be run with:
java parser

The same file, but using the semantic_type option, as in
yacc -j -s double tf.y
is available here.

Command Line Options

• -j  Switches from C/C++ to Java output
• -f<classname> Changes the name of the Java class (and .java file) to classname
• -x<extendname> Changes the class the parser extends from the default Thread to extendname.
• -s<semantic_type> Changes the semantic (value of the rules' variables) type to semantic_type. No extra class will be generated.

User-Supplied Methods

• void yyerror(String msg) -- This method is expected by BYACC/Java, and is used to provide error messages to be directed to the channels the user desires.
• int yylex() -- This method is the one where BYACC/Java expects to obtain its input tokens. Wrap any file/string scanning code you have in this function. This method should return <0 if there is an error, and 0 when it encounters the end of input. See the examples to clarify what we mean.

About the Generated Java File

Normally, the class generated is made an extension of Thread, as a convience, so that parsing may be performed as a background thread, allowing the current execution to continue unimpeded.  A run() method and a constructor are also inserted into the code.

However, it may occur that the programmer needs to extend a different class.  In this case, the -x<classname> option is provided, which will create an alternate extension.  Since it is impossible to predict the needs of the other class, the run() and constructor will be omitted.

About the parserval (previously 'semantic') class

Starting with this version, the semantic value is stored in a public class called parserval, which is defined thusly:
 

public class parserval  {    public int ival;    public double dval;    public String sval;    public Object obj;    public parserval(int val)    {      ival=val;    }    public parserval(double val)    {      dval=val;    }    public parserval(String val)    {      sval=val;    }    public parserval(Object val)    {      obj=val;    }  }//end class

So now a semantic value can be an int, a double, a String, or an Object. In your scanner (or something that yylex() calls), you may use this like:
 

yylval = new parserval(doubleval); yylval = new parserval(integerval); ...or even something like... yylval = new parserval(new myTypeOfObject());

And on the Left Hand Side (the YACC side) you can use the values of the $ and the $$ just as easily:
 

$$.ival = $1.ival + $2.ival;  $$.dval = $1.dval - $2.dval;

 

A side effect of using this inner class is that the default parser no longer fits into one .class file, however, the resulting parserval.class is extremely small.

Why the name change from "semantic" to "parserval"? Do you hate the users or what?

Examples

• Here is an example of what a 3d object file might look like. A corresponding bare-bones YACC parser is implemented in Java! This is also a good example of how to read a file from a URL.

Why?

Credits

Availability

Also, a couple of binaries for BYACC/Java can be obtained here. These are native console applications, so they do not require any class libraries to work. Of course, you will need a Java development environment to process the generated source files.
And remember, this version of YACC also parses "standard" C/C++ YACC source files!

Questions

Links

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	Intelligent Computer-Aided Training - The Training Technology!
	LinCom Avionics Systems Group  Our home page. Please visit!
	Gamelan - The Mecca of Java

An excellent place to look for Java resources: