MiniTutorial on Customizing Error Messages

This minitutorial is under preparation.

This file provides an overview of how the user can handle error
messages instead of using the default reporting mechanisms generated
by Jack.  Jack generated files report error messages at two places -
in the generated lexical analyzer and the generated parser.

Customizing Parser Error Messages:

The generated parser contains a method called "token_error" with no
parameters with a standard implementation.  You can create a
subclass of the generated parser and override this method to customize
error reporting.  To facilitate the writing of your customized error
routines, the following primitives are available for you use (from
the generated parser class):

	int error_line;
	// The line where the error was detected.

	int error_column;
	// The column where the error was detected.

	String error_string;
	// The offending string.

	String[] expected_tokens;
	// The list of tokens that were expected instead of
	// the offending string.

When you subclass the parser, you will also have to provide constructors
with parameters similar to the ones you use in the parser class.

As an example, here is how you may customize error reporting for the
Simple1 parser in the examples directory:

	// First subclass the parser class.
	public class MySimple1 extends Simple1 {

	  // Now define the constructor you wish to use and call
	  // the corresponding constructor in the parser class.
	  public MySimple1(java.io.InputStream stream) {
	    super(stream);
	  }

	  // Your main program which replaces the one in the parser
	  // class.  Note this calls the constructor in this class.
	  public static void main(String args[]) throws ParseError {
	    MySimple1 parser = new MySimple1(System.in);
	    parser.Input();
	  }

	  // Finally the customized error reporting mechanism.  In this case
	  // we simply report the error in a more abrupt way.
	  protected void token_error() {
	    System.out.println("Line: " + error_line);
	    System.out.println("Column: " + error_column);
	    System.out.println("Encountered: \"" + error_string + "\"");
	    System.out.print("Expected: ");
	    for (int i = 0; i < expected_tokens.length; i++) {
	      System.out.print(expected_tokens[i] + "; ");
	    }
	    System.out.println("");
	  }

	}

You can try adding this file to the examples directory, and do the following:

	jack Simple1.jack
	javac *.java
	java MySimple


Customizing Lexical Analyzer Error Messages:

The generated lexical analyzer contains a method called "LexicalError"
with no parameters with a standard implementation.  You can create a
subclass of the generated lexer and override this method to customize
lexical error reporting. You have the following info available:

	int error_line;
	// The line where the error was detected.

	int error_column;
	// The column where the error was detected.

	String error_after;
	// The partial string that is seen AFTER the previous matched token (note
	// that this can also be a IGNORE_IN_BNF token).

	char curChar;
	// The offending character.

The lexer subclass should also have all the different constructors
that the generated lexer has with identical parameters.

As an example, here is how you may customize error reporting for the
Simple1TokenManager in the examples directory:

	// First subclass the lexer class.
	public class MySimple1TokenManager extends Simple1TokenManager {

	  // Now define the constructor you wish to use and call
	  // the corresponding constructor in the lexer class.
	  public MyLexer(ASCII_CharStream stream) {
	    super(stream);
	  }

	  // Customized error reporting.
	  protected void LexicalError() {
	    System.out.println("Lexical error at line " + error_line + ", column " + error_column);
	    System.out.println("Encountered: \"" + String.valueOf(curChar) + "\"");
	  }

	}

	// Your main program which replaces the one in the parser
	// class.  Note this calls the constructor in this class.
	public static void main(String args[]) throws ParseError {
	  Simple1TokenManager tm = new MySimple1TokenManager(
	                           new ASCII_CharStream(System.in, 1, 1));
	  Simple1 parser = new Simple1(tm);
	  parser.Input();
	}

You can try adding this file to the examples directory, and do the following:

	jack Simple1.jack
	javac *.java
	java Simple1