Issues of Error handling with ANTLR3 - antlr

Question

I tried error reporting in following manner.
#members{
public String getErrorMessage(RecognitionException e,String[] tokenNames)
{
List stack=getRuleInvocationStack(e,this.getClass().getName());
String msg=null;
if(e instanceof NoViableAltException){
<some code>
}
else{
msg=super.getErrorMessage(e,tokenNames);
}
String[] inputLines = e.input.toString().split("\r\n");
String line = "";
if(e.token.getCharPositionInLine()==0)
line = "at \"" + inputLines[e.token.getLine() - 2];
else if(e.token.getCharPositionInLine()>0)
line = "at \"" + inputLines[e.token.getLine() - 1];
return ": " + msg.split("at")[0] + line + "\" => [" + stack.get(stack.size() - 1) + "]";
}
public String getTokenErrorDisplay(Token t){
return t.toString();
}
}
And now errors are displayed as follows.
line 6:7 : missing CLOSSB at "int a[6;" => [var_declaration]
line 8:0 : missing SEMICOL at "int p" => [var_declaration]
line 8:5 : missing CLOSB at "get(2;" => [call]
I have 2 questions.
1) Is there a proper way to do the same thing I have done?
2) I want to replace CLOSSB, SEMICOL, CLOSB etc. with their real symbols. How can I do that using the map in .g file?
Thank you.

Answer 1

1) Is there a proper way to do the same thing I have done?
I don't know if there is a defined proper way of showing errors. My take on showing errors is a litmis test. If the user can figure out how to fix the error based on what you have given them then it is good. If the user is confued by the error message then the message needs more work. Based on the examples given in the question, symbols were only char constants.
My favorite way of seeing errors is with the line with an arrow pointing at the location.
i.e.
Expected closing brace on line 6.
int a[6;
^
2) I want to replace CLOSSB, SEMICOL, CLOSB etc. with their real symbols. How can I do that using the map in .g file?
You will have to read the separately generated token file and then make a map, i.e. a dictionary data structure, to translate the token name into the token character(s).
EDIT
First we have to clarify what is meant by symbol. If you limit the definition of symbol to only tokens that are defined in the tokens file with a char or string then this can be done, i.e. '!'=13, or 'public'=92, if however you chose to use the definition of symbol to be any text associated with a token, then that is something other than what I was or plan to address.
When ANTLR generates its token map it uses three different sources:
The char or string constants in the lexer
The char or string constants in the parser.
Internal tokens such as Invalid, Down, Up
Since the tokens in the lexer are not the complete set, one should use the tokens file as a starting point. If you look at the tokens file you will note that the lowest value is 4. If you look at the TokenTypes file (This is the C# version name) you will find the remaining defined tokens.
If you find names like T__ in the tokens file, those are the names ANTLR generated for the char/string literals in the parser.
If you are using string and/or char literals in parser rules, then ANTLR must create a new set of lexer rules that include all of the string and/or char literals in the parser rules. Remember that the parser can only see tokens and not raw text. So string and/or char literals cannot be passed to the parser.
To see the new set of lexer rules, use org.antlr.Tool –Xsavelexer, and then open the created grammar file. The name may be like.g . If you have string and/or char literals in your parser rules you will see lexer rules with name starting with T .
Now that you know all of the tokens and their values you can create a mapping table from the info given in the error to the string you want to output instead for the symbol.
The code at http://markmail.org/message/2vtaukxw5kbdnhdv#query:+page:1+mid:2vtaukxw5kbdnhdv+state:results
is an example.
However the mapping of the tokens can change for such things as changing rules in the lexer or changing char/string literals in the parser. So if the message all of a sudden output the wrong string for a symbol you will have to update the mapping table by hand.
While this is not a perfect solution, it is a possible solution depending on how you define symbol.
Note: Last time I looked ANTLR 4.x creates the table automatically for access within the parser because it was such a problem for so many with ANTLR 3.x.

Answer 2

Bhathiya wrote:
*1) Is there a proper way to do the same thing I have done?
There is no single way to do this. Note that proper error-handling and reporting is tricky. Terence Parr spends a whole chapter on this in The Definitive ANTLR Reference (chapter 10). I recommend you get hold of a copy and read it.
Bhathiya wrote:
2) I want to replace CLOSSB, SEMICOL, CLOSB etc. with their real symbols. How can I do that using the map in .g file?
You can't. For SEMICOL this may seem easy to do, but how would you get this information for a token like FOO:
FOO : (X | Y)+;
fragment X : '4'..'6';
fragment Y : 'a' | 'bc' | . ;