[antlr-interest] string interpolation

[Rodrigo B. de Oliveira]

I'm building a very simple parser that recognizes (and interprets) simple programs such as:

    a = 5+3
    b = 102-5
    print "a: ${a}, b: ${b}, a+b: ${a+b}"

The main thing I'm trying to achieve with this toy is to get the string interpolation thing right before integrating it in a larger parser project I'm working on.

The SI thing works by transforming expressions such as:

    "a+b: ${a+b}, b: ${b}"

in:

    "a+b: {0}, b: {1}" a + b ESEPARATOR b

before they get to the parser (ESEPARATOR is an imaginary token).

I have successfully put the parser to work by combining two lexers, a TokenStreamSelector along with a custom TokenStream implementation (TokenStreamRecorder).

My problem now is to make sure the lexer rejects invalid sequences such as:
        
       "${ ${}"

       "${"

       "${

with a SemanticException. Unfortunately the exception is not getting through...

I have already set the defaultErrorHandler to false in the parser and in both lexers.

The grammar for the second lexer (the one parsing the expression inside ${}) follows:
<grammar>
options
{
 language="CSharp";
}
class ExpressionLexer extends Lexer;
options
{
 importVocab = SI;
 defaultErrorHandler=false;
}
{
 public override void uponEOF()
 {
  // should never see EOF
  Error();
 }

 void Error()
 {
  throw new SemanticException(
   "Invalid expression!",
   getFilename(),
   getLine(),
   getColumn()
   );
 }
}
ID: ID_LETTER (ID_LETTER | DIGIT)*;

INT: (DIGIT)+;

ASSIGN: '=';

SUM_OPERATOR : '+' | '-' ;

WS: (' ' | '\t' | '\r' | '\n' { newline(); }) { $setType(Token.SKIP); };

RBRACE: '}';

DQ: '"' { Error(); };
 
protected
ID_LETTER : ('_' | 'a'..'z' | 'A'..'Z' );

protected
DIGIT : '0'..'9';
</grammar>

I can send the other 2 files (the main grammar file and the custom TokenStream implementation) if someone is willing to take a look at them also...

BTW, I'll post the whole project to the antlr site upon completion, I think it's a good example of how to combine TokenStreamSelector with custom TokenStream implementations...

Thanks in advance for any help,
Rodrigo
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