[antlr-interest] Embedding expressions in plain text -- how to?

[Rob Greene]

How should I go about embedding an expression language in plain text.  The
expression is wrapped in '${' and '}' but may occur zero to many times
within plain text.  When in plain text, I want to keep the whitespace, but
within the expression, it should be ignored.

I've currently placed a boolean into a ThreadLocal and check that before
calling skip() in my whitespace rule.  But, that causes issues with the
actual expressions.

For instance, "${true}" returns Boolean.TRUE while "${ true }" returns a
null.  If I take out the test in WS, I can have those spaces within an
expression, but the plain text loses all white space.

I'm certain this is a simple fix, but I haven't a clue what to do!

I'm using ANTLR 3.0.1.

Thanks!
-Rob

/**
 * Language is the simple expression language used throughout the XML
Template Engine.
 * General language constructs:
 * - functionName(argument, argument, ...)
 * - object.method.method
 * - 'string constant'
 * - "string constant"
 * - 1234.56
 * - true
 * - false
 * - null
 * - [ array, values, ... ]
 */
grammar Language;

/**
 * Use the embeddedExpression entry for expressions that are embedded within
other
 * forms of text.
 * <p>
 * Note that unknown magic occurs because the plainText is somehow
 * glommed together into one string instead of character
 * by character.
 */
embeddedExpression returns [List<Object> list]
@init { list = new ArrayList<Object>(); }
    :    (    '${' expression '}'    { list.add($expression.value); }
        |    plainText            { list.add($plainText.value); }
        )*
    ;

/**
 * Handle plain text that wraps around the expression.
 */
plainText returns [String value]
@init { ThreadState.current().ignoreWhitespace = false; }
@after { ThreadState.current().ignoreWhitespace = true; }
    :    '\\' '$'                { $value = "$"; }
    |    '\\' b=~('$')            { $value = $b.text; }
    |    a=~('$')                { $value = $a.text; }
    ;

/**
 * This is the bare expression evaluation.  Evaluates one expression and
returns
 * an Object.
 */
expression returns [Object value]
    :    function            { $value = $function.value; }
    |    constant            { $value = $constant.value; }
    |    reference            { $value = $reference.value; }
    ;

/**
 * Evaluate a function invocation, along with parameters and return the
Object result.
 */
function returns [Object value]
    :    ID '(' arguments ')'        { $value =
Helper.invoke($ID.text,$arguments.value);
}
    ;
catch [LanguageException le] {
    emitErrorMessage(le.getMessage());
    throw new RuntimeException(le);
}

/**
 * Evaluate a constant value and return that value.
 */
constant returns [Object value]
    :    STRING                { $value =
$STRING.text.substring(1,$STRING.text.length()-1); }
    |    NUMBER                { $value = Double.valueOf($NUMBER.text); }
    |    'true'                { $value = Boolean.TRUE; }
    |    'false'                { $value = Boolean.FALSE; }
    |    'null'                { $value = null; }
    |    '[' arguments ']'        { $value = $arguments.value; }
    ;

/**
 * Evaluate an object reference and any nested evaluations and return that
value.
 */
reference returns [Object value]
    :    a=ID                { $value = Helper.find($a.text); }
        ('.' b=ID            { $value = Helper.reflect($value,$b.text); }
         )*
    ;
catch [LanguageException le] {
    emitErrorMessage(le.getMessage());
    throw new RuntimeException(le);
}

/**
 * A comma-separated argument list.  Use for the function invocation and
array list.
 */
arguments returns [List<Object> value]
@init { value = new ArrayList<Object>(); }
    :    (a=expression            { $value.add($a.value); }
        (',' b=expression        { $value.add($b.value); }
         )* )?
    ;

ID    :    Char (Char|Digit)*
    ;
STRING    :    '"' ~('"')* '"'
    |    '\'' ~('\'')* '\''
    ;
NUMBER    :    ('-'|'+')? Digit+ ('.' Digit+)?
    ;
WS
    :    (' '|'\t'|'\r'|'\n')+    { if
(!ThreadState.current().ignoreWhitespace) skip(); }
    ;

fragment Char
    :    'a'..'z' | 'A'..'Z' | '_'
    ;
fragment Digit
    :    '0'..'9'
    ;
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