[antlr-interest] Newbie Issues: MismatchedTokenExceptions and FailedSyntacticPredicates

[Benji Smith]

Hi. I'm new to this list, and new to ANTLR in general, though I have
quite a bit of experience on a previous project implementing a DSL
compiler with JavaCC (I'm switching to ANTLR primarily because of
support for C#).

I'm currently developing a grammar for stock-market indicators. For
example, when complete it'll allow a user to define an indicator like
this:

<code>
   // In technical analysis, the "MACD" symbol represents a "Moving Average
   // Convergence Divergence" indicator. This indicator is a simple arithmetic
   // function over a "slow" exponential moving average (EMA), and a "fast"
   // exponential moving average.

   indicator MACD[m, n] : EMA[m](CLOSE) - EMA[n](CLOSE)
</code>

But I'm getting ahead of myself. For now, I'm just trying to get the
left-hand side of the assignment expression to parse correctly. The
attached grammar file (VTExpression.g) was written to parse syntax
like "indicator MACD[m, n]", as well as "indicator EMA[3]" or
"indicator XYZ[true, 5, 3 + (n % 2) ]".

But even on the simplest stuff ("indicator EMA[3]"), I get a
MismatchedTokenException from the ANTLRWorks interpreter at the top
level of the expression. I tried rewriting the grammar
(VTExpression2.g) to use string literals in the rule definitions
instead of tokens, and the error went away. (Why? Shouldn't the lexer
& parser be identical at this point, whether the grammar uses tokens
or string literals in the rules?)

Now, using the tokenless grammar (VTExpression2.g), the interpreter
gets inside the left-bracket, but it fails to recognize the "3" as an
IntegerLiteral, instead throwing a FailedPredicateException (between
primaryExpression and synpred19).

When I do a google search on syntactic predicates, I can see that
they're defined like this:

   (...)=>

But I'm not doing that. Whenever I've seen these failed predicate
problems, it's from ordinary stuff like this:

   A : B ( op_a B)* ;
   B : C ( op_b C)* ;

I don't understand where all of these synpred objects come from. As
far as I can tell, I'm never actually using a syntactic predicate.

I feel a little silly grappling with errors parsing such simple little
expressions. The stuff I used to do in JavaCC was significantly more
complex than this. Is it just me, or do many other people find ANTLR a
lot more difficult to use?

Anyhow, thanks for whatever help you can offer. I appreciate it.

--benji smith
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