[antlr-interest] Implementation decision help

[Jim Idle]

On Jul 8, 2009, at 4:59 AM, Gustaf Johansson <gustaf.j at gmail.com> wrote:

> Hello!
>
> I am trying to implement the ETSI TTCN-3 BNF into Antlr.
> The major part is done and working as it should but there are some
> quirks which i don't know how to solve.
>
> I have rules like this:
> stm : stmGroup1 | stmGroup2 ... | stmGroupX;
>
> And all/some of these stmGroup's can start with the same type of
> token, but they are distinguishable by some token along the line
> further down the parse tree.
>
> I have tried to solve this by using backtrack=true which didn't work.
>
> My current solution is a specific order of the stmGroup's and a few
> syntactic predicates. But i don't like this solution since i don't
> know if it will work for all possible input (the grammar is really
> complex).
>
> Basically what i want to accomplish is make Antlr try all of the rules
> in "stm" and only report error if none of them matches. Currently it
> reports errors even though a rule later in the list will match the
> input completely, just because the rule reporting the error matches it
> partly.
>
> The only solution other then syntactic predicates and backtrack i can
> think of is to combine the stmGroup's into a rule which has truly
> distinctive paths depending on the next token.
> Is this the way to do it?

Yes.

> It will require a huge amount of work since all the stmGroup's are
> quite large and complex themselves. Also it will make the grammar
> almost unreadable.

I think you will find that it doesn't result in an readable grammar in  
fact. Formatting is your friend here.

I keep trying to influence people to practice by presenting an  
approach to writing such grammars. But I think people basically  
download Works and start typing. There are lots of posts about left  
factoring, not using backtrack and so on. Basically the normative spec  
is rarely written with the parser programmer in mind and you must  
interpolate.

Generally you can get backtrack to work as you want but with terrible  
error messages. If you can't then I would suggest starting again and  
use the recommended incremental approach to building grammars. You  
already have a lexer so you can cut and paste parser pieces until you  
get it correct.


Jim

>
>
>
> An example of a problematic rule is:
> http://www.trex.informatik.uni-goettingen.de/trac/wiki/ttcn-3_4.0.4#FunctionStatement
>
> These are two parts of that problem:
> -> ConfigurationStatements -> StartTCStatement
> -> TimerStatements -> StartTimerStatement
>
> PS. backtrack is not used at all in the grammar.
>
> BR Gustaf
>
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