ANTLR: using stringTemplate

Question

(I'm a Noob with Antlr)... I'm having difficulties getting my grammar with StringTemplates. Basically I'm trying to write a little DSL. I can get my grammar the way I want (it parses correctly), but I can't get the generation of the target code to work with templates. So here's a snippet of my grammar:

grammar Pfig;

options { 
    output=template;  
 language=CSharp2;
 }

conf 
    : globalName
    ;


globalName 
    : 'GlobalName:'  ID
     -> localConf(name ={$ID.text})
    ;

I simplified it quite a bit just to get the essence across. Basically, when the lex/parse comes across `GlobalName: Foo' I want it to spit out text based on the StringTemplate called 'localConf'. Super straightforward.

So now, let's fire up the parser in a test app and have it process an input file.

// C# processing a file with the lex/parser.
// the 'app.pfig' file just has one line that reads 'GlobalName: Bla'
using (FileStream fs = File.OpenRead("c:\\app.pfig"))
        {
            PfigParser parser = new PfigParser(new CommonTokenStream(
                new PfigLexer(new ANTLRInputStream(fs))));

            using (TextReader tr = File.OpenText("./Pfig.stg"))
            {
                parser.TemplateLib = new StringTemplateGroup(tr);
            }

            var parseResult = parser.conf();
            string code = parseResult.Template.ToString(); // Fail: template is null
        }

I can step through the parser code and see that it correctly identifies my text and applies the stringTemplate correctly. The problem is that since this 'globalName' rule is a subrule of 'conf' it doesn't get executed directly--the method just finds it and returns. But the calling 'Conf' method does not keep the return value from the subrule--it goes to thin air. This means that my resultant template on the last line is null.

If I get rid of the 'conf' rule in my grammar and call 'globalName' directly, it will work (since it's the only rule on the stack). But I obviously want more than one rule. I've generated the parser in Java and it does the same thing:

// antlr generated parser code
public PfigParser.conf_return conf() // throws RecognitionException [1]
{   
    PfigParser.conf_return retval = new PfigParser.conf_return();

    try 
 {
        {
         PushFollow(FOLLOW_globalName_in_conf30);
         globalName(); // <- it calls globalName() but doesn't keep the return.
         state.followingStackPointer--;

        }

        retval.Stop = input.LT(-1);

    }

// snip

It's simple to see I don't get some basic concept with how the Template approach is supposed to work with Antlr. I'm quite sure this is my problem but I'm a loggerheads to know what I'm doing wrong... the examples I've seen don't really show real-world template emission of code.

Answer 1

I'm having the same problem, using Antlr 3.2, I have both Antlr books, still need to find an example of a simple DSL in a single grammar that produces output from more than one rule via stringtemplate.

I explored the parser return value datastructure (in C# with Visual Studio 2010) to no avail, I can only find the stringtemplate output on the main grammar rule.

How does one get at the stringtemplate output of the other rules that fired?

Has anyone got an example grammar & code snippet for this? Thanks!

Answer 2

Basically, you need to explicitly forward the template output to the subrule:

conf 
    : a=globalName -> {$a.st}
    ;

It is not beautiful, but works.