ANTLR: using stringTemplate

Posted by Kevin Won on Stack Overflow See other posts from Stack Overflow or by Kevin Won
Published on 2010-05-11T01:58:59Z Indexed on 2010/05/11 2:04 UTC
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(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 { 

    : 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();

         globalName(); // <- it calls globalName() but doesn't keep the return.


        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.

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