Question

I’m writing an interpreter for a simple DSL and wondering how to implement if-then-else evaluator elegantly. I found one example online from antlr.org but it actually used getNodeIndex() from CommonTreeNodeStream, which is protected, so no use. The best I could do was the following:

ifblock
@init
{
  int t = 0;
  int f = 0;
}
@after
{
  if(c)
    stream.push(t);
  else
    stream.push(f);
  statements(); // handle the statements in then or else branch
  stream.pop()
}
  : ^(IF c=condition {t = input.mark();}. {f = input.mark();}.)
;

This is working, but I am not really satisfied somehow. Is there a better way?

Answer 1

I would separate the logic from your tree grammar and create custom node-classes that are responsible for evaluating the input. If you keep it all inside the grammar, it will soon become a big pile of spaghetti, which is hard to maintain, IMO. Especially when you extend the language.

For an example of how to do this, see thie previous Q&A: if then else conditional evaluation

Some more related links:

• Scott Stanchfield’s excellent ANTLR v3 video blog series: http://javadude.com/articles/antlr3xtut/
• my blog about creating a small dynamic language with ANTLR: http://bkiers.blogspot.com/2011/03/creating-your-own-programming-language.html