ANTLR AST outputEdit

These notes are about ANTLR's output = AST; option in the context of the C language target.

Differences

These examples describe the entry points in a sample parser, pWalrusParser, generated with and without the output = AST; option:

# without output = AST;
void pWalrusParser->template(pWalrusParser)

# with output = AST
WalrusParser_template_return pWalrusParser->template(pWalrusParser)

When to use output = AST;

You specify output = AST; when you plan on passing the returned AST on to a tree parser for further manipualtion.

When not to use output = AST;

Given that your entry points have a void return type in this case, non-AST parsers are generally best suited for:

Situations where you merely want to recognize input as valid or invalid

Although the parser entry points don't return anything, you can still check the parser for errors:

if (parser->pParser->rec->errorCount > 0)
{
    // do something here to handle the errors
}

Directly translating from within actions

If your translation requirements are simple you may be able to merely print results out from within rule actions. Or if you don't want to print out the results, you may be able to store them somewhere else from within your actions; the generated parsers have custom fields that you can use to store points to other storage.