How can I parse nested source files with ANTLR4 - Trying one more time - antlr4

Question

I found the code (reproduced below) in an article from Terrence Parr showing how INCLUDE files could be handled in ANTLR3 for Java. I tried to add this to a grammar I use with ANTLR4 (with a C++ target) but when I tried to generate a parser, I got the errors
error(50): : syntax error: '^' came as a complete surprise to me
error(50): : syntax error: mismatched input '->' expecting SEMI while matching a rule
error(50): : syntax error: '^' came as a complete surprise to me
error(50): : syntax error: '(' came as a complete surprise to me while matching rule preamble
and I have no idea what these error means. Can anyone explain and perhaps show me the way forward?
(NB: I'm not wild about polluting the grammar file with code, I'm using the visitor pattern but I'll take it if I can!)
Thanks
include_filename :
('a'..'z' | 'A'..'Z' | '.' | '_')+
;
include_statement
#init { CommonTree includetree = null; }
:
'include' include_filename ';' {
try {
CharStream inputstream = null;
inputstream = new ANTLRFileStream($include_filename.text);
gramLexer innerlexer = new gramLexer(inputstream);
gramParser innerparser = new gramParser(new CommonTokenStream(innerlexer));
includetree = (CommonTree)(innerparser.program().getTree());
} catch (Exception fnf) {
;
}
}
-> ^('include' include_filename ^({includetree}))
;

Answer 1

Starting with ANTLR4 it is no longer possible to manipulate the generated parse tree with grammar rules. In fact ANTLR3 generated an AST (abstract syntax tree), which is a subset of a parse tree (as generated by ANTLR4). That in turn means you cannot keep the tree rewrite syntax (the part starting with ->). Hence you should change the code to:
include_statement
#init { CommonTree includetree = null; }
:
'include' Include_filename ';' {
try {
CharStream inputstream = null;
inputstream = new ANTLRFileStream($include_filename.text);
gramLexer innerlexer = new gramLexer(inputstream);
gramParser innerparser = new gramParser(new CommonTokenStream(innerlexer));
includetree = (CommonTree)(innerparser.program().getTree());
} catch (Exception fnf) {
;
}
}
;