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Does Antlr support an annotated parse tree that would allow for round-tripping? I didn't see anything in the documentation, at a quick glance. Even then, formulating a grammar (especially one that also catches whitespace and comments) seems like a pretty daunting task.

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Catching whitespace and comments is no problem in Antlr[1]. I'm not exactly sure what you mean by round-tripping. In terms of how daunting it is, it's much easier than trying to use Flex/Bison or lex/yacc, that's for sure.

1: At the bottom of the grammar, there are rules to intentionally skip over whitespace and comments, but it could have just as easily captured them and parsed them further. https://github.com/antlr/grammars-v4/blob/master/c/C.g4

What I mean by "round-tripping" is that I would have to be able to read a .f90 file into an AST structure, then write out the tree to a new file, and end up with the exact same file content as the original. This would be a prerequisite for a refactoring tool, which has to preserve comments and code formatting.

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