alt Hacker NewsOf course this grammar does not work; you violated the two rules of writing PEGs:
• do not use left-recursive rules;
• put alternatives in such an order that none can be a prefix of a subsequent one.
These may seem limiting, but can always be fixed by a simple local change. In contrast, transforming a PEG into a conventional grammar often requires complex, wide-scoped changes. I’ve had the Tree-sitter compiler “shit itself” many times at grammars that PEG accepted with no problem, and had to introduce several ugly hacks to work around the problem of Tree-sitter not allowing ambiguous grammars.
> Of course this grammar does not work [in PEG]
That's the critique, yes. If I put this grammar into a known good parser, it just works. I have to repeat this to hammer the point home.
A user should not have to waste time to find work-arounds for the undocumented limitations. Since there are many more limitations than just the one example I showed, you should realise that there is not much value explaining the particular limitations to me; all the limitations and the required work-around steps should rather go into the Janet documentation so that all users can see them and make use of them. But that's still a crappy developer experience, I would rather see Janet simply adopt a parser that is free of this kind of limitations.
> can always be fixed by a simple local change
I sceptical of that. I claim once the grammar is of the size required to model real-world problems, say about dozens of production rules, fixes become complex, wide-scoped. In the spirit of HN curiosity, I am willing to cooperate with you by conducting an experiment that is designed to change my mind. I would show a grammar that is of the type which is in common use everywhere, and you would apply the fixes to make it work in Janet/PEG, and then we examine whether the changes are always simple and local. Are you willing?
> [PEG grammar in Tree-sitter]
That reads rather bizarre to me because you describe the opposite direction. I have not had that train of thought because in all of my experience and those of the people I know it has always been the case that one receives a grammar that is of the type which is in common use everywhere. And when we try to express it in PEG, it does not work at all, no one knows what to do to make it work, experts who might help cannot be found, and the solution (after wasting a lot of time) is to either give up or try a different parser.
As an aside, I have not examined Tree-sitter yet, and its documentation does not tell me the information I need, so I cannot put it into the category of known good parsers.