while not totally perfectly related, i have wondered if something like the the recent pika parsing paper https://arxiv.org/abs/2005.06444 could be used to provide better error recovery from layout related errors in haskell and similar languages.
On Sun, Apr 4, 2021 at 6:19 PM Alexis King <lexi.lam...@gmail.com> wrote: > On 4/4/21 1:52 PM, Iavor Diatchki wrote: > > Hi Alexis, > > I wasn't sure what the "alternative layout" is either and did some > googling, and it appears that it is something that was never really > documented properly. The following link contains pointers to the commit > that introduced it (in 2009!) (not the main ticket but some of the > comments) > > Thanks, that’s a helpful pointer, though of course it still doesn’t > explain very much. I’m still interested in understanding what the purpose > of “alternative layout” is and how it operates, if anyone else has any idea. > > Overall, I do think that Haskell's layout rule is more complicated than it > needs to be, and this is mostly because of the rule that requires the > insertion of a "virtual close curly" on a parse error. > > Yes, this does seem to be by far the trickiest bit. But I’d be sad not to > have it, as without it, even simple things like > > let x = 3 in e > > would not be grammatically valid. > > My feeling is that it'd be pretty tricky to do layout in the parser with > grammar rules, but you may be able to do something with the parser state. > > Yes, I have some vague ideas, but none of them are particularly fleshed > out. It’s entirely possible that I just don’t understand the relationship > between the lexer and the parser (which seems somewhat obscured by the > “alternative layout” stuff), and the ideas I have are what’s already > implemented today. I’ll have to study the implementation more closely. > > In any case, thank you for your response! The ALR-related pointer > certainly clarifies at least a little. > > Alexis > _______________________________________________ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs >
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