How does GHC implement layout?

[Carter Schonwald]

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.lambda at 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
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