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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