How does GHC implement layout?

Carter Schonwald carter.schonwald at
Mon Apr 5 19:42:02 UTC 2021

while not totally perfectly related, i have wondered if something like the
the recent pika parsing paper 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> 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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