[Haskell-cafe] Question on Lexing Haskell syntax

[Isaac Elliott]

I like the approach from "Principled Parsing for Indentation-Sensitive
Languages" (https://michaeldadams.org/papers/layout_parsing/)

On Wed, Nov 1, 2023 at 8:32 AM Travis Athougies <travis at athougies.net>
wrote:

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> According to the Haskell report [1] (See Note 5), a virtual `}` token
> is inserted if parsing the next token would cause a parse error and the
> indentation stack is non-empty.
>
> I'm trying to lex and parse Haskell source and this sort of interplay
> (which requires two-way communication between lexer and parser) makes
> it very difficult to write a conformant implementation.
>
> I can't change the standard (obviously), but I'm wondering if this is
> actually what GHC (de facto the only Haskell compiler) does, or if it
> applies some other rule. If so, does anyone know the exact mechanism of
> its implementation?
>
> I've been programming Haskell for more than a decade, and while I have
> an intuitive understanding of the indentation rules, I would have
> assumed the source could be lexed without also having a parser. In
> particular, the note seems to imply that the main purpose of this is to
> properly lex `let`/`in` bindings. Perhaps there's an alternate
> equivalent rule?
>
> Curious to hear other's thoughts.
>
> Travis
>
> [1]
>
> https://www.haskell.org/onlinereport/haskell2010/haskellch10.html#x17-17800010.3
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