[Haskell-cafe] Question on Lexing Haskell syntax

[Travis Athougies]

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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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