[Haskell-cafe] Hard cases for "Note 5" in Haskell layout parsing

[Brandon Allbery]

Consider the case where someone has done: let x = do ...

There's also the opposite edge case, which people doing one-liners in ghci
or lambdabot run into fairly often: ... do ...; let x = 5; ...
Where the semicolon continues the let bindings, not the do; you must use
braces to disambiguate.

On Mon, Mar 11, 2019 at 8:56 PM Chris Smith <cdsmith at gmail.com> wrote:

> Quick question.  I'm trying to resolve Haskell layout in order to provide
> better syntax highlighting, but without committing to building a full
> Haskell parser.  The reason this is hard at face value is because of the
> "Note 5" in the relevant part of the Haskell Report, which says that a
> layout context closes whenever (a) the next token without closing the
> layout context would not be the start of any valid Haskell syntax, but (b)
> the current statement followed by an implicit '}' could be valid Haskell
> syntax.  This rule is why it's okay to write "let x = 5 in x^2", even
> though let introduces a new layout syntax: the "in" implicitly closes it
> because "x = 5 in" isn't the start of any valid syntax, so the layout
> context is implicitly closed before the "in".
>
> Now for my question.  Does anyone know other cases besides let/in where
> this commonly comes up?  Everywhere I've seen this before uses let/in as an
> example, but then concludes that full parsing is needed and gives up on
> simpler answers.  But the specific example with let/in is easily handled
> with a special-purpose rule that closes layout contexts as needed when an
> "in" keyword shows up.  I cannot seem to construct any other examples,
> because other layout-introducing keywords have their contents at the end of
> syntactic element.
>
> Is there something I've missed here?  I don't even care if it's a
> situation where something is technically valid but a horrible edge case.
> I'm interested in realistic counterexamples.
>
> Thanks,
> Chris
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-- 
brandon s allbery kf8nh
allbery.b at gmail.com
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