ArgumentDo proposal updated

[Manuel Gómez]

On Wed, Jul 13, 2016 at 5:42 AM, C Maeder <chr.maeder at web.de> wrote:
> seeing
>
>  aexp -> qvar                              (variable)
>       |  gcon                              (general constructor)
>       ...
>       |  qcon { fbind1 … fbindn }          (labeled construction)
>       |  aexp { fbind1 … fbindn }          (labelled update)
>
> and
> https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-220003
>
> I realise that the update requires at least one field binding whereas
> for a construction "C {}" (n = 0) could be used.  ("C {}" makes sense
> for patterns!)
>
> And due to the meta-rule a labelled update is not possible for a lambda
> abstraction, let expression, or conditional (as aexp), but it is for
> case (and do if the record type happens to be a monad). So a further
> less obvious example is:
>
>   case e of
>     p -> r
>   { f = v }
>
> that will be parsed as: (case e of p -> r) { f = v }
>
> (I'm sure the grammar could be fully disambiguated, but this would not
> improve readability. Preferring shift over reduce is common and fine for
> such cases.)

Upon reading this example, I believed this to be simply a matter of
the layout rule.

  case e of
    p -> r
  { f = v }

would become

  case e of {
    p -> r
  } { f = v }

This, on the other hand

  case e of p -> r { f = v }

would be equivalent to

  case e of { p -> (r { f = v }) }

I just tested this after writing the preceding as I was confused about
what you found confusing, and I am surprised that the example you
showed does indeed yield a parse error.  I very much expected this to
be valid Haskell:

  data X = X { x :: Bool }
  someX = X True

  foo =
    case () of
      _ -> someX
    { x = False }

Am I alone in my surprise?