On Wed, Jul 13, 2016 at 5:42 AM, C Maeder <chr.mae...@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?
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