Re: ArgumentDo proposal updated

Hello Manuel, this is exactly the change that is being discussed: currently a `case` expression is not considered to be atomic (`aexp`), which is why it can't appear in a record update without parens. The proposed change, as I understand it, is to make `case` (and `do`) into atomic expressions as they are "parentesized" by the key-word (`case` or `do`) at the start, and the closing `}` at the end. Making this change would allow the kind of thing you were expecting, which to me makes sense. Others seem to find it confusing :-) -Iavor On Wed, Jul 13, 2016 at 10:35 AM, Manuel Gómez wrote:

On Wed, Jul 13, 2016 at 5:42 AM, C Maeder 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? _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users