[Hugs-users] Field names starting Upper [Was: [haskell-cafe/ghc-proposals] Allow reserved identifiers as fields ...

[Anthony Clayden]

I said (
https://mail.haskell.org/pipermail/haskell-cafe/2024-December/136986.html)

On Wed, 25 Dec 2024 at 16:49, Anthony Clayden <anthony.d.clayden at gmail.com>
wrote:

> ...
>
> [**] I'm particularly keen on field names starting upper case, because
> they're injective/part of the structure, like data constructors.
>
>
>
And previously with Hugs (
https://mail.haskell.org/pipermail/hugs-users/2020-May/000927.html)


> * I've taught Trex to support field labels starting upper-case, because,
>
well for me they're more like constructors than variables.
>
>

So can I combine that work? Yes, providing I use Upper-case labels only
within `{ ... }`. These can combine lower with Upper field names within the
same data type; can pun on the lower.

>
>     data Upup a = MkUp{ down :: a, Up :: Int }   deriving (Eq, Show)
>
>     anUp  = MkUp{ down = 'a', Up = 5 }      -- build
>     recUp = anUp{ Up = 7 }              -- field update by Up label,
record syntax
>
>     succUp (MkUp {down, Up = u} ) = MkUp{ down = succ down, Up = succ u }
>

You can't use a field selection function: 'ERROR - "Up" is not a data
constructor'.
(It looks like Hugs does generate such a function; this error is a syntax
rejection in trying to use it.)
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