[ghc-steering-committee] #380 GHC2021: Let's deprecate ExistentialQuantification

Iavor Diatchki iavor.diatchki at gmail.com
Fri Dec 4 16:15:57 UTC 2020

I am strongly against deprecating ExistentialQuantification and prefer it
over the GADT notation for non-GADT datatypes.  I find GADT notation nice
when the types of the result change, but I don't think it is great for
normal types (which are the large majority of types I use) for these
   - It duplicates the declaration of the types many types, but often it is
not obvious that you have the same type
   - The disconnect between the type variables in the data declaration and
the constructors is confusing, which is why I prefer the GADT from where
you declare the kind of the GADT, but that's quite ugly and verbose.

On Fri, Dec 4, 2020 at 7:25 AM Joachim Breitner <mail at joachim-breitner.de>

> Hi,
> Am Freitag, den 04.12.2020, 15:47 +0100 schrieb Spiwack, Arnaud:
> > I'm just saying: let's _not_ add ExistentialQuantification in GHC2021
> > (because it ought, in my opinion, to be considered deprecated).
> I’m convinced!
> >  Let's not go into talking about chopping the feature into smaller
> > pieces.
> Not for GHC2021! But if GADTs don’t make it this round, we can ask the
> question if ExistentialVariables is a feature worth having on its own,
> of if anyone who wants to use that should just go for GADTs.
> Cheers,
> Joachim
> --
> Joachim Breitner
>   mail at joachim-breitner.de
>   http://www.joachim-breitner.de/
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