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

Spiwack, Arnaud arnaud.spiwack at tweag.io
Fri Dec 4 14:47:28 UTC 2020

I'm just saying: let's _not_ add ExistentialQuantification in GHC2021
(because it ought, in my opinion, to be considered deprecated). Let's not
go into talking about chopping the feature into smaller pieces. I do agree
that one of my arguments in the original email has been shown to be
incorrect, though. But I still think that my suggestion holds.

On Fri, Dec 4, 2020 at 3:34 PM Richard Eisenberg <rae at richarde.dev> wrote:

> I do use the data Ex = forall a. ... syntax occasionally, but only when
> I'm emphasizing that I'm not building a GADT. I would entertain the thought
> of removing it. Yet, I don't think we can do so as part of this process, as
> we don't have a combination of extensions that allows GADT-syntax
> existentials but not this other syntax. (Even -XGADTs
> -XNoExistentialQuantification doesn't eliminate it.)
> In any case, I've voted against GADTs, and so if you want to disentangle
> this one piece of syntax, that would also be a vote against GADTs, followed
> up with a proposal to spin out this syntax into its own extension.
> Richard
> On Dec 4, 2020, at 9:14 AM, Spiwack, Arnaud <arnaud.spiwack at tweag.io>
> wrote:
> Ah one clarification, GADTSyntax alone does not enable existential types.
>> You need to enable either ExistentialQuantification or full GADTs.
> It makes more than perfect sense, but I wasn't aware of this. A
> consequence of always enabling -XGADTs, I suppose :-) .
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