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

Alejandro Serrano Mena trupill at gmail.com
Fri Dec 4 13:41:11 UTC 2020

Agree! Try to steering people to either use the "simple ADT" or go to "full
GADT" syntax seems a good plan to me.

El vie, 4 dic 2020 a las 14:23, Spiwack, Arnaud (<arnaud.spiwack at tweag.io>)

> Dear all,
> ExistentialQuantification is one of the extensions that we are considering
> adding to GHC2021.
> ExistentialQuantificaton enables the following syntax:
> data Dyn = forall a. Typeable a => MkDyn a
> Has any of us actually used this syntax in recent years? I, personally,
> only use the equivalent GADT syntax
> data Dyn where
>   MkDyn :: forall a. Typeable a => a -> Dyn a
> It’s barely more verbose, and it’s less quirky.
> Besides, the GADT syntax is there to stay (and GADTSyntax at the very
> least, is one of these extensions where there seems to be no doubt will be
> in GHC2021).
> I’d say there is such a thing as too much syntax. And, in my view, the
> GADT syntax makes the ExistentialQuantification syntax redundant. I’d much
> rather we didn’t add it in the defaults so that it can be on its slow path
> to deprecation.
> /Arnaud
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