[ghc-steering-committee] #380 GHC2021: Current status

Spiwack, Arnaud arnaud.spiwack at tweag.io
Fri Dec 4 08:22:57 UTC 2020

On Thu, Dec 3, 2020 at 6:31 PM Richard Eisenberg <rae at richarde.dev> wrote:

> The problem I described would arise when someone who does not know about
> GADTs and type inference accidentally writes a GADT. But this cannot happen
> easily today, precisely because of the need to write the extension.
> Useful, popular, and stable all help argue for an extension (and I agree
> here!), but I'm more concerned about error messages and the beginner
> experience, captured in our Criterion 2 of
> https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0372-ghc-extensions.rst#criteria
> .

This is not a very believable objection in my opinion. GADTs are guarded by
a different syntax which isn't used by someone who doesn't know about
GADTs. So writing a GADT by accident is exceedingly unlikely. Besides,
Ocaml has GADTs by default (with a similar syntax split as Haskell's). I
don't believe I've ever heard anybody complain about GADTs since they've
landed, certainly of anybody writing one by mistake.
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