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

[Iavor Diatchki]

Hello,

Joachim, would the voting system get a lot more complicated if we added a
bit of ranked voting (e.g., instead of no,maybe,yes) we went (-2: strong
reject, -1: reject, 0: don't care, 1: accept, 2: strong accept)?   At least
in my votes, I have a few extensions that I certainly feel strong about,
and for some of them I just picked something but I don't really care either
way, and I don't think that's captured in the votes.  Let me know if that's
OK, and I'll find some time to update my votes.

Also, I've been busy with work, so when there are a bunch of e-mails like
today (which is fun!), it is hard to keep up with what people have said
about the many threads going on.  If we adopt the haddock like commenting
system, would it be possible to scrape the e-mails and auto generate a
summary of what people have said for various things?  This could also help
with a final document where we not only pick the extensions, but also have
some provenance about what people thought about the various extensions.

-Iavor








On Thu, Dec 3, 2020 at 8:58 AM Simon Peyton Jones via
ghc-steering-committee <ghc-steering-committee at haskell.org> wrote:

> I voted against GADTs and am not yet inspired to change that vote: GADTs
> cause trouble for type inference. For example:
>
>
>
> Yes, but there is no prospect (that I know of) of a substantial
> improvement in this – and what we have does not seem to cause problems in
> practice.   And they are jolly useful and popular!
>
>
>
> Simon
>
>
>
> *From:* ghc-steering-committee <ghc-steering-committee-bounces at haskell.org>
> *On Behalf Of *Richard Eisenberg
> *Sent:* 03 December 2020 16:32
> *To:* Alejandro Serrano Mena <trupill at gmail.com>
> *Cc:* ghc-steering-committee at haskell.org; Joachim Breitner <
> mail at joachim-breitner.de>
> *Subject:* Re: [ghc-steering-committee] #380 GHC2021: Current status
>
>
>
>
>
>
>
> On Dec 3, 2020, at 4:40 AM, Alejandro Serrano Mena <trupill at gmail.com>
> wrote:
>
>
>
> - GADTs:
>
>   - Stable and well documented,
>
>   - Adding indices to types is one of the main reasons one would like to
> have MultiParamTypeClasses and TypeFamilies on,
>
>   - I find the GADT syntax much nicer (but this is an extremely personal
> choice.)
>
>
>
> I voted against GADTs and am not yet inspired to change that vote: GADTs
> cause trouble for type inference. For example:
>
>
>
> data T a where
>
>   MkT :: Int -> T a
>
>
>
> foo (MkT x) = x
>
>
>
> GHC can infer that foo :: T a -> Int.
>
>
>
> But if I change this to
>
>
>
> data T a where
>
>   MkT :: Int -> T Int
>
>
>
> foo (MkT x) = x
>
>
>
> (where T is now a GADT) GHC can no longer infer my type. It complains
> about untouchable variables. This is a case of a "bad failure mode", where
> a simple error in input can lead to a very complicated error message in
> output. I thus think that users should knowledgeably opt into GADTs. Maybe
> if we had a much gentler error message in place here, I could be convinced
> otherwise. But, for now:
>
>
>
> Vote against GADTs!
>
>
>
> Richard
>
>
>
> PS: A counterpoint is RankNTypes, which I voted for. It would be hard to
> accidentally write a higher-rank type, and I find the error messages that
> arise are much clearer. Yes, it's an advanced feature, but one that is
> appropriately (in my opinion) gated in the code.
> _______________________________________________
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>
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