A few things make this not a straightforward thumbs up for me, though I'm not strongly against.

What is the interaction with GHC20xx? Presumably we want to say something like GHC20xx will never include any Deprecated or Legacy extensions? What about Unsable? if an extension transitions from Stable -> Legacy, would we remove it from the next GHC20xx?

For existing, or future, language sets such as GHC2021 or Haskell98, it is expected that none of the contained extensions would be Unstable. However, this proposal does not seek to impose any particular policy on the inclusion of extensions into language sets - the developers and the steering committee are always in the best position to make a decision about a concrete extension and extension set.

Something doesn't feel quite right about the warning system. If a module can start with

{-# OPTIONS_GHC -Wno-XDeprecated #-}

{-# LANGUAGE OverlappingInstances #-}

and silently use an extension that the {build system, user, project} wanted to disallow, have we achieved anything? Compare this to the current situation, where the environment can say -XNoOverlappingInstances and code can override that with {-# LANGUAGE OverlappingInstances #-} - there's essentially no difference, we just added another layer of disable/override that isn't buying us anything.

(note I'm viewing this through the spectacles of -Werror, because I've come to believe that warnings are essentially not useful unless given teeth with -Werror.)

Cheers

Simon

On Fri, 1 Sept 2023 at 13:18, Vladislav Zavialov <vlad.z.4096@gmail.com> wrote:

I agree that we need a categorisation of extension language flags, but I'm not convinced that {Stable, Unstable, Deprecated, Legacy} is the right set of labels. In fact, I wouldn't want to commit to any particular categorisation before we actually go through all the extensions in GHC and see for ourselves that they can be adequately categorized according to the proposed system.

The proposal says "classifications of individual language extensions will be left to a future proposal". Well, I am skeptical that this separation makes sense. I would much prefer if we were discussing a concrete categorisation proposal, not just a set of four labels whose implications I can't fully grasp.

Vlad

On Fri, Sep 1, 2023 at 11:37 AM Simon Peyton Jones <simon.peytonjones@gmail.com> wrote:

Dear Simon, Vlad, Eric, Chris, Moritz

I would love to hear from you about this proposal.  Please.

I plan to accept it unless I hear dissent.  But I would much rather have an explicit response from you than take silence as assent.  You are a member of the committee, after all!

My apologies if I have missed your reply

Simon