Support of old GHC

[Shimuuar]

Below is list of things we do to support old GHCs and what could we do if raise minimum supported version. This is not a call to action but information to aid decision making.

• GHC 8.2:
  • First version shipped with deepseq-1.4.3.0. We could drop support for deepseq<1.4.3
• GHC 8.6 (base-4.12):
  • Has support for DerivingVia. We could replace a lot of CPP for deriving Unboxed vectors with DerivingVia
• GHC 8.8 (base-4.13):
  • Drop CPP around MonadFail/fail
  • Type application in rewrite rules is supported: "unstreamM[ST]" forall s. forall. unstreamM @(ST s) = unstreamPrimM
• GHC 9.0
  • conditional compilation around unsafeWithForeignPtr

[lehins]

I don't think we should drop ghc-9.0 quite yet, especially since this point "conditional compilation around unsafeWithForeignPtr" is not such a big deal.

With respect to dropping support for ghc < 8.10 I am all in favor 👍

[Shimuuar]

As I said it's not a proposal to do anything. GHC 9.0 is simply last version requiring CPP. GHC 9.12 would certainly end up on this list if it needed CPP.

I think good point for having such discussion would be preparing next release

[Shimuuar]

While working on #540 I looked at implementation of unboxed vectors and remembered that it's terrible CPP mess. So it would be good idea to drop support for old GHC and use GND+DerivingVia for deriving necessary instances.

I propose to drop support for GHC<8.8. It's 6 year old at this point, shipped with Debian oldoldstable and quite reasonable version to support.

[Shimuuar]

We won't need to think about dropping support for older GHC for few years. So closing