GHC 9.4 checklist

[RyanGlScott]

This issue serves as a reminder to do certain things before we ship a new major release that supports GHC 9.4:

• Support DefaultD
• Support OpaqueP
• Support PromotedInfixT/PromotedUInfixT
• Support \cases
• Use withDict instead of unsafeCoerce to implement bindIP (EDIT: Never mind, see Use withDict to implement bindIP on recent GHCs #166 (comment))

[RyanGlScott]

At this point, the only thing holding back a new release is the th-lift-instances dependency. It currently does not build with containers-0.6.6 (bundled with GHC 9.4) due to bennofs/th-lift-instances#32.

[ysangkok]

@RyanGlScott Is that dependency still needed? I see in the README that the minimum required version of GHC is 8.0.

Seems like the library is only needed with really old versions of GHC, as can be seen in phadej/file-embed-lzma#11.

This patch seems to make it compile with 9.4.1:

diff --git a/Language/Haskell/TH/Desugar/Lift.hs b/Language/Haskell/TH/Desugar/Lift.hs
index 50bb7e9..980bd24 100644
--- a/Language/Haskell/TH/Desugar/Lift.hs
+++ b/Language/Haskell/TH/Desugar/Lift.hs
@@ -20,7 +20,6 @@
 module Language.Haskell.TH.Desugar.Lift () where
 
 import Language.Haskell.TH.Desugar
-import Language.Haskell.TH.Instances ()
 import Language.Haskell.TH.Lift
 
 $(deriveLiftMany [ ''DExp, ''DPat, ''DType, ''DForallTelescope, ''DTyVarBndr
diff --git a/th-desugar.cabal b/th-desugar.cabal
index 234b8a7..59226fc 100644
--- a/th-desugar.cabal
+++ b/th-desugar.cabal
@@ -45,7 +45,7 @@ library
   build-depends:
       base >= 4.9 && < 5,
       ghc-prim,
-      template-haskell >= 2.11 && < 2.19,
+      template-haskell >= 2.11 && < 2.20,
       containers >= 0.5,
       mtl >= 2.1 && < 2.4,
       ordered-containers >= 0.2.2,

The tests don't compile though, since th-orphans depends on th-lift-instances.

[RyanGlScott]

I'm a bit confused by #157 (comment), as Language.Haskell.TH.Instances comes from th-orphans, not th-lift-instances. (Confusing, I know.) We certainly can't remove the th-orphans dependency, as it provides Quasi instances that are used in Language.Haskell.TH.Desugar.Reify.

[ysangkok]

Oh, I think this might be a cabal-install bug. It seems it does find the conflict if I provide --constraint='containers==0.6.6'. Sorry for this noise. I suppose one option would be to make the dependency conditional in th-orphans, but that is not a discussion to be had here.

[ysangkok]

Th-orphans was updated to not require th-lift-instances for ghc versions relevant to th-desugar: mgsloan/th-orphans#38

[RyanGlScott]

Thanks!

I've uploaded th-desugar-1.14 to Hackage, which is compatible with GHC 9.4.