GHC 8.10 checklist

[RyanGlScott]

This issue serves as a reminder to do certain things before we ship a new major release that supports GHC 8.10:

• Update th-desugar (see GHC 8.10 checklist th-desugar#114).
• Give Figure out how to promote visible type application sensibly #378 some serious consideration
  • After some serious consideration, I have come to the conclusion that a full fix for Figure out how to promote visible type application sensibly #378 just isn't possible at the moment—see my explanation at Figure out how to promote visible type application sensibly #378 (comment). I have, however, documented the fact that singletons doesn't fully support TypeApplications in the README.
• Some simplifications can be made using visible dependent quantification (VDQ):

  • This comment in Data.Singletons.Promote.Defun suggests some code cleanup that would be possible with VDQ:

    singletons/src/Data/Singletons/Promote/Defun.hs

    Lines 405 to 412 in ea63077

    	Having to explicitly annotate each occurrence of every kind variable on
    	the LHS like this is a bit tiresome, especially since we don't have to
    	do this in the return kind. If GHC had syntax for visible dependent
    	quantification, we could avoid this step entirely and simply write:
    	data SymmetrySym1 :: forall k (t :: k). forall (a :: Proxy t) -> ...
    	Until GHC gains this syntax, this is the best alternative.

  • Current, Sigma is defined to be:

    singletons/src/Data/Singletons/Sigma.hs

    Line 34 in ea63077

    data Sigma (s :: Type) :: (s ~> Type) -> Type where

    Why not:

    type Sigma :: forall s -> (s ~> Type) -> Type
    data Sigma s t where

  • Currently, we need ImpredicativeTypes in Data.Singletons.Sigma due to this type synonym:

    singletons/src/Data/Singletons/Sigma.hs

    Line 39 in ea63077

    type Σ = Sigma

    However, we could eliminate the use of ImpredicativeTypes altogether by just giving Σ a standalone kind signature:

    type Σ :: forall s -> (s ~> Type) -> Type
    type Σ = Sigma

• Future-proof singletons against the deprecation of the CUSKs extension by giving more declarations standalone kind signatures:

  • ApplyTyCon:

    type ApplyTyCon :: (k1 -> k2) -> (k1 ~> unmatchable_fun)
    type family ApplyTyCon where ...

  • SndSigma:

    type SndSigma :: forall s t. forall (sig :: Sigma s t) -> t @@ FstSigma sig
    type family SndSigma sig where ...

[RyanGlScott]

Now that GHC 8.10.1 is out, I plan to release th-desugar-1.11 and singletons-2.7 some time in the next few days (unless there are any last-minute objections).

[goldfirere]

No objections from me, certainly!

[RyanGlScott]

Fixed with the Hackage release of singletons-2.7.