Commits on Jun 30, 2024

1. Give helper type families more precise kinds …

   When promoting a class or instance method, we generate a "helper" type family
   definition that contains the actual implementation of the class or instance
   method. Prior to this patch, it was possible that the kind of the helper type
   family could be more polymorphic than desired. For instance, `singletons-th`
   would promote this:
   
   ```hs
   $(promote [d|
     type MyApplicative :: (Type -> Type) -> Constraint
     class Functor f => MyApplicative f where
       ap :: f (a -> b) -> f a -> f b
   
       rightSparrow :: f a -> f b -> f b
       rightSparrow x y = ap (id <$ x) y
     |])
   ```
   
   To this:
   
   ```hs
   type PMyApplicative :: (Type -> Type) -> Constraint
   class PMyApplicative f where
     type Ap (x :: f (a ~> b)) (y :: f a) :: f b
     type RightSparrow (x :: f a) (y :: f b) :: f b
     type RightSparrow x y = RightSparrowDefault x y
   
   -- The helper type family
   type RightSparrowDefault :: forall f a b. f a -> f b -> f b
   type family RightSparrowDefault x y where
     RightSparrowDefault x y = Ap (IdSym0 <$ x) y
   ```
   
   Note that GHC would generalize the standalone kind signature of
   `RightSparrowDefault` to:
   
   ```hs
   type RightSparrowDefault :: forall {k} (f :: k -> Type) (a :: k) (b :: k).
                               f a -> f b -> f b
   ```
   
   This is more general than intended, as we want `f` to be of kind `Type -> Type`
   instead. After all, we said as much in the standalone kind signature for
   `MyApplicative`! This excessive polymorphism doesn't actively cause problems in
   today's GHC, but they will cause problems in a future version of GHC that
   implements [GHC#23515](https://gitlab.haskell.org/ghc/ghc/-/issues/23515).
   (See #601.)
   
   This patch resolves the issue by propagating kind information from the class's
   standalone kind signature (or, in the case of instance declarations, the
   instance head) to the helper type family declarations. After this patch, we now
   generate the following kind for `RightSparrowDefault` (as verified by the new
   `T601a` test case):
   
   ```hs
   type RightSparrowDefault :: forall (f :: Type -> Type) a b.
                               f a -> f b -> f b
   ```
   
   This piggybacks on machinery that was added in #596 to do most of the heavy
   lifting.
   
   Resolves the "Overly polymorphic promoted class defaults" section of #601.

   

   RyanGlScott committed Jun 30, 2024
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