Stock `Eq` instances have changed

[Lysxia]

They now use dataToTag#... https://gitlab.haskell.org/ghc/ghc/-/merge_requests/6955

[Lysxia]

I think I want to change the generic implementation to match it...

[pwm]

Does this issue cause the following?

data Foo a = Bar [a] [a]
  deriving stock (Eq, Generic1)
  deriving Eq1 via (Generically1 Foo)

leads to

No instance for ‘Eq1
                     (GHC.Generics.D1
...
arising from the 'deriving' clause of a data type declaration
  Possible fix:
    use a standalone 'deriving instance' declaration,
      so you can specify the instance context yourself
• When deriving the instance for (Eq1 Foo)typecheck(-Wdeferred-type-errors)

[Lysxia]

Try importing Generic.Data.Orphans.

[pwm]

That solved it, thank you.

Out of curiosity: is there any pros/cons using Generic.Data vs. say Data.Functor.Classes.Generic?

Say I have:

data Expr a
  = Var String
  | Lam String a
  | App a a
  deriving stock (Eq, Ord, Show, Functor, Foldable, Traversable, Generic1)

then I can either do

instance Eq1 Expr where
  liftEq = liftEqDefault

instance Ord1 Expr where
  liftCompare = liftCompareDefault

instance Show1 Expr where
  liftShowsPrec = liftShowsPrecDefault

or

deriving (Eq1, Ord1, Show1) via (Generically1 Expr)

the latter is nice for brevity but I don't know if there are any other differences one should be aware of?

[Lysxia]

I'm assuming you mean Data.Functor.Classes.Generic from transformers-compat. As far as I can tell these are equivalent. Note that with the upcoming GHC 9.12 you will be able to do deriving via for Eq1 and others directly without dependencies outside of base.