Previously, in order to use quantified constraints on typeclasses like Ord
, you had to include the superclass in the instance like so:
newtype A f = A (f Int)
deriving instance (forall a. Eq a => Eq (f a)) => Eq (A f)
deriving instance (forall a. Eq a => Eq (f a), forall a. Ord a => Ord (f a)) => Ord (A f)
(This is in fact precisely the solution given in this question).
In GHC 9, however, the above code doesn't work. It fails with the following error:
• Could not deduce (Eq (f a))
from the context: (forall a. Eq a => Eq (f a),
forall a. Ord a => Ord (f a))
bound by a stand-alone deriving instance declaration:
forall (f :: * -> *).
(forall a. Eq a => Eq (f a), forall a. Ord a => Ord (f a)) =>
Ord (A f)
or from: Eq a
bound by a quantified context
• In the ambiguity check for a stand-alone deriving instance declaration
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the stand-alone deriving instance for
‘(forall a. Eq a => Eq (f a), forall a. Ord a => Ord (f a)) =>
Ord (A f)’
Unfortunately the AllowAmbiguousTypes
suggestion doesn't work. (you get the same error, followed by the same error for every method on the class)
Does anyone know of a workaround for this?
CodePudding user response:
One simple way to solve it is to change the second deriving clause to:
deriving instance (Eq (A f), forall a. Ord a => Ord (f a)) => Ord (A f)
I don't yet have a good explanation for why this is happening though.