T8030.hs:9:3: error:
    • Couldn't match expected type ‘Pr a’ with actual type ‘Pr a0’
      NB: ‘Pr’ is a non-injective type family
      The type variable ‘a0’ is ambiguous
    • In the ambiguity check for ‘op1’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method:
        op1 :: forall k (a :: k). C a => Pr a
      In the class declaration for ‘C’

T8030.hs:10:3: error:
    • Couldn't match type ‘Pr a0’ with ‘Pr a’
      Expected type: Pr a -> Pr a -> Pr a
        Actual type: Pr a0 -> Pr a0 -> Pr a0
      NB: ‘Pr’ is a non-injective type family
      The type variable ‘a0’ is ambiguous
    • In the ambiguity check for ‘op2’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the class method:
        op2 :: forall k (a :: k). C a => Pr a -> Pr a -> Pr a
      In the class declaration for ‘C’