Covalent: heterogenous traversal

Covalent is an analogue of Traversal where its targets may be any type satisfying the specified constraint.

class Covalent c a where
  traverseCommon :: Applicative f => (forall x. c x => x -> f x) -> a -> f a

An instance of Covalent can be generically derived as long as all the fields satisfy the constraint:

data Numeral = NumInt Int | NumDouble Double | NumPair Double Int deriving Generic

instance Covalent Show Numeral
instance Covalent Num Numeral

Using the Covalen Num Numeral instance, you can write a function that adds 1 regardless of the constructor.

add1 :: Numeral -> Numeral
add1 = overCommon @Num (+1)