Modifiers for types, i.e. newtype wrappers where the values satisfy some constraint (non-empty, positive etc.). Suggestions on useful types are appreciated.

To apply the modifiers types you can use the record label. For instance:

  data C a = C [a] [a] deriving Typeable
  instance Enumerable a => Enumerable (C a) where
     enumerate = unary $ funcurry $ 
       \xs ys -> C (nonEmpty xs) (nonEmpty ys)

Alternatively you can put everything in pattern postition:

  instance Enumerable a => Enumerable (C a) where
     enumerate = unary $ funcurry $ 
       \(Free (NonEmpty xs,NonEmpty ys)) -> C xs ys)

The first approach has the advantage of being usable with a point free style: \xs -> C (nonEmpty xs) . nonEmpty .