Or, alternatively, some common class that lets me express that a type is boring (i.e., inhabited by precisely one fully-defined value)? lens has Settable, whose law ensures the type involved has a boring representation (in the sense of representable functor), but is there a more fundamental way?

class Boring x where
  inhabitant :: x
instance Boring () where
  inhabitant = ()
instance Boring (Proxy a) where
  inhabitant = Proxy
instance Boring y => Boring (x -> y) where
  inhabitant = const inhabitant
instance (Boring x, Boring y) => Boring (x, y) where
  inhabitant = (inhabitant, inhabitant)
instance Boring x => Boring (Const x y) where
  inhabitant = Const inhabitant
instance Boring x => Boring (Identity x) where
  inhabitant = Identity inhabitant
...

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