Why is Distributive a superclass constraint for Representable?

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1

The definition of Representable constraints the implementor to provide an instance for Distributive first.

I wondered why that may be, in the definition of Representable, there is no mention of any Distributive functions: source.
In fact, the imports from Distributive are only ever used for re-exporting and other instance definitions.

Additionally, I wanted to use an instance I defined like this:

instance Representable w => Distributive w where
  distribute :: Functor f => f (w a) -> w (f a)
  distribute f = tabulate (\ i -> fmap (flip index i) f)

which sadly doesn’t work, or at least I can’t get it to work, because Distributive is a superclass constraint for Representable.
GHC and HLS offer me some options about UndecidableInstances but I’m not experienced enough to successfully use these.

Do you know of a reason for this constraint?
Could I expose the instance I defined maybe using a newtype?

2

You can derive your desired instance via Co:

data W a 
  -- ...
  deriving Distributive W via Co W

instance Representable W where
  -- ...