There are two ways to use this library: the way with extra boilerplate, or the way with a leaky abstraction. A truly good solution would fix it all with Template Haskell; but the author is not a truly good Haskeller, so this will have to do for now.

The boilerplate way:

data Hideous = Hideous { yak :: [String] }
instance ToJSON Hideous where
  toJSON x = object [ "yak" .= hairy (yak x) ]
instance FromJSON Hideous where
  parseJSON (Object o) = Hideous <$> (shave <$> o .:? "yak")

>>> encode $ Hideous ["foo","bar"]
"{\"yak\":[\"foo\",\"bar\"]}"
>>> encode $ Hideous ["baz"]
"{\"yak\":\"baz\"}"
>>> encode $ Hideous []
"{\"yak\":null}"
>>> yak <$> decode "{\"yak\":[\"foo\",\"bar\"]}"
Just ["foo", "bar"]
>>> yak <$> decode "{\"yak\":\"baz\"}"
Just ["baz"]
>>> yak <$> decode "{}"
Just []

The leaky way:

data Abhorrent = Abhorrent { yak :: Yak String }
$(deriveJSON defaultOptions{omitNothingFields=True} ''Abhorrent)

>>> encode . Abhorrent . hairy $ ["shaven","shorn","sheared"]
"{\"yak\":[\"shaven\",\"shorn\",\"sheared\"]}"
>>> shave . yak <$> decode "{}"
Just []

Which to prefer depends on how many yaks you need to deal with vs. how much you hate cleaning up yak droppings in the rest of your codebase.