ambiguities.hs

newtype A = A Int
newtype B = B Int

type Database = String
type JSON = String

class ParseDatabase a where parse :: Database -> Maybe a
instance ParseDatabase A where parse db = A <$> readMaybe s
instance ParseDatabase B where parse db = B <$> readMaybe s

class ToJSON a where toJSON :: a -> JSON
instance ToJSON A where toJSON (A n) = show n
instance ToJSON B where toJSON (B n) = "{\"this one uses an object, not a bare number\": " ++ show n ++ "}"

database :: Database
database = "3"

-- The thing you are asking for.
foo :: Database -> Maybe JSON
foo db = toJSON <$> parse db

-- Now I ask you: which of these two JSON objects do you expect to get from foo database, and why?
-- 3
-- {"this one uses an object, not a bare number": 3}