Single tag constructors in Aeson - haskell

Question

I have a data type like this:
data A = A T.Text deriving (Generic, Show)
instance A.ToJSON A
If I use A.encode to it:
A.encode $ A "foobar" -- "foobar"
Then I use singleTagConstructors on it:
instance A.ToJSON A where
toEncoding a = A.genericToEncoding $ A.defaultOptions { A.tagSingleConstructors = True }
A.encode $ A "foobarquux" -- "{tag: A, contents: foobarquux}"
At some point I made another data type:
newtype Wrapper a = Wrapper
{ unWrap :: a
} deriving (Show)
instance A.ToJSON a => A.ToJSON (Wrapper a) where
toJSON w = A.object [ "wrapped" A..= unWrap w ]
Here's the part where I get confused:
A.encode $ Wrapper $ A "foobar" -- "{wrapped: foobar}"
How do I get the result to be like this?
"{wrapped: {tag: A, contents: foobarquux}}"

Answer 1

To answer the question directly, you can always implement the Wrapper instance with tagSingleConstructors = False, like this:
instance Generic a => A.ToJSON (Wrapper a) where
toJSON w = A.object [ "wrapped" A..= encA (unWrap w) ]
where
encA = A.genericToEncoding $ A.defaultOptions { A.tagSingleConstructors = False }
But I don't see why you'd want to do that.
If you control the API, then you don't need the tag field: the expected type of the wrapped value is already statically known, so tag would not be helpful.
And if you don't control the API, I would recommend representing it very explicitly, for example as a record that exactly matches the API shape. Otherwise you run a risk of accidentally breaking the API by making unrelated changes in remote parts of the codebase.

Answer 2

The issue is how you implemented your custom ToJSON instance.
instance A.ToJSON A where
toEncoding a = A.genericToEncoding $ A.defaultOptions { A.tagSingleConstructors = True }
Since you do not implement toJSON directly default implementation from typeclass definition is used.
class ToJSON a where -- excerpt from Data.Aeson.Types.ToJSON
-- | Convert a Haskell value to a JSON-friendly intermediate type.
toJSON :: a -> Value
default toJSON :: (Generic a, GToJSON' Value Zero (Rep a)) => a -> Value
toJSON = genericToJSON defaultOptions
Effectively, you have the following instance:
instance A.ToJSON A where
toJSON = genericToJSON defaultOptions
toEncoding a = A.genericToEncoding $ A.defaultOptions { A.tagSingleConstructors = True }
While toEncoding uses expected tagged encoding, toJSON uses default encoding (which does not tag single constructors). This inconsistency is the root cause of the confusion. Later, in wrapper's ToJSON instance .= operator used. Internally, it uses toJSON and not toEncoding:
class KeyValue kv where
(.=) :: ToJSON v => Text -> v -> kv
infixr 8 .=
instance KeyValue Pair where
name .= value = (name, toJSON value)
As a solution, you should either define only toJSON and keep default toEncoding implementation (that uses toJSON) or implement both.