Tidy serialisation of ADTs to JSON.

[chrisdew]

Crossposted: http://stackoverflow.com/questions/31439991/how-to-write-haskell-adts-as-tidy-json-using-aeson

(What is tidy is in the eye of the beholder, but I believe: {"attach":{"tel":"+447890"}} is tidier than {"tag":"AttachMsg","contents":{"tel":"+447890"}}.)

I've spent some time playing around with Aeson, but I can't get Algebraic Data Types to serialise nicely.

What I've tried is:

data Attach = Attach { tel :: String }
              deriving (Show)
$(deriveJSON defaultOptions ''Attach)

data Fix = Fix { lat :: Double, lng :: Double }
              deriving (Show)
$(deriveJSON defaultOptions ''Fix)

data MsgIn = AttachMsg Attach
           | FixMsg    Fix
           deriving (Show)
$(deriveJSON defaultOptions ''MsgIn)

data MsgIn2 = MsgIn2 { attach :: Maybe Attach, fix :: Maybe Fix }
            deriving (Show)
$(deriveJSON defaultOptions ''MsgIn2)

someFunc :: IO ()
someFunc = do
  let attach = Attach "+447890"
  let reply = AttachMsg attach
  BL.putStrLn (encode reply)
  let reply2 = MsgIn2 (Just attach) Nothing
  BL.putStrLn (encode reply2)

The output is:

{"tag":"AttachMsg","contents":{"tel":"+447890"}}
{"attach":{"tel":"+447890"},"fix":null}

The output I'm looking for is:

{"attach":{"tel":"+447890"}}

but from the MsgIn type, rather than MsgIn2.

(The output of MsgIn2 gets quite close, but it's got an explicit null.)

Is there a way of doing this in Aeson?

[chrisdew]

Useful answers on SO.

[Gekkio]

Use custom options instead of defaultOptions. You can get the right structure by using sumEncoding = ObjectWithSingleField, which reduces your first example to {"AttachMsg":{"tel":"+447890"}}. You can then customize the constructor tags by using constructorTagModifier = myConstructorTag and by writing a function myConstructorTag that customizes the names to your liking (e.g. AttachMsg -> attach).

As an example, you'll get the output you want by writing this into a separate module, importing it, and using myOptions instead of defaultOptions:

myConstructorTag :: String -> String
myConstructorTag "AttachMsg" = "attach"
myConstructorTag x = x

myOptions :: Options
myOptions = defaultOptions {sumEncoding = ObjectWithSingleField, constructorTagModifier = myConstructorTag}

A separate module is needed here because of Template Haskell. There's probably a way to define myConstructorTag and myOptions in a better way to satisfy the needs of TH, but I have absolutely no idea how to do that.

Edit: I cross-posted my answer to SO as well