tagged-aeson

tagged-aeson provides tagged FromJSON and ToJSON classes and TH generators that use those instances instead of Aeson's ones.

Any function expecting a normal FromJSON or ToJSON constraint can work with tagged-aeson instances by means of the TaggedAeson newtype wrapper.

Is it usable already?

Somewhat, but it's better to wait until an official release. The API is in flux, and feedback is welcome!

See also: monadfix/jijo, a more radical instance-less approach to bidirectional JSON encoding/decoding.

Usecase: avoid orphan instances

You have a URI in your config type and you want to autoderive a FromJSON instance for the config. Without tagged-aeson, you'd write an orphan instance:

{-# OPTIONS_GHC -Wno-orphans #-}

instance FromJSON URI where
    parseJSON = withText "URI" $
        maybe (fail "invalid URI") return . parseURIReference . unpack

data Config = { ... }
deriveFromJSON defaultOptions ''Config

decodeConfig :: ByteString -> Maybe Config
decodeConfig = decode

With tagged-aeson, you can write a non-orphan instance – and only use it for FromJSON Config without letting it escape into the rest of your program:

instance FromJSON Config URI where
    parseJSON = withText "URI" $
        maybe (fail "invalid URI") return . parseURIReference . unpack

data Config = { ... }
deriveFromJSON ''Config defaultOptions ''Config

decodeConfig :: ByteString -> Maybe Config
decodeConfig = fmap (fromTaggedAeson @Config) . decode

Usecase: mark some instances as dangerous

You have a request type with a plaintext password in one of the fields. You are prudent and don't have a ToJSON instance for your PlainTextPassword type – after all, you only need to parse requests, not generate them.

That is, until you start writing tests.

Without tagged-aeson: you would define orphan instances for all your request types in the testsuite. Or you would write functions someRequestToJSON, otherRequestToJSON, etc, and import them when necessary.

With tagged-aeson:

data TestOnly a

instance FromJSON Api PlainTextPassword
instance ToJSON (TestOnly Api) PlainTextPassword

instance FromJSON Api SomeRequest
instance ToJSON (TestOnly Api) SomeRequest

Usecase: versioned APIs

Coming soon!

Usecase: modify Aeson-provided instances

You are working with a weird API that represents UTCTime as /Date(1302547608878)/. By default, the serialization for UTCTime is not what you want, but Aeson provides a newtype to handle this case:

data User = User { name :: Text, created :: UTCTime }

instance FromJSON User where
  parseJSON = withObject "User" $ \o ->
    name    <- o .: "name"
    created <- fromDotNetTime <$> o .: "created"
    pure User{..}

However, you can forget to use the newtype wrapper, leading to hard-to-find bugs. With tagged-aeson, though, you can just change the instance:

data WeirdAPI

instance FromJSON WeirdAPI User where
  parseJSON = withObject "User" $ \o ->
    name    <- o .: "name"
    created <- o .: "created"
    pure User{..}

instance FromJSON WeirdAPI UTCTime where
    parseJSON = fmap fromDotNetTime . using @Aeson parseJSON

You can also write a stub instance with a TypeError and direct users towards one of several existing newtypes.

Usecase: burn down Aeson-provided instances

Let's say you really don't like Aeson instances for Maybe.

tagged-aeson provides no built-in instances; you can always lift instances from Aeson, but you don't have to. Or you can write a stub instance – and, again, add a TypeError so that nobody would be able to use it. At last!