Add option tagAsContentsFieldName to TaggedObject SumEncoding

[wi11dey]

Hello,

Thanks for making aeson.

I've added a flag tagAsContentsFieldName which will allow encoding sum types as

{
  "tag": "constructorName"
  "constructorName": { ... contents ... }
}

This style is used in type encoding standards such as https://palantir.github.io/conjure/#/docs/spec/wire?id=_54-union-json-format.

Initially I was thinking to make contentsFieldName be a String -> String and provide it the constructor name, which would be flexible enough to allow the default behavior of const "contents" and also be able to dynamically use the constructor name (and even modify it), but this would break backcompat and also not be serializable to the TH implementation.

Please let me know if you'd like any changes to the proposed API

[julmb]

I could use something like this in one of my projects. However, the constructor

TaggedObject { tagFieldName :: String, contentsFieldName :: String, tagAsContentsFieldName :: Bool }

admits the semantically questionable value TaggedObject "type" "item" True. If tagAsContentsFieldName is True, then the value in contentsFieldName is meaningless. I feel like it would be better to have

data ContentsFieldName = FromTag | Fixed String
... TaggedObject { tagFieldName :: String, contentsFieldName :: ContentsFieldName } ...

Alternatively, instead of ContentsFieldName, we could also use the isomorphic Maybe String, with a documentation note that a contentsFieldName value of Nothing means that the contents field gets the tag value as its name.

We could be even more extreme and not change any of the types in TaggedObject and simply treat contentsFieldName being the empty string as an indicator for using the tag value as content field name.

[wi11dey]

We could be even more extreme and not change any of the types in TaggedObject and simply treat contentsFieldName being the empty string as an indicator for using the tag value as content field name.

I like this. I've updated to the PR to this behavior. Let me know if it looks good to you