Add allNullaryConstructorTagModifier option

[wi11dey]

Often, enums (all nullary contructors) are encoded differently than sum types.

For example, if I have

data State = Launching | Launched | Terminated | MarkedForDeletion

I might want to have these states encoded as LAUNCHING, LAUNCHED, TERMINATED, MARKED_FOR_DELETION, respectively.

However, using the existing constructorTagModifier is not satisfactory for this because I don't necessarily want my sum types, e.g.

data Employee = Manager { subordinates :: [Employee] } | Junior

to be encoded the same way as an enum.

I've added an optional field allNullaryConstructorTagModifier (it's Maybe so that there's no behavior change to any existing usages of Aeson, in case someone is depending on constructorTagModifier also applying to all-nullary datatypes). When it's Nothing, everything is the same as today. When it's non-Nothing it's used instead of constructorTagModifier for all-nullary datatypes, for the purposes of handling enums differently than sum types.

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

[julmb]

Since your State and Employee types are different types, why not just use a different Options value (with a different constructorTagModifier) for each of the types? Is it important to you that the same Options value can act differently on different types?

[wi11dey]

@julmb That's fair, though yes I would like to reuse the same Options value as I'm using DerivingVia to derive FromJSON for a bunch of different types. I think the same logic of just using a different Options value could also apply to the existing allNullaryToStringTag, where you could just set your sum encoding to UntaggedValue for data types you know have all nullary constructors. If you feel strongly about not adding this though, I can live without it and close

[julmb]

If you feel strongly about not adding this though, I can live without it and close

Oh, this might be a misunderstanding, I am not a maintainer of aeson and I have no say in whether this PR gets accepted. I was just asking out of my own curiosity.