[JSON.jl] About empty structs

General Usage
1

Hello.

The package JSON.jl made the interesting design decision to make singleton structs unserializable.
This behavior is implemented in this function in the source code.

function lower(a)
    if nfields(a) > 0
        CompositeTypeWrapper(a)
    else
        error("Cannot serialize type $(typeof(a))")
    end
end

However, I find this behavior interesting and I was wondering why this was the case. Empty Dicts and named tuples,are handled and return a simple {} JSON object. Empty tuples and arrays return []. Shouldn’t singletons return {} by default?

Thank you!

2

If you want to serialize a struct, you usually have to preserve the type information since

struct A
     x
end

struct B
    x
end

are different types and must be treated differently even if they have the same structure. In that case you have to provide JSON.lower(s::A), JSON.lower(s::B) … methods to make them distinguishable, even more so for an empty struct C end where you have only type information.

I’m no JSON expert, but for me the cited design decision makes sense. It is described also in the README.

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