Relax requirements for `vibe.data.json` using `toJson`?

[vpanteleev-sym]

Currently, vibe.data.json will only call toJson if both toJson and fromJson is defined.

However, not all applications require both serialization and deserialization.

I'm also wondering if it would make sense to simply check if something called toJson exists on the object. The reason is that the current method, which performs speculative compilation, hides any errors in the implementation of toJson and fromJson - if the method exists, but the speculative call fails for any reason, then it is treated as if it's absent, which usually leads to incorrect serialization instead of an explicit error. This is especially relevant when these methods are templated, which can be useful when we have many types, out of which only some will actually need to be serialized, to reduce the total amount of compiled code.

[Geod24]

I've been hit by this also. I usually just add a assert(0) fromJson method. But relaxing the requirement would make sense, sometimes you just want to serialize (or deserialize).

[s-ludwig]

One approach that would work like this without resulting in mismatching serialize/deserialize procedures would be to only check for a toJson member in both cases and simply assume that fromJson exists for the deserializeJson side. That way serialization would work as expected, but deserialization would fail with an error instead of falling back to per-field serialization.

However, while this would probably work for toJson/fromJson and toRepresentation/fromRepresentation without breaking valid code, it definitely wouldn't for toString. So implementing it like this would introduce an inconsistency.