ˋArraysˋ is not the protocol though – ˋArrays.Protocolˋ is. Your public API is not your protocol, and given the constraint you mentioned it cannot be. Same for e.g. ˋEnumˋ btw. The protocol is ˋEnumerableˋ while ˋEnumˋ uses it to provide a nice API. But on the other hand take a look at ˋAccessˋ, which is actually a behaviour (even when often mentioned to be a protocol), which you need to implement to have datatypes be accessible as data containers.
You‘re obviously in a situation where you need both the behaviour version of datatypes providing certain functions as well as the protocol side. What I suggested would at least remove the need of explicitly defining the protocol implementation unless something truly custom needs to happen. The downside to behaviours however is that they cannot be implemented in userland like protocols can.
In the end I question if you actually need ˋemptyˋ on the protocol, because how useful is ˋArray.Protocol.empty(…)ˋ if there‘s nothing to dispatch on. For ˋEnumˋ one would also do ˋMapSet.new(list) |> Enum.map(…)ˋ as well.
