I stumbled across the desire to want to use get_in while accessing deeply nested parts of a custom struct while needing to provide a default value. I could make a simple defp function to achieve this, but I wanted to see how to handle it using the Access protocol, so I tried something like this:

get_in(my_struct, [:x, Access.key(:y, "my_default)])

Where my struct was shaped something like

%MyStruct{x: %{y: "My Value"}}
# or maybe 
%MyStruct{x: %{}}

This results in an error:

** (UndefinedFunctionError) function MyStruct.fetch/2 is undefined (MyStruct does not implement the Access behaviour)

I have 2 followup questions to this:

1. Is there an example somewhere in some library that demonstrates how to implement the Access behaviour?
2. I’ve read somewhere that structs don’t implement this behaviour for performance reasons – what considerations should be made before implementing the behavior in a struct?

Thanks as always!