yeah, actually this can happen in certain use case and we will definitely look into that. But the idea of custom directive is to let app developers choose their query/mutations and modify/add whatever they want. May be in some use case developers wants to use only certain functions and not the inbuilt ones.
So to cover both cases we support inbuilt and custom (you can say user defined) functions or structures and it adds lots of flexibility into the design.
And the developers only uses those custom functions then i guess it’s not big problem for them to know what function they defined and to use it.
But thanks for suggestion and we will look for use case where this can create problem and work on it.