Several years down the line and finally here is the Godbolt explaining it all. Hopefully.
[Note: metacall base article]
I am currently in the stage of searching for simple but valid examples of metacall validity (aka: usefulness) as interfacing concept and programming idiom.
And, I think I have found one. metacall + project Euler. The mathematical nature of some computational problems make them a great fit for the functional paradigm. For those of you who don’t know, “Project Euler is a series of challenging mathematical/computer programming problems…” (ProjectEuler.net). I have concluded I could use the simple and interesting solution for a problem from “Project Euler” to show metacall in action.