Generics Dismantled

While I was bouncing back and forth between Go, JavaScript, and some Elixir, some patterns and thoughts emerged; and please bear with me even if there are better solutions to these samples. The solutions provided are intended merely to get across the concepts under discussion – so they are demonstrative pseudocode.
Puzzle Pieces
The problem: we need to apply some functionality/computation to sets of data in a uniform way. So, to take an example, let’s write a map utility for slices, which will map/apply a function to all the items in a slice of any kind, and gives us a slice of the same length with items of another kind. How about: