"append","delete","copy" in Go is parametric polymorphism hardcoded in the compiler. append can be called with many different array types. There is nothing complicated with that.

go can iterate over arrays irregardless of the element type the same way with the same construct using "range", therefore "range" is polymorphic.

The real debate here is should the compiler be "privileged" or should there be API in userland to implement these behavior.