[delayed]

> NULL is a valid instance of T

This by the way is Tony Hoare's famous "Billion Dollar Mistake"

A NULL pointer for error would be a safe interface where you get a trap when there is no value.

Minor nit, strings are never nil in Go, so that would be "" instead. The point stands though, a common pattern is "defer foo.Close()", the Close() signature almost always returns an error and almost no one checks it...which is probably fine, but not automatically fine.