First, openArray stands for array or seq, not for string. Well I hope that is still the case, it was when I read the tutorial five years ago. I guess one can cast a seq[char] to string.
Note that Nim is statically typed, so the return type is well defined at compile time. But array type and seq type is very different, you may read that section already in my kids book, but it may contain some errors still. So you can not have a single proc that can return array or seq. Basically you need two different procs. We have "when" keyword in Nim as a compile time "if", and "auto" keyword as a possible return type. Both may help that we have to write only one proc, but the compiler will instantiate two for us. We have also or-types in Nim, we can write "int or float" in the parameter list and the compiler will instantiate one proc with int and one with float for us, and in the proc we can test with something like "when typeof(x) is float:" But arrays are not a type but a typeclass, as arrays with same base type but different length are incompatible. So I think there is no solution for you. Can you tell us what you want to archive?
