New object versus mutating the current object depends on convention - in Python, there is a "replace" function on the native string type that returns a copy (http://docs.python.org/library/stdtypes.html#string-methods); in C++, std::string::replace mutates the string in place.

I'm not sure what you mean by not mentioning what is passed in. Keep in mind: I have never programmed in Objective-C. I am going on intuition alone.

No namespace support is a bummer - it means you're going to end up with long identifiers all over the place.