Objective-C data structures are already immutable by default. What do these persistent data structures bring to the table that the default immutable NSDictionary, NSArray, NSSet, etc., don't?
One thing to remember is that getting a new version of the data structure does not change old ones at all. In order to change state, in Clojure for example, you use constructs such as atoms to "advance" a persistent data structure forward. Think about it like advancing through your git commit history - efficiently and by only applying the delta for the next "change".
Your classes are 10-20x slower in every other way, which definitely would offset any imaginary expense you think calling `mutableCopy` costs.
I have to tell you in 10+ years of writing Objective-C where I was like [myArray mutableCopy] is super slow, I need a replacement. I've wished for an NSOrderedDictionary though.
Then,
// yourDictionary is NSMutableDictionary, [yourDictionary count] > 20
NSMutableDictionary *mutable = [yourDictionary mutableCopy];
mutable[@"old_key"] = @"new_value";
NSDictionary *changedDict = [NSDictionary dictionaryWithDictionary:mutable];
// yourDictionary is AAPersistentHashMap, [yourDictionary count] > 20
AAPersistentHashMap *changedDict = [yourDictionary setObject:@"new_value" forKey:@"old_key"];