Why is there no mapKeys in the Data.Hashmap?

Question

I want to use a function like mapKeys to map for keys only. There is a mapKeys function in mapKeys , but there are no analogues in Data.Hashmap . Moreover, in Data.Hashmap I did not find anything about key manipulation. Is there only an incompleteness of the Data.Hashmap module or are there any ideological reasons for not doing this?

Answer 1

A hash map provides good performance by locating a value by applying a hash function to the key. Key mapping will pull each item out of the map and reinstall it using a hash of the new key. Of course, all this is doable, but it is quite a lot of work, and therefore it is best avoided.