ffracture@c25l I didn't know there were people still using it. nice. I don't see a direct reason why a collection couldn't be used as a hash key. I have worked with libraries that have allowed this. It isn't really anything special. A collection or any other composite type (class/record/etc) aren't really that different. The one thing you may consider is that hashmaps are usually mutable. If you mutate a hashmap after using it as a key, you can run into issues.
@c25l But this is true of any object you might use as a key to map.
@c25l I remembered where I saw use of hash maps as keys. It was the scala collections library.
Here is a link to where hashCode is defined for most maps:
Which leads to:
https://github.com/scala/scala/blob/v2.12.1/src/library/scala/util/MurmurHash.scala#L1
Which leads to a git hup repo: