I'm a minimalist too, but this time I don't agree. You can't have this feature at all if it is not implemented in the core, not in O(1), and is general enough,

I'm a minimalist too, but this time I don't agree. You can't have this feature at all if it is not implemented in the core, not in O(1), and is general enough, hash.random_key() will not really bloat the language and it is very clear what it does. If there is 'partition' in Ruby's array I think random_key is not so strange to see inside, especially given that you can't implement it in Ruby itself.