One thing about functional programming languages that is source of frustration is immutable data structures. In Clojure there are a host of data structures that allow you change the data in place. This is possible because the operation is wrapped in a transaction of sorts that will guarantee it will work or everything will be reverted.

One description that might be helpful is that Clojure uses locks by default. Any data you store is immutable and therefore locked. You can always make a copy efficiently and you are provided some tools to unlock the data when necessary.

I’m definitely not used this model by any stretch, but it seems the transactional paradigm along with efficient copies makes for a good balance of functional values along side practical requirements.