Immutability can be nice when writing server logic. Suppose all you data structures are immutable and the only mutable state you have is a pointer to the root

Immutability can be nice when writing server logic. Suppose all you data structures are immutable and the only mutable state you have is a pointer to the root of your immutable data structures. To create a new state, you build a new state by allocating what you have to and using pointers into the old state for those things that haven't changed, and then you update the pointer to point at the new state. Depending on the data structures used, updating state could require a lot of allocations. However, there are a couple really compelling benefits. Error handling is very easy because you can bail out at any point before swapping pointers and the server state remains unaffected. Also, readers can continue to read the old state without being blocked by writers -- basically, all reader threads can just go right in and read the state with no locking at all. I've used this approach before in Haskell and it seemed to work pretty well. I used the lens library to make data structure updates easier (syntactically, they look a lot like the sort of data structure updates you'd make in an imperative language, except that they do a copy-on-write behind the scenes), and also paired it with the acid-state library to make the Haskell data structures persistent across application restarts.