Sharing mutable data without a mutex (which suffers from unbounded contention) is hard. Approaches that work include updating persistent data structures and sharing the new copy, or sharing diff objects over a lock-free queue.
Sharing the copy is not what's solved by lenses. In a certain sense lenses are just a way to try to do away with the boilerplate of updating nested immutable data structures. It is very easy to write this boilerplate (in the sense that it is straightforward and hard to mess up). It's just mind-numbingly tedious.
Although I don't know what you mean by growing lenses.
Mutex + inner mutation is no easier than CAS (which is the usual solution with concurrent writing of immutable data structures) a la Java AtomicReference or STM (another popular one) and in my opinion significantly harder as soon as you have multiple mutexes.
CAS is a massive pain in the ass for complex changes. Lenses are a way to point to part of the object which can in theory make it less painful as you can redo a change without redoing the work to setup the change necessarily. Growing just refers to growing a codebase using them by adding them is all.
I am not against FP I think the point of "it is good if you can fix performance" is very accurate. I just think it is also important to acknowledge that the paradigm can be more complex in situations where it is supposed to help leading to a mixed bag.
Similar to distributed databases. Your database can now be phenomenally powerful but you can't do a stored proc anymore without losing that power.
It can be a very effective and totally worth it but it isn't a pure win necessarily.
What are you CASing? If the object is too large you will have contention to the point that you might as well be single threaded. If the object is too small you now have to CAS multiple things which is far from trivial.
CAS is a primitive, it isn't complex itself but it can be complex to work with once you are talking non trivial work. Just like locks. A global lock is dumb simple but a real locking system can be as complex as you will let it.
> If the object is too large you will have contention to the point that you might as well be single threaded. If the object is too small you now have to CAS multiple things which is far from trivial.
Right but both of those things are true for locks too right? CAS seems no harder than locks.
Retry logic is hidden for locks while in your face for CAS.
Additionally while multiple locks is annoying it is way easier than multiple CAS. "Have a global order for locks" is the hard but solvable problem for multiple locks. For CAS if you need to CAS two dependent things you... I don't know it depends.
> Retry logic is hidden for locks while in your face for CAS.
It's hidden in both cases (usually CAS instructions are hidden behind an `update` interface as in Java's Atomic* family rather than directly used in the same way that generally a lock is an interface for a TAS instruction/spin lock + upgrading to wait queue).
> For CAS if you need to CAS two dependent things you... I don't know it depends.
You use nested atomic references. Just like locks it's not a great way of doing things, and an analogous problem to ordering locks rears its head (by virtue of nesting you cannot mess up ordering in the strict sense, but you can accidentally "cross the boundaries" of two atomics in an update that goes against the nesting order), but it's doable in the same way as locks.
The usual CAS-like but better approach is STM (which is where immutability really shines).
I'm still not seeing how CAS is any harder than locks.
IMO the hard part of immutable structures is mutation (solved by https://lib.rs/crates/im), the hard part of diffing is writing one "command" subclass per type of operation (which can be more or less manageable), and the hard part of mutexes is remembering to lock the mutex or rwlock in every single access (Rust fixes this), avoiding priority inversions (Rust doesn't help), and avoiding deadlocks (Rust doesn't help).
