[skybrian]

This article explains the problems pretty well, but they are old problems and functional programmers have various solutions that the author seems unaware of.

I'm not a Haskell programmer but I do know about monads and have heard of lenses.

[charcircuit]

Lenses don't help. They just make copying and updating large data structures cleaner to write. They don't solve the performance problem of copying everything.

[WkndTriathlete]

Incorrect. Optics implementations can ensure updating a node in a large balanced tree is O(log n) instead of O(n), trivially, and I'm sure there are sufficiently clever collections with optics that would enable much better performance than the trivial optimizations I can think of.

The same holds true for large structure updates.

[charcircuit]

Not all data structures are a large balanced tree and a O(1) update is going to be faster than O(log n) updates. There is still going to be a significant performance hit from having to recreate a whole node instead of just updating a single value. While yes you can say large data structures are like a big node of a tree, having to copy all of the different records from the old one to a new one is going to be considerably slower than just modifying a single record.

It does help, but it's still going to be significantly slower than just modifying a single record in a data structure. It is a still a problem that will be slowing down your program.