1) change in cardinality is such a change in domain that lenses won’t solve it. There will be much bigger changes in business logic probably making original code obsolete regardless of used pattern.
2) deep tree modifications of the kind that you mentioned indicate problems with architecture. Why would you need the whole typed tree (not DOM or something, but object tree) to modify a tiny leaf of it? If you touch multiple leafs with root as closest integration point, why your data model is designed like that, pointing to strong coupling of different contexts?
3) most importantly the code that can be reused, can be extracted to a private method of an entity where the change occurs. Calling nested constructors on the root object breaks encapsulation — it should pass the message to nested objects instead.
If you have an immutable structure, you can't just modify the leaf of it. Unless you're arguing that deep, immutable structures are an issue in themselves?
Oh, we cannot modify it, of course, but we are talking about the ways of producing a modified copy.
The part of the argument about deep immutable structures is just a side remark: building them for modification is likely pointing to a problem with design, but that’s not the most important part.
The important part is that it is better to call business methods than externalizing the whole thing with deconstruction-modification-construction to a lens or some other code, when you have the full power of OOP to do it right.
1) change in cardinality is such a change in domain that lenses won’t solve it. There will be much bigger changes in business logic probably making original code obsolete regardless of used pattern.
2) deep tree modifications of the kind that you mentioned indicate problems with architecture. Why would you need the whole typed tree (not DOM or something, but object tree) to modify a tiny leaf of it? If you touch multiple leafs with root as closest integration point, why your data model is designed like that, pointing to strong coupling of different contexts?
3) most importantly the code that can be reused, can be extracted to a private method of an entity where the change occurs. Calling nested constructors on the root object breaks encapsulation — it should pass the message to nested objects instead.