Doesn't it depend on the data structure? Eg prepending to a list is actually cheaper with immutable data structures: you keep the original list and add a new he

Doesn't it depend on the data structure? Eg prepending to a list is actually cheaper with immutable data structures: you keep the original list and add a new head pointing to its head. Now you have two lists available in your program, but only one stored in memory. Yay!