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kotlinlang

Hi, Kotliniers. I have struggle (phylosophical). What is correct more "functional"  way of doing stuff
var immutableList = ...
or
val muttableList = ...
Or I shouldn't do either, but always go from the top of structure and copy whole structure whenever even one element of list is added/removed? But what if the structure is nontrivial - list of maps of lists of... (usually wrapped into that pesky concept - class) But If I want to write multithreaded functional code in kotlin? Is it even possible?