You're probably thinking of storing it as some linked list but that doesn't work with mathematical abstractions (which is my point).

For example in mathematics if you write "C = A + B" and then you ask what A is, it's still always going to be the same.

To make this work in programming you basically have to make all your data structures persistent[1]. AFAIK you would need a store the list as a finger tree or something if you need fast concatenation and that will still only be O(log(n)) [2].

[1] https://en.wikipedia.org/wiki/Persistent_data_structure

[2] http://hackage.haskell.org/package/fingertree-0.1.4.2/docs/D...

[3] https://stackoverflow.com/questions/28406512/why-does-concat...