Hello,

> The function (++) :: [x] -> [x] -> [x] has O(n) complexity.
>
> If somebody were to invent some type that looks like [x] but actually
> uses some sort of tree rather than a linked list, you should be able to
> get O(1) concatenation. Has anybody ever implemented such a thing?
>
You can also do this with a functional representation of lists (i.e. lists are functions); this idea is usually called difference lists. There is a nice dlist library on hackage.

-Jeff

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