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It makes a List of Lists, then combines those into a List.

[5,3,4] becomes [f(5), f(3), f(4)], but f(x) must return a List. Those inner Lists are then concatenated.

When I say "original data structure", I mean the type of data structure is the same (in this case, List).

What is the data structures don't support a join operation, like a tree?

We would need a way to transform a Tree of Trees into a single Tree. The inner Trees would be at leaf nodes of the outer Tree, so they could be attached in place.

Although I recall monads need to satisfy certain laws, and I don't recall if Trees satisfy them or not. Just because something is a data structure does not necessarily imply it is a monad.

A rose tree is a monad.

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