I only have a small bit of experience with functional languages, so I don't get to have much of an opinion. I can say, however, that immutability and no side-effects are great ideas, and that I've borrowed them for my OO.
My initial goal for every new object I write is that it not change, and that it not have side effects. This obviously can't suit every object, but I've been pleasantly surprised about how much can be done under these constraints, and how much the constraints simplify code.
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