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Comment by joshuamorton | Hacker News Reader

Actually my response would be that you just have a plain-ish vanilla list of `Component`, and a function `TotalCost(List[Component]) -> float`, which is implemented as, approximately, lambda arr: sum(x.cost for x in arr). This maintains the invariant without state.

There's a potential performance cost here, but that usually doesn't matter a whole lot.

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That's true - but where do you put the function? In a complex system I can end up with loads of these totalling functions scattered about. I might have gross and net versions of the total and, given human nature, they get created near the use of them. The variants then get duplicated. The objects give you a modular structure which means you can see the interface.There is a disconnect when you have to shift to the object - but you can also see when to refactor.

There are a few answers for this: you might include common ones in the same file as the struct (see rust for how this looks, usually).

> This maintains the invariant without state.

Yes, but this is not possible in general. For instance, what if you want to endow your List with a "MaxTotalCost", set at creation. You can't ensure that your list respects that invariant, other than by encapsulating it behind some custom interface.

That's true, although it raises a question: is the maximum you're willing to spend a trait of your bill of materials (I'd argue no), or of some other thing, for example your overall project. In that case you could have a method that validates your BOM against your project requirements.

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