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> Create a subclass called NumberArray that can only hold numbers and has specific methods for operating on them.

I can't see anything wrong with this for some cases. Another way to get there would be to define a NumericCollection module that can be used to extend any Array.

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I would tend toward that idea: a module that can be included. Injecting a sum method into the object's ghost class also maintains scope of responsibility, but feels a bit more magic. I'd opt for less magic. At first, anyway. :)

How about create a method called detectSum that accepts a block as an argument. The block will return the specific field to use when summing.

This lets you skip subclassing.

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