Calling [1,2,3].inject(0) {|sum, ii| sum + ii } has a cleaner smell, but it's a lot more typing.

I wonder if the solution might be to have all arrays support sum (in the responds_to() sense) only if they can can actually perform the operation.

Thus

[1, 2, 3].respond_to?(:sum) => true

[1, [2, 3]].respond_to?(:sum) => false

I can come up with an O(1) way to do this where we assume that an array does respond to sum, and degrades to not supporting sum when a non numeric is inserted ... but getting the array to support sum again if/when the non-numeric is removed... I'd have to think for a bit to come up with something better than O(n)...