I agree with you if you mean that an array should be able to abstractly reduce its elements, but I'd argue that the specifics of how the array gets reduced are

I agree with you if you mean that an array should be able to abstractly reduce its elements, but I'd argue that the specifics of how the array gets reduced are generally outside of the responsibility of the array class. In Ruby, this is accomplished through Array#inject, which allows you to specify the procedure that gets applied as you're joining. This makes it easy to do summing, string concatenation, etc, etc.