Why is [1,"two"].sum any different from 1 + "two"? That is, 1.respond_to?(:+) is always going to be true, and yet sometimes sending the + message to a number w

Why is [1,"two"].sum any different from 1 + "two"? That is, 1.respond_to?(:+) is always going to be true, and yet sometimes sending the + message to a number will give a type error. I don't buy into this idea of interface as binary - that if you respond_to? the message, it's always appropriate, and if you don't, it's not. Interfaces are something you look at when you're writing the program, and so they are interpreted by humans and can be necessarily fuzzy: "#sum will give you the total of the elements in the array, unless they aren't homogenous, in which case you'll likely get an error". Fine. If you don't know enough about the array in question, don't send #sum. Similarly, if you don't know that this is a chequing account, it would be odd to ask it to write a cheque. But at runtime, you send the message, and you maybe get an error. Whether this is TypeError or NoMethodError seems entirely irrelevant. Incidentally, my objection to Array#sum is that you don't know what to use as the "0" element (what if the elements implement +, but aren't numbers), though I'd be fine with it being called #numeric_sum or the like.