Lovely little discussion which would be great for beginning OO programmers to see. The question of whether to return 0 for an empty collection is interesting. While other things can be summed, we tend to think of sums returning numbers. In that sense, 0 is a great default return. On the other hand, having a specific object for a generic sum function is problematic (+1). There is a real tension and that would be a great discussion for people to have. Either way, the need for sumIfEmpty: is obvious.

Cheers,

Jeff

On Tue, Dec 1, 2015 at 11:19 AM Ben Coman <btc@openinworld.com> wrote:

On Tue, Dec 1, 2015 at 10:24 PM, Sven Van Caekenberghe <sven@stfx.eu> wrote:
> Doru,
>
>> On 01 Dec 2015, at 15:11, Tudor Girba <tudor@tudorgirba.com> wrote:
>>
>> Hi,
>>
>>> On Dec 1, 2015, at 12:52 PM, Sven Van Caekenberghe <sven@stfx.eu> wrote:
>>>
>>>
>>>> On 01 Dec 2015, at 12:45, Stephan Eggermont <stephan@stack.nl> wrote:
>>>>
>>>> On 01-12-15 11:46, Sven Van Caekenberghe wrote:
>>>>> The basic question for me is, what should
>>>>>
>>>>> #() sum
>>>>>
>>>>> return. Right now, it is an error, I would very much like that for this common case the result would be 0. There is a lot of power (easy of use) in a unary selector, we should not destroy that with semantics that force a test before using it.
>>>>
>>>> I like the error, it aligns with most of our collection protocol.
>>>
>>> I hate the error, a lot ;-)
>>>
>>>> It shows the need for #sum:ifEmpty: though
>>>
>>> Yes, as long as #() sum == 0
>>
>> That won��t work :).
>
> Why ? Please explain.
>
>>> I want the simplest case to be simple, having a non-0 default is a special case IMHO
>>
>> That is why you have sumNumbers:. We could also add Collection>>sumNumbers.
>>
>> We had this discussion at length before Pharo 4, and this is when we agreed to add sumNumbers: and let sum: be generic (like the name says it should be) and not assume that it should work with Numbers.
>
> It is not about numbers or not, you are still using #+ in your generic case, that is numeric in my book. It is about the zero element and how to deal with an empty collection.
>
> The current solution, which was indeed recently added, leaves the problem of what to do with an empty collection. There is no general solution, you have to specify a zero element.
>
> But for most people in most cases that will be effectively 0, so lets make the unary #sum respect that. (And unary #sum will also work for non-empty non-zero based objects). The less common case can then use the longer message.

https://en.wikipedia.org/wiki/Empty_sum