2015-12-22 10:37 GMT+01:00 Sven Van Caekenberghe <[email protected]>:
>
> > On 22 Dec 2015, at 10:22, Nicolas Cellier <
> [email protected]> wrote:
> >
> >
> >
> > 2015-12-22 3:46 GMT+01:00 Ben Coman <[email protected]>:
> > On Wed, Dec 2, 2015 at 3:38 AM, Tudor Girba <[email protected]>
> wrote:
> > > I am saying that if you want sum: to be generic, it cannot assume a
> *specific* Zero object.
> > > And sum: should be generic because of its name.
> >
> > This seems the crux of the disparate viewpoints, which is why I
> > suggested return a *generic* Zero object as follows...
> >
> > > On 04 Dec 2015, at 01:49, Ben Coman <[email protected]> wrote:
> > > do something like...
> > >
> > > Collection>>sum
> > > | sum sample |
> > > self isEmpty ifTrue: [ ^ ArithmeticZero ].
> > > sample := self anyOne.
> > > sum := self inject: sample into: [ :accum :each | accum + each ].
> > > ^ sum - sample
> >
> >
> > Nice, but generic zero can't work: you sometimes need to decide if it is
> the zero of vector space, or of associated field.
> > The generic zero can't behave as both say a matrix and a scalar...
>
> Indeed, that is what I said the first time Ben proposed this.
>
> But I am curious, Nicolas, what you think of this, with your math
> background/interest ?
>
>
This is what I had in old app (in the 90s):
SequenceableCollection>>sum: aBlock
"answer the sum of all elements after applying a Block"
| sum |
self isEmpty ifTrue: [^0].
sum := aBlock value: (self at: 1).
2 to: self size do: [:i | sum := sum + (aBlock value: (self at: i))].
^sum
Collection>>sum: aBlock
"answer the sum of all elements after applying a Block"
| sum |
sum := 0.
self do: [:e | sum := sum + (aBlock value: e)].
^sum
I find sum: much more expressive than inject:into:
If we really find that it adds semantic value to inject:into: we may also
implement optionnal:
Collection>>sum: aBlock to: initialValue
"answer the sum of all elements after applying a Block"
| sum |
sum := initialValue.
self do: [:e | sum := sum + (aBlock value: e)].
^sum
Collection>>sum: aBlock
^self sum: aBlock to: 0
This way, no (#anyOne,#-) trick required, that's why I prefer sum:to: to
sum:ifEmpty:...
For example, I had symbolic algebra, and the #- trick would have required
further simplification of the sum.
So I don't have any mathematical solution, just pragmatic ones:
- I agree on this: no need for many selectors, one or maximum two should be
enough
- I wouldn't care too much and would just answer 0 for empty collection
because we have inject:into: or optionnally sum:to: for exotic cases
