Re: [Pharo-dev] #sum:, #detectSum:, #sumNumbers:

Dec. 22, 2015

      2015-12-22 10:37 GMT+01:00 Sven Van Caekenberghe <sven@stfx.eu>:
...
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On 22 Dec 2015, at 10:22, Nicolas Cellier <
nicolas.cellier.aka.nice@gmail.com> wrote:
2015-12-22 3:46 GMT+01:00 Ben Coman <btc@openinworld.com>:
On Wed, Dec 2, 2015 at 3:38 AM, Tudor Girba <tudor@tudorgirba.com>
wrote:
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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...
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On 04 Dec 2015, at 01:49, Ben Coman <btc@openInWorld.com> 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