Am 02.08.15 um 15:23 schrieb webwarrior:
On 02.08.2015 15:44, jtuchel [via Smalltalk] wrote:
> I'd say it is questonable if SequenceableCollections should be
> comparable by default.
>
> is (a b c) equal or lower than (b a c) ?
> Doesn't this depend heavily on what is in the Collection and what the
> meaning of the Sequence of two Collections is?
(a b c) = (b a c) if a = b
(a b c) < (b a c) if a < b
The semantics are well defined.
Are they? Why didn't the programmer use a Set then? She wanted to
preserve the order of objects, right? So the order if objects matters to
her, how does this weigh into your suggestion of comaprison? Don't get
me wrong: your suggestion sounds logical and is correct. But so are
others...
Of course, it depends on what's in collection! That's the point - we
compare collections by their content.
If some elements are not comparable, we'll get DoesNotUnderstand -
expected behavior when comparing non-comparable objects.
I am only talking about the sequence of objects. Ceteris paribus, what
does the fact that one of the objects has a higher index in one
collections mean in terms of being greater/equal/lower?
It's the same story as with equality.
so at first sight, you are correct: (a b c) = (b a c) if a = b
But you aren't, because we are comparing Collections that only exist
because the order of objects matter. For whatever reason that we can
guess or not.
> So I'd say there is not much use in putting comparisons into the base
> class library, because there is a ton of assumptions about the
> Collections in whatever implementation one can imagine. I would assume
> that this is the job of some business object that holds objects in a
> SequenceableCollection. Don't you think?
There is only one assumption - that elements of the first collection are
comparable to the elements of the second collection.
Only technically speaking.
But if you wanted to only make sure you have the same objects in two
collections, you'd probably use a Set or a Bag, no?
And then, looking deeper:
* What does it mean if element at position 3 in A is greater than B(3)
and A(4) is lower than B(4) and all others are equal?
* If collection A has 4 elements and B has 5. The first 4 are equal. Is
B greater or lower than A?
And I think that we shouldn't multiply entities beyond necessity by
creating a class when using simple datastructure would be sufficient.
I am trying to make a point about exactly this not being true. The
semantics of the sequence in a SequenceableCollection is completely out
of scope of the Collection class. SequenceableCollections make sure your
given ordering is preserved. But it has absolutely no idea why you chose
that order and what it is good for.
So while you are correct that we shouldn't build classes for no special
reason other than that we can, the example at hand is not an argument to
support it. It the sequence matters, then the semantics for the sequence
needs to be modelled. There needs to be a class that models the
knowledge about what the sequence is good for and - if necessary - can
decide whether it is greater or lower than another instance of that very
class. The SequenceableCollection is a data structure, not a business
object. If there is semantics to a sequence, it is business knowledge.
> The fact that other languages provide such an implementation doesn't
> mean much to me, to be honest. I choose a SequencableCollection in
order
> to keep objects in a certain order, nothing more, nothing less.
That's why we have methods such as polynomialEval: and
asDigitsToPower:do: (and I'm not talking about extension methods) ?
Maybe these methods have gotten where they are for the same reasons
people use Associations and Dictionary all over the place. Laziness.
Most of the times, this smells like technical debt.
Joachim
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