ANN: intervalset Was: Set type for datetime intervals

[Michael Selik]

It seems coding a generic interval and intervalset will bring a variety of difficult design choices. If I were you, I'd forget making it generic and build one specifically for the application you have in mind. That way you can ignore most of these feature discussions.

> On Apr 5, 2016, at 2:59 PM, Random832 <random832 at fastmail.com> wrote:
> 
>> On Tue, Apr 5, 2016, at 04:11, Nagy László Zsolt wrote:
>> But there is still need to check begin <= end. But maybe
>> you are right: if we suppose that nobody will try to use an interval
>> where end < begin then we can use plain tuples. But then the user *must*
>> make sure not to use invalid intervals.
> 
> The IntervalSet constructor and add method could switch them or throw an
> exception. But I'm not saying to use plain tuples. I'm saying to make
> Interval a "dumb" object, which *only* supports being constructed and
> added to a set, rather than putting a bunch of set-like operations on
> it.
> 
>> It is blurred by design. There is an interpretation where an interval
>> between [0..4] equals to a set of intervals ([0..2],[2..4]).
> 
> No, because 2.5 is in one and not the other. Floats are orderable with
> ints, so it's not reasonable to say "this is an int intervalset so no
> floats are contained in it". But that's another discussion.
> 
> But why would you want to use [0..4] directly instead of ([0..4])?
> 
>> Actually,
>> you can ask: "is an element within the interval?" The same question can
>> be asked for an Interval set. So the same type of value can be
>> "contained" within an interval and also witin an interval set. In the
>> current implementation, if you try to create a set with these two
>> intervals [0..2] and [2..4], then they are unified into a single element
>> on purpose: to make sure that there is only a single official
>> representation of it. E.g. ([0..2],[2..4]) is not a valid set, only
>> ([0..4]) is.
> 
> Uh, exactly. I don't understand why any of this makes it useful to have
> these operations on interval objects.
> 
>> By definition, a set is given with its elements, e.g. for any possible
>> item, you can tell if it is part of the set or not. So if
>> ([0..2],[2..4]) and ([0..4]) have exactly the same elements (by the
>> given definition), then they are not just equal: they are the same set.
>> The same reasoning is used in math when they say: there cannot be
>> multiple empty sets. There exists a single empty set. It happens that we
>> can only represent a finite number of elements in any set on a computer.
>> I wanted to have a well defined single way to represent any given set.
> 
> What? *I* am the one who wants a well defined single way to represent a
> given set. *You* are the one who wants to make [0..4] a set-like object
> that is different from ([0..4]) and doesn't quite support the same
> operations.
> 
>> I can also see your point. From another point a view, a set and a set of
>> sets is not the same type.
> 
> I'm saying an interval is _not_ a set. It is merely part of the way of
> describing the set. Just like the word "of" isn't a set just because
> "the set of all even numbers" is a set.
> 
> You could consider "the set of [dates|numbers|etc] in a single interval"
> to be a type of set, certainly. But there's no reason to have such a set
> _not_ fully support set operations even when those operations will not
> return a set of a single interval.
> 
>> However, I do not share this view, because if
>> we start thinking this way, then operations like the one below does not
>> make sense:
>> 
>> [0..4]) - (1..2] == [0..1] | [2..4]
>> 
>> If you make a strinct distinction between intervals and interval sets,
>> then the above equation does not make sense, because you cannot remove
>> an element from a set that is not an "element" of it. The above equation
>> makes sense only if the "intervalset" is a general representation of
>> possible values, and the "interval" type is a special representation of
>> (the same) possible values.
> 
> I don't understand why you want to let the user use the "interval" type
> for purposes other than adding to an intervalset in the first place
> Nothing you've posted has explained this.
> 
>> It is clear when you ask the question: "is 3
>> in the interval?" and you can also ask "is 3 in the interval set?"
>> - so
>> the value of the same type can be contained in the interval and also in
>> the interval set. Maybe the naming is bad, and it should be named
>> "Intervals" instead of IntervalSet?
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