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 <random...@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? > -- > https://mail.python.org/mailman/listinfo/python-list -- https://mail.python.org/mailman/listinfo/python-list
