Range API

[Brian Goetz]

Sorry for the not-good news, but I'm not too surprised.  Computational 
domains like "32 bit integers" seem like they should have a lot in 
common with algebraic structures like groups and rings, but when you 
start poking at them, the compromises we make to fit them into hardware 
registers start to bite.  (And let's not get started on floating 
point...) Lots of research into numeric towers in various languages, or 
capturing fundamental properties in type classes like Haskell's `Eq` and 
`Ord`, offers plenty of compromise to go with its promise.

I think a big part of what you are running into is that you've started 
with a _concept_ (a deceptively simple one, at that), rather than 
_requirements_.  And it is the open-endedness of this concept (discrete 
vs continuous, bounded vs half-open, including endpoints or not, etc) 
that resists abstraction.  Plus, without clear requirements, you will be 
subject to an endless barrage of "what about my pet use case" (e.g., 
"what about the numbers zero to ten, advancing by two").  Meanwhile, 
domain-specific libraries such as java.time will invent their own 
domain-specific answers, like Interval.

Rather than starting from the algebraic properties, perhaps start from 
the other end: what are the use cases where the lack of a range 
abstraction is problematic.  I get that

     for (int i=0; i<100; i++) { ... }

is uglier and less abstract than

     for (int i : Range.of(0, 100)) { ... }

but I also don't sense people beating down the doors for that (even if 
the language had range literals, like `0..<100`).

Where I do see people having trouble is that many range computations are 
error prone.  For example, `String::indexOf` returns the starting index 
of a match; if you want to actually iterate over the characters of such 
a match, you have to do something like

     for (int j=index; j<index+target.length(); j++)

and you are at risk for fencepost errors when recreating the range.  
Whereas an indexOf method (under a more suitable name) that returned a 
range, would be more amenable to downstream processing.  Similarly, I 
see errors in API usage because sometimes we specify range by (start, 
end) and sometimes by (start, length), and since both are ints, we get 
no type checking when you pass the wrong kinds of ints to such a method.

But, the mere existence of a Range type would do little to help String, 
Arrays, and other range-happy APIs, because we would have to update them 
to include new overloads that dispense and consume ranges.  So that's a 
big project.

Still, I think investigating use cases involving libraries that work 
intensively with ranges like this would likely yield useful information 
for what a Range type would want to provide.

HTH,
-Brian






On 9/26/2024 9:07 AM, Olexandr Rotan wrote:
> Researching the of deriving some iterable representations from ranges, 
> and I am not here with the good news.
>
> Unlike range algebra and boolean operations, which generalize 
> extremely well, iterability of ranges... Well, it's safe to say it 
> doesn't generalize at all. Analyzing key features people expect 
> iterable ranges to have, I ended up concluding there are basically two 
> groups / two use cases for them. First is plain and simple, arguably 
> the most popular one: iterating over a range of integer numbers, i.e. 
> `for (i : Range.of(1, 10))`. Another use case is for more complex 
> iterations over ranges of reference types, most commonly dates/time.
>
> There are two groups of values by their nature: discrete and 
> continuous. Most of the types belong to the second group, as there is 
> no direct increment AND decrement for them (we will omit hardware 
> limitations for simplicity), such as floating point values. What is 
> the increment of 1,3? 1.31 or 1.30000000001, or maybe something even 
> more unreadable? On the other hand, the increment of LocalDate in 
> context of range iteration that represents today is rather obvious - 
> it is tomorrow.
>
> There is a pretty limited number of discrete types in jdk. Dates, 
> whole numbers and basically that's it. The discrete types that are not 
> present in jdk can be really various. For example, users can define a 
> comparable type "F1Team" and compare them based on their position in 
> the last race. There, increment would most likely be the next team in 
> rating. There are many domain-specific cases like this.
>
> This is where the problem comes from. If the user would always have to 
> pass a comparator to create a range, it would be consistent to make 
> the user define increment/decrement as well. But we don't want users 
> to pass a comparator if the type is already comparable. Similarly, we 
> don't want users to define increment/decrement if there is already one 
> in the language! I think defining increments for dates (say 
> LocalDate.plusDays(1)) would be acceptable, even defining increments 
> for floats in context of ranges might be acceptable, but making people 
> define increments for integers is, in my opinion, completely not. 
> Besides performance impact, this is a terrible user experience.
>
> There are a few solutions to this:
> 1) Define ton of overrides for factory methods and specialized types 
> for this (uhh, sounds awful)
> 2) Introduce new interface, say Discrete<T>, that defines T 
> increment() (and possible T decrement()) methods. From now on, there 
> are 2 branches:
> 2.1) Leave things as is, allow users to define incrementation logic 
> for their types, but don't touch integers and other built-ins.I see 
> this option as extremely inconsistent and not solving the main issue, 
> which is iterability of integers.
> 2.2) Retrofit (scary) existing types to implement this interface. This 
> should not have any compatibility nor security implications, but still 
> sneaking into java.lang every time we need some new API to be more 
> user-friendly is obviously not a way to go. This basically comes down 
> to a question about how deep we want to integrate ranges into 
> language, and is range generalization even worth the invasion into the 
> core of language (imo yes).
> 3) Leave things as they are, just let users derive iterables using 
> something like range.asIterableWithStep(IncremetStartegy increment). I 
> think this would make an API too narrow as no one will use it for 
> routine tasks the same way people do in Rust, Kotlin and other languages.
>
> I would love to hear community opinion on this matter. Which option is 
> the most preferable, maybe some compromise between a few of them, or 
> maybe there is a better way to go that I didn't mention here?
>
> Best regards
>
> On Tue, Sep 24, 2024 at 5:11 PM Alan Snyder <javalists at cbfiddle.com> 
> wrote:
>
>     I have another example: I have a datatype that represents a region
>     of an audio track, for example, one tune in a medley of tunes. I
>     allow the region to
>     specify both a start and end time, but the end time is optional
>     (and mostly not used). When the end time is not specified, the
>     region ends at the start of the next region, or at
>     the end of the track if there is no next region. The latter case
>     is useful because the exact track length may not be known. The
>     optionality of the end time
>     is not represented in the type system.
>
>     Having said that, I’m not sure that a general abstract interface
>     would be useful for this example.
>
>>     On Sep 24, 2024, at 2:13 AM, Olexandr Rotan
>>     <rotanolexandr842 at gmail.com> wrote:
>>
>>     As part of the redesigning  process , I am researching whether or
>>     not there are use cases that require asserting that the range is
>>     exactly half-bounded. This is important because I plan to switch
>>     to BoundedAtEnd/BoundedAtStart sealed interfaces instead of flags
>>     and runtime checks: Here is what I gathered for now.
>>
>>       * *Date/Time Handling (Historical or Forecast Data)*: When
>>         dealing with events that started at a specific time but have
>>         no known end (e.g., open-ended employment contracts or
>>         ongoing subscriptions)
>>       * *Stream Processing (Real-time Event Streams)*: In real-time
>>         systems, you might process data that has a start time but no
>>         defined end, such as monitoring a live video feed or logging
>>         system. The range is bounded at the start and unbounded at
>>         the end as more data will continuously arrive.
>>       * *Data Pagination (Fetch Until Condition)*: When implementing
>>         pagination, sometimes you might want to fetch items starting
>>         from a specific index up to an unbounded limit (e.g.,
>>         fetching all items after a certain point until memory runs
>>         out or a condition is met).
>>       * *Auditing and Monitoring*: In systems where audit trails or
>>         logging data should capture all events after a certain point
>>         (bounded start) with no foreseeable end (unbounded end), such
>>         as monitoring changes to records in a database starting from
>>         a fixed timestamp.
>>       * *Scientific or Statistical Ranges*: When modeling physical
>>         systems or statistical ranges, you might want to capture
>>         measurements that begin at a known threshold but
>>         theoretically have no upper or lower bound. For example,
>>         recording temperature data starting at absolute zero and
>>         increasing without any known upper limit.
>>       * *Inventory or Resource Allocation*: Resource allocation
>>         policies, such as those for virtual machines, may be based on
>>         known minimum allocation thresholds but have flexible or
>>         unbounded resource caps, depending on availability.
>>
>>         I am writing to ask whether anyone who worked with such
>>         systems could confirm/deny that those are real use cases. If
>>         so, would it be satisfying enough to assert one-way
>>         unboundness with instanceof checks, i.e. range instanceof
>>         UnboundedEndRange && !(range instanceof UnboundedStartRange).
>>         Would appreciate any feedback.
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/core-libs-dev/attachments/20240926/a78070c3/attachment-0001.htm>