Feature #20027: Add Range Deconstruction

• Status: Rejected
• Assignee: -
• Target version: -

[stuyam (Stuart Yamartino)]

Ranges are a powerful tool in ruby. A common range I use is a date range such as (Date.yesterday..Date.tomorrow). A range will often be passed around to methods because the dates hold meaning together such as a timeframe for a table filter.
Often I want to grab the original values out of a range like:

timeframe = (Date.yesterday..Date.tomorrow)
start_date = timeframe.begin
end_date = timeframe.end

#=> start_date = yesterday
#=> end_date = today

Similar to array and hash deconstruction I thought it would be useful to support range deconstruction like this:

start_date, end_date = (Date.yesterday..Date.tomorrow)

#=> start_date = yesterday
#=> end_date = today

This would also work for endless or beginless ranges since the beginning and end are just nil in those cases:

start_date, end_day = ..Date.tomorrow

#=> start_date = nil
#=> end_date = tomorrow

You could do this now using to_a like:

start_date, *middle_dates, end_date = (Date.new(2000,1,1)..Date.new(2023,1,1).to_a

However this has unnecessary performance issues by converting the range to an array especially if the range spans a large period, middle_dates would hold a very large array. Also if the range resulted in an array with 2 values, end_date would be nil and this wouldn't actually work to get the begin and end values.

I think this provides a simple interface for a common pattern of deconstructing ranges into their beginning and end values. It would be useful for ranges regardless of date ranges or other types of ranges since they are essentially tuples. Would love to know what others think about this <3

[stuyam (Stuart Yamartino)]

• Subject changed from Range Deconstruction to Add Range Deconstruction

Update title

[shan (Shannon Skipper)]

Pattern matching would be another option though Range doesn't implement #deconstruct or #deconstruct_keys by default.

class Range def deconstruct = [self.begin, self.end] end

42..420 => low, high

Just an aside, but you could avoid the performance issues of splatting #to_a by using Range #begin and #end.

low, high = (42..420).then { [_1.begin, _1.end] }

[mame (Yusuke Endoh)]

• Status changed from Open to Rejected

We discussed this at the dev meeting and decided to reject it.

Currently, assignment deconstruction is available only for Arrays (note that assignment deconstruction is not pattern matching). Such special handling for Ranges is not justified by the expected frequency of use.

This is my personal opinion, but it might be conceivable to introduce some method that returns Range#begin and #end as arrays.

p (1..100).begin_and_end #=> [1, 100]

start_date, end_date = (Date.yesterday..Date.tomorrow).begin_and_end

[stuyam (Stuart Yamartino)]

Thanks for the thoughtful responses @shan (Shannon Skipper) and @mame (Yusuke Endoh)!

Shan, you make great points. With the next it syntax coming in ruby 3.3 you could also do:

low, high = (42..420).then { [it.begin, it.end] }

Mame, thank you for discussing this proposal! I like your suggestion of the #begin_and_end method, that solves the performance issues, it also then relies on solely array deconstruction doing it's own job which is nice! I could see #begin_and_end being useful for something like an ORM where you need the endpoints to do a BETWEEN in SQL or something and it would allow you to more easily deconstruct a range when a range is being used more as an object to store the endpoints rather than an object to ask questions about things in between the endpoints etc.

[Dan0042 (Daniel DeLorme)]

(1..42).minmax #=> [1, 42]
(42..1).minmax #=> [nil, nil]

[stuyam (Stuart Yamartino)]

Yeah great point @Dan0042 (Daniel DeLorme), #minmax almost works, but as you point out it does not work because if the range is a reverse range you get nil for both values. Interesting!

(1..42).minmax #=> [1, 42]
(42..1).minmax #=> [nil, nil]

(1..42).then { [_1.begin, _1.end] } #=> [1, 42]
(42..1).then { [_1.begin, _1.end] } #=> [42, 1]

[stuyam (Stuart Yamartino)]

Conversation moved to new issue for implementation of #begin_and_end method on Range: #20080