[ruby-core:87318] [Ruby trunk Feature#14799][Assigned] Startless range

[mame@ruby-lang.org]

Issue #14799 has been updated by mame (Yusuke Endoh).

File beginless-range.patch added
Status changed from Open to Assigned
Assignee set to matz (Yukihiro Matsumoto)

I tried begin-less range once, and it caused many parser conflicts, so I gave up.

However, I've tried it again and created a patch with no conflicts.
A proof-of-concept patch is attached.  It uses the damn lexer state, so I'd like nobu and yui-knk to review it.
Note that the main focus of this patch is parse.y, so we need more work to let many builtin methods to support begin-less range.
 
Matz, what do you think?

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Feature #14799: Startless range
https://bugs.ruby-lang.org/issues/14799#change-72311

* Author: zverok (Victor Shepelev)
* Status: Assigned
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
----------------------------------------
On introduction of endless range at #12912, "startless range" was discussed this way:

> @sowieso: Not having the opposite (`..5` and `..-2`) feels like this is rather a hack than a thoroughly planned feature.

> @duerst: I don't understand the need for a `..5` Range. The feature is called "endless range". Although mathematically, it's possible to think about startless ranges, they don't work in a program. Maybe some programming languages have `..5` as a shortcut for `0..5`, but that's in any way a usual, bounded, range with a start and an end. It's conceptually totally different from `5..`, which is a range with a start but no end, an unbound range.

In the context of that ticket (ranges used mostly for slicing arrays) having `..5` was indeed hard to justify, but there are other cases when `..5` being `-Infinity..5` is absolutely reasonable:

```ruby
case release_date
when ..1.year.ago 
  puts "ancient"
when 1.year.ago..3.months.ago
  puts "old"
when 3.months.ago..Date.today
  puts "recent"
when Date.today..
  puts "upcoming"
end

log.map(&:logged_at).grep(..Date.new(1980)) # => outliers due to bad log parsing...
```

E.g., whenever case equality operator is acting, having startless range to express "below this value" is the most concise and readable way. Also, for expressing constants (mostly decorative, but very readable):

```ruby
# Celsius degrees
WORK_RANGES = {
   ..-10 => :turn_off,
  -10..0 => :energy_saving,
  0..20 => :main,
  20..35 => :cooling,
  35.. => :turn_off
}
```

In addition, my related proposal #14784 suggests that this kind of ranges could be utilized by more powerful clamp too:

```ruby
updated_at.clamp(..Date.today)
```

**Uncertainty points:**

* Would it be hard to add to parser? I am not sure, I am not very good at it :(
* Should `..` be a thing? I guess not, unless there would be convincing real-life examples, which for me it is hard to think of.

---Files--------------------------------
beginless-range.patch (3.55 KB)


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