[ruby-core:114845] [Ruby master Feature#13933] Add Range#empty?

[ruby-core:114845] [Ruby master Feature#13933] Add Range#empty?

[Dan0042 (Daniel DeLorme) via ruby-core]

Issue #13933 has been updated by Dan0042 (Daniel DeLorme).


Since 6 years ago, beginless and endless ranges have been added, so `#none?` is no longer sufficient.

```ruby
(nil..0).none?  #=> TypeError (can't iterate from NilClass)
```

And as mentioned above `Range#size` doesn't work for String ranges.

So I think it would be good to have `Range#empty?`, with cover-based semantics rather than succ-based:

```ruby
(1..0).empty?                #=> true
(1..1).empty?                #=> false
(1...1).empty?               #=> true
("a".."b").empty?            #=> false
("b".."a").empty?            #=> true
(nil..0).empty?              #=> false  
(..-Float::INFINITY).empty?  #=> false  
(...-Float::INFINITY).empty? #=> true
```

----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104692

* Author: ted (Ted Johansson)
* Status: Open
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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[ruby-core:114853] [Ruby master Feature#13933] Add Range#empty?

[nobu (Nobuyoshi Nakada) via ruby-core]

Issue #13933 has been updated by nobu (Nobuyoshi Nakada).


Dan0042 (Daniel DeLorme) wrote in #note-5:
> ```ruby
> (...-Float::INFINITY).empty? #=> true
> ```

How do you define this?
`Range#empty?` must know if the element is a `Float`, and negative infinity?

----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104699

* Author: ted (Ted Johansson)
* Status: Open
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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[ruby-core:114855] [Ruby master Feature#13933] Add Range#empty?

[Dan0042 (Daniel DeLorme) via ruby-core]

Issue #13933 has been updated by Dan0042 (Daniel DeLorme).


It was an ad-hoc example that just seemed to me like it should return true.

As to how to define it... maybe we can say that numeric ranges are equivalent to `begin == -Float::INFINITY` if beginless, and `end == +Float::INFINITY` if endless. Then it would be like saying `(k...k).empty?` with `k == -Float::INFINITY`

----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104702

* Author: ted (Ted Johansson)
* Status: Open
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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[ruby-core:114862] [Ruby master Feature#13933] Add Range#empty?

[akr (Akira Tanaka) via ruby-core]

Issue #13933 has been updated by akr (Akira Tanaka).


I described that this is difficult to implement in https://bugs.ruby-lang.org/issues/19839#note-18.

The problem is not only with -Float::INFINITY.
There are many minimum values other than -Float::INFINITY : [], "", true, false, Object.new, etc.
Of course, a user may define a class with a minimum value.

Range#empty? should work with them.



----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104710

* Author: ted (Ted Johansson)
* Status: Open
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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[ruby-core:114863] [Ruby master Feature#13933] Add Range#empty?

[Dan0042 (Daniel DeLorme) via ruby-core]

Issue #13933 has been updated by Dan0042 (Daniel DeLorme).


akr (Akira Tanaka) wrote in #note-8:
> Range#empty? should work with them.

Ok for `Float::INFINITY, [], ""`, but I don't think `true, false, Object.new` are relevant. A Range object normally has a begin and end, and if the values are not comparable (like `"a"..1`) we get "ArgumentError (bad value for range)". With `true..true` we happen to coincidentally pass that validation because `true<=>true` is 0 (and arguably it should be nil but that's another topic). But semantically, as a range, `true..true` doesn't mean anything. The introduction of beginless/endless ranges also mean that we can now create all kinds of ranges that skip the normal validation.

`(true..true)` and `(true..)` and `(..true)` are not semantically meaningful ranges, there is no use case for them, and therefore it doesn't matter that `#include?` raises an error, and it shouldn't matter what is returned by `#empty?`

But unlike `true`, arrays truly are comparable. The Comparable module is not mixed in, but <=> can return -1 and 1. So `[]` is truly a minimum value for arrays. It could be special-cased. Although I'm not sure it's worth the trouble. Or maybe we can define a range "r" as empty if `r.begin.nil? and r.exclude_end? and r.end.respond_to?(:empty?) and r.end.empty?`

Random idea: beginless/endless ranges may only be created for objects that mix in `Comparable`. So `(Time.now..)` is ok but `(Object.new..)` raises "ArgumentError (bad value for range)"

----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104711

* Author: ted (Ted Johansson)
* Status: Open
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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[ruby-core:114869] [Ruby master Feature#13933] Add Range#empty?

[duerst via ruby-core]

Issue #13933 has been updated by duerst (Martin Dürst).





Dan0042 (Daniel DeLorme) wrote in #note-9:



> because `true<=>true` is 0 (and arguably it should be nil but that's another topic).



Are you saying that true shouldn't be equal to true? That would be strange, at least to me.



----------------------------------------

Feature #13933: Add Range#empty?

https://bugs.ruby-lang.org/issues/13933#change-104717



* Author: ted (Ted Johansson)

* Status: Open

* Priority: Normal

----------------------------------------

Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)







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[ruby-core:114879] [Ruby master Feature#13933] Add Range#empty?

[Dan0042 (Daniel DeLorme) via ruby-core]

Issue #13933 has been updated by Dan0042 (Daniel DeLorme).





duerst (Martin Dürst) wrote in #note-10:

> Are you saying that true shouldn't be equal to true? That would be strange, at least to me.



I understand that it appears logical, but <=> is a *comparison* operator, used to compare with *other* values during sort/min/max. It's pretty much useless if the only valid comparison is with itself. What's the use case for `[true,true].min == true` but `[true,anything_else].min #=> ArgumentError` ? I can't think of anything reasonable. At least if `true<=>true` was nil, that would tell us this value is not comparable with any other value. But I'm not suggesting we change this now, it seems too backward incompatible.



----------------------------------------

Feature #13933: Add Range#empty?

https://bugs.ruby-lang.org/issues/13933#change-104732



* Author: ted (Ted Johansson)

* Status: Open

* Priority: Normal

----------------------------------------

Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)







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[ruby-core:114894] [Ruby master Feature#13933] Add Range#empty?

[akr (Akira Tanaka) via ruby-core]

Issue #13933 has been updated by akr (Akira Tanaka).


Dan0042 (Daniel DeLorme) wrote in #note-9:
> akr (Akira Tanaka) wrote in #note-8:
> > Range#empty? should work with them.
> 
> Ok for `Float::INFINITY, [], ""`, but I don't think `true, false, Object.new` are relevant. A Range object normally has a begin and end, and if the values are not comparable (like `"a"..1`) we get "ArgumentError (bad value for range)". With `true..true` we happen to coincidentally pass that validation because `true<=>true` is 0 (and arguably it should be nil but that's another topic). But semantically, as a range, `true..true` doesn't mean anything. The introduction of beginless/endless ranges also mean that we can now create all kinds of ranges that skip the normal validation.
> 
> `(true..true)` and `(true..)` and `(..true)` are not semantically meaningful ranges, there is no use case for them, and therefore it doesn't matter that `#include?` raises an error, and it shouldn't matter what is returned by `#empty?`

There is an actual example of `(x <=> x) = nil`, `Float::NAN`.

```
% ruby -e 'x = Float::NAN; p x <=> x'    
nil
```

From experience with `Float::NAN`, I think it is not a good idea to expand such objects.
It makes the behavior of `Array#<=>` inconsistent.

```
% ruby -e 'a = [Float::NAN]; p a <=> a'                  
0
```

Since the (only) element of the array, `Float::NAN`, is not comparable,
the array should not be comparable.
But `a <=> a` returns 0 which means it is comparable and equal.
It is because `Array#<=>` returns 0 if the given argument is the receiver itself, regardless of its elements.
We need to choose efficiency or consistency.
Ruby chooses efficiency here: `Array#<=>` returns 0 without examining its elements.
Expanding `Floag::NAN`'like behavior makes this inconsistency more visible.

I agree that objects comparable only with itself have not much usage for range.
But their order is well-defined as a partial order.
There is no reason to break it.
 
> But unlike `true`, arrays truly are comparable. The Comparable module is not mixed in, but <=> can return -1 and 1. So `[]` is truly a minimum value for arrays. It could be special-cased. Although I'm not sure it's worth the trouble. Or maybe we can define a range "r" as empty if `r.begin.nil? and r.exclude_end? and r.end.respond_to?(:empty?) and r.end.empty?`

You ignored the user-defined classes I mentioned.
A user may define a class with a minimum value.
The minimum value may not have `empty?` method.


----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104752

* Author: ted (Ted Johansson)
* Status: Open
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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[ruby-core:114895] [Ruby master Feature#13933] Add Range#empty?

[Dan0042 (Daniel DeLorme) via ruby-core]

Issue #13933 has been updated by Dan0042 (Daniel DeLorme).


akr (Akira Tanaka) wrote in #note-12:
> From experience with `Float::NAN`, I think it is not a good idea to expand such objects.
> It makes the behavior of `Array#<=>` inconsistent.

I agree but I think this is a bit different from what I was talking about. Float::NAN is a float that is not comparable with other floats. This is not the same case as a value that is not comparable with anything else.

> There is no reason to break it.

I agree; as I said, I'm not suggesting we change this now.

> > Or maybe we can define a range "r" as empty if `r.begin.nil? and r.exclude_end? and r.end.respond_to?(:empty?) and r.end.empty?`
> 
> You ignored the user-defined classes I mentioned.
> A user may define a class with a minimum value.
> The minimum value may not have `empty?` method.

How do you define which is the minimum value? I suggested the above as a *posssible* way to define the minimum value. So a custom class would need to have #empty? that returns true in order to define "this is the minimum". But it was just a suggestion. Or maybe each class could have a MINIMUM constant like `String::MINIMUM = ""`, etc. More importantly, I think that `(...minimum).empty?` is such an edge case of an edge case that it's not worth worrying too much. Are you suggesting that `Range#empty?` should not exist just because it's not possible to perfectly handle this extreme edge case for every possible class? Would it really be so bad if it returned `true` for user-defined classes?

----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104753

* Author: ted (Ted Johansson)
* Status: Open
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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[ruby-core:114902] [Ruby master Feature#13933] Add Range#empty?

[akr (Akira Tanaka) via ruby-core]

Issue #13933 has been updated by akr (Akira Tanaka).


Dan0042 (Daniel DeLorme) wrote in #note-13:
> akr (Akira Tanaka) wrote in #note-12:
> > From experience with `Float::NAN`, I think it is not a good idea to expand such objects.
> > It makes the behavior of `Array#<=>` inconsistent.
> 
> I agree but I think this is a bit different from what I was talking about. Float::NAN is a float that is not comparable with other floats. This is not the same case as a value that is not comparable with anything else.
> 

Float::NAN is not comparable with anything else, not only floats.

```
% ruby -e 'p Float::NAN <=> "foo"'               
nil
% ruby -e 'p Float::NAN <=> []'     
nil
```

> > > Or maybe we can define a range "r" as empty if `r.begin.nil? and r.exclude_end? and r.end.respond_to?(:empty?) and r.end.empty?`
> > 
> > You ignored the user-defined classes I mentioned.
> > A user may define a class with a minimum value.
> > The minimum value may not have `empty?` method.
> 
> How do you define which is the minimum value? I suggested the above as a *possible* way to define this minimum value. So a custom class would need to have #empty? that returns true in order to define "this is the minimum". But it was just a suggestion. Or maybe each class could have a MINIMUM constant like `String::MINIMUM = ""`, etc.

I described an idea in https://bugs.ruby-lang.org/issues/19839#note-18 :
`minimum?` method.

I feel `empty?` can sometimes be a wrong name.
`minimum?` represent the property directly.

> More importantly, I think that `(...minimum).empty?` is such an edge case of an edge case that it's not worth worrying too much. Are you suggesting that `Range#empty?` should not exist just because it's not possible to perfectly handle this extreme edge case for every possible class? Would it really be so bad if it returned `true` for user-defined classes?

I think a bug report like my comment will coming someday.
Ruby developers (especially matz) need to respond to it.

So, we compare the usefulness of the method and the difficulties of responding to the bug report.

Also, this problem can be expanded if we introduce `Range#exclude_begin?` (with neko operator).
https://bugs.ruby-lang.org/issues/12133
In that case, we need `maximum?` as well.
But we also need a method to check the existence between two objects because `[1] ^..^ [1, ""]` is empty (no objects between `[1]` and `[1, ""]`).


----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104767

* Author: ted (Ted Johansson)
* Status: Open
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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[ruby-core:114907] [Ruby master Feature#13933] Add Range#empty?

[Dan0042 (Daniel DeLorme) via ruby-core]

Issue #13933 has been updated by Dan0042 (Daniel DeLorme).


akr (Akira Tanaka) wrote in #note-14:
> Float::NAN is not comparable with anything else, not only floats.

Sorry, maybe I didn't express myself well, I meant that floats are usually comparable with other floats, with the exception of NaN. Whereas one `Object.new` is never comparable with another `Object.new`

> I described an idea in https://bugs.ruby-lang.org/issues/19839#note-18 :
> `minimum?` method.
> 
> I feel `empty?` can sometimes be a wrong name.
> `minimum?` represent the property directly.

Yes, `minimum?` is cleaner and "the right way"; `empty?` is more like a hack that works with existing builtin classes with no changes needed. All 3 options you described are perfectly fine.

> I think a bug report like my comment will coming someday.
> Ruby developers (especially matz) need to respond to it.

By "not worth worrying too much" I did not mean we should ignore or postpone it. I just meant we can pick *any* of the options without worrying too much about getting the perfect one.
* `empty?` method
* `minimum?` method
* `MINIMUM` constant
* hard code minimum values
* document the limitation  (even this, while not ideal, is "good enough" imho)
* a mix of the above (ex: hardcode for builtins, `minimum?` method for user-defined classes)

> Also, this problem can be expanded if we introduce `Range#exclude_begin?` (with neko operator).
> https://bugs.ruby-lang.org/issues/12133
> In that case, we need `maximum?` as well.
> But we also need a method to check the existence between two objects because `[1] ^..^ [1, ""]` is empty (no objects between `[1]` and `[1, ""]`).

Huh, "neko" operator... that's cute 🐱


----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104774

* Author: ted (Ted Johansson)
* Status: Open
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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[ruby-core:114988] [Ruby master Feature#13933] Add Range#empty?

[kyanagi (Kouhei Yanagita) via ruby-core]

Issue #13933 has been updated by kyanagi (Kouhei Yanagita).


While this is slightly off-topic, it's relevant to `MINIMUM`, so I'm noting it here.

`0.0..Float::INFINITY` represents the entire range of floating-point numbers that is greater or equal to 0.0.
Therefore, semantically, the range represented by `0.0..Float::INFINITY` should be the same as `0.0..`,
but the following results contradict this:

```
(0.0..Float::INFINITY).cover?(0.0..) # => false
(0.0..).cover?(0.0..Float::INFINITY) # => true
```

To make this work "correctly", we need the information that `Float::INFINITY` is the maximum value.

If there is any handling regarding the minimum value, it might be necessary to have a similar approach for
the maximum value as well.

----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104858

* Author: ted (Ted Johansson)
* Status: Open
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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[ruby-core:115012] [Ruby master Feature#13933] Add Range#empty?

[matz (Yukihiro Matsumoto) via ruby-core]

Issue #13933 has been updated by matz (Yukihiro Matsumoto).

Status changed from Open to Rejected

One drawback of `Range#empty?` is that we have several corner cases that cannot determine emptiness correctly (e.g., regarding exclude_end ranges or infinity).
Upon the fact we don't see the real-world use-case yet, we reject the proposal for the moment.

FYI, `Range#overlap?` has similar corner cases, it's better to document clearly somewhere.

Matz.


----------------------------------------
Feature #13933: Add Range#empty?
https://bugs.ruby-lang.org/issues/13933#change-104882

* Author: ted (Ted Johansson)
* Status: Rejected
* Priority: Normal
----------------------------------------
Range already responds to #size. It would be nice if it also responded to predicate #empty? :-)



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