Issue #15864 has been updated by mrkn (Kenta Murata).


Other candidates from mathematical terms:

- `unbounded?` for checking whether `begin` or `end` is `nil`
- `lower_unbounded?`, `left_unbounded?`, or `unbounded_below?` for checking whether `begin` is `nil`
- `upper_unbounded?`, `right_unbounded?`, or `unbounded_above?` for checking whether `end` is `nil`

----------------------------------------
Feature #15864: Proposal: Add methods to determine if it is an infinite range
https://bugs.ruby-lang.org/issues/15864#change-80903

* Author: osyo (manga osyo)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------

## Summary

Proposal to add methods to determine if it is an infinite range.


## Current status

```ruby
# begin / end return nil
p (..10).begin  # => nil
p (1..).end     # => nil

# But, first / last raise an exception
p (..10).first  # Error: in `first': cannot get the first element of beginless range (RangeError)
p (1..).last    # Error: in `last': cannot get the last element of endless range (RangeError)

# Returns Infinity if it is a Numeric
p (1..10).size      # => 10
p (1..).size        # => Infinity
p (..1).size        # => Infinity

# Otherwise returns nil
p ("a".."z").size   # => nil
p ("a"..).size      # => nil
p (.."z").size      # => nil
```

* `(..10).begin` return `nil`, but `(..10).first` raise an exception.
* `(1..).size` return `Infinity`, but `("a"..).size` return `nil`.
* Behavior changes depending on the state of Range and the called method.
* It is difficult to determine if it is an infinite range.


## Proposal methods

* `Range#beginless?`
  * return `true` if `begin` is `nil`
* `Range#endless?`
  * return `true` if `end` is `nil`
* `Range#infinite?`
  * return `true` if `begin` is `nil` or `end` is `nil`
* `Range#finite?`
  * return `true` if `begin` is not `nil` and `end` is not `nil`


## Example

```ruby
p (1..10).beginless?  # => false
p (1..10).endless?    # => false
p (1..10).infinite?   # => false
p (1..10).finite?     # => true

p (..10).beginless?   # => true
p (..10).endless?     # => false
p (..10).infinite?    # => true
p (..10).finite?      # => false

p (1..).beginless?    # => false
p (1..).endless?      # => true
p (1..).infinite?     # => true
p (1..).finite?       # => false

# NOTE: Float::INFINITY is not support
p (1..Float::INFINITY).beginless?    # => false
p (1..Float::INFINITY).endless?      # => false
p (1..Float::INFINITY).infinite?     # => false
p (1..Float::INFINITY).finite?       # => true
```


## Use case

### before

```ruby
def search_in(range)
  query = "/search"
  if !(range.begin.nil? || range.end.nil?)
    "#{query}?from=#{range.begin}&to=#{range.end}"
  elsif range.begin.nil?
    "#{query}?to=#{range.end}"
  elsif range.end.nil?
    "#{query}?from=#{range.begin}"
  else
    query
  end
end

p search_in("2019-1-1".."2019-4-30")
# => "/search?from=2019-1-1&to=2019-4-30"

p search_in(.."2019-4-30")
# => "/search?to=2019-4-30"

p search_in("2019-5-1"..)
# => "/search?from=2019-5-1"
```

### after

```ruby
def search_in(range)
  query = "/search"
  if range.finite?
    "#{query}?from=#{range.begin}&to=#{range.end}"
  elsif range.beginless?
    "#{query}?to=#{range.end}"
  elsif range.endless?
    "#{query}?from=#{range.begin}"
  else
    query
  end
end

p search_in("2019-1-1".."2019-4-30")
# => "/search?from=2019-1-1&to=2019-4-30"

p search_in(.."2019-4-30")
# => "/search?to=2019-4-30"

p search_in("2019-5-1"..)
# => "/search?from=2019-5-1"
```


## Memo

* Check whether the tip is infinite only with `nil`.
* `Float::INFINITY` is not supported.
  * I think that there is no relation between `Float::INFINITY` and infinite Range.
* I would like an opinion on how to determine if it is infinite.
  * see `range.begin.infinite?` ?
* Uhether there is a better name for the method name.
  * e.g. `#infinite?` to `#infinity?`.


Thank you.

github pull request: https://github.com/ruby/ruby/pull/2196




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