Ruby 2.7 adds Beginless Range

Ruby 2.7 added support for Beginless Range which makes the start of range an optional parameter.

(..100) is a Beginless Range and it is equivalent to (nil..100).

Let's see how Beginless Range could be used.

> array = (1..10).to_a

# Select first 6 elements
> array[..5]
=> [1, 2, 3, 4, 5, 6]

# Select first 5 elements
> array[...5]
=> [1, 2, 3, 4, 5]

# grep (INFINITY..5) in (1..5)
> (1..10).grep(..5)
=> [1, 2, 3, 4, 5]

# (..100) is equivalent to (nil..100)
> (..100) == (nil..100)
=> true

Here is another example where in the case statement the condition can be read as below the specified level.

case temperature
when ..-15
  puts "Deep Freeze"
when -15..8
  puts "Refrigerator"
when 8..15
  puts "Cold"
when 15..25
  puts "Room Temperature"
when (25..)   # Kindly notice the brackets here
  puts "Hot"
end

It can also be used for defining constants for ranges.

TEMPERATURE = {
  ..-15  => :deep_freeze,
  -15..8 => :refrigerator,
  8..15  => :cold,
  15..25 => :room_temperature,
  25..   => :hot
end

Using Beginless Range in DSL makes it easier to write conditions and it looks more natural.

# In Rails
User.where(created_at: (..DateTime.now))
# User Load (2.2ms)  SELECT "users".* FROM "users" WHERE "users"."created_at" <= $1 LIMIT $2  [["created_at", "2020-08-05 15:00:19.111217"], ["LIMIT", 11]]


# In RubySpec
ruby_version(..'1.9') do
# Tests for old Ruby
end

Here is the relevant commit and discussion regarding this change.