What is the difference between `Range#include?` and `Range#cover?`?

Question

Edit Fixed following toro2k\'s comment.

Range#include? and Range#cover? seem to be different as seen in the source code 1,

Answer 1

The two methods are designed to do two slightly different things on purpose. Internally they are implemented very differently too. You can take a look at the sources in the documentation and see that .include? is doing a lot more than .cover?

The .cover? method is related to the Comparable module, and checks whether an item would fit between the end points in a sorted list. It will return true even if the item is not in the set implied by the Range.

The .include? method is related to the Enumerable module, and checks whether an item is actually in the complete set implied by the Range. There is some finessing with numerics - Integer ranges are counted as including all the implied Float values (I'm not sure why).

These examples might help:

('a'..'z').cover?('yellow')
# => true

('a'..'z').include?('yellow')
# => false

('yellaa'..'yellzz').include?('yellow')
 => true

Additionally, if you try

('aaaaaa'..'zzzzzz').include?('yellow')

you should notice it takes a much longer time than

('aaaaaa'..'zzzzzz').cover?('yellow')

Answer 2

The main difference is that include is checking whether object is one of range element, and cover is returning whether object is between edge elements. You can see that:

('a'..'z').include?('cc')     #=> false
('a'..'z').cover?('cc')       #=> true

Answer 3

date_range = {:start_date => (DateTime.now + 1.days).to_date, :end_date => (DateTime.now + 10.days).to_date}                
date_range_to_check_for_coverage = {:start_date => (DateTime.now + 5.days).to_date, :end_date => (DateTime.now + 7.days).to_date}                

(date_range[:start_date]..date_range[:end_date]).include?((DateTime.now + 5.days).to_date)                
#true        
(date_range[:start_date]..date_range[:end_date]).cover?((DateTime.now + 5.days).to_date)                
#true        
(date_range[:start_date]..date_range[:end_date]).include?(date_range_to_check_for_coverage[:start_date]..date_range_to_check_for_coverage[:end_date])                
#true        
(date_range[:start_date]..date_range[:end_date]).cover?(date_range_to_check_for_coverage[:start_date]..date_range_to_check_for_coverage[:end_date])                
#false        

Shouldn't the last line return true ?

The reason I am asking is rubocop flags a conflict when I use include? in place of cover?. And clearly, my logic (to check if the range is included in another range) does not work with cover?.