The Comparable mixin is used by classes whose objects may be ordered. The class must define the <=> operator, which compares the receiver against another object, returning -1, 0, or +1 depending on whether the receiver is less than, equal to, or greater than the other object. Comparable uses <=> to implement the conventional comparison operators (<, <=, ==, >=, and >) and the method between?.

   class SizeMatters
     include Comparable
     attr :str
     def <=>(anOther)
       str.size <=> anOther.str.size
     def initialize(str)
       @str = str
     def inspect

   s1 = SizeMatters.new("Z")
   s2 = SizeMatters.new("YY")
   s3 = SizeMatters.new("XXX")
   s4 = SizeMatters.new("WWWW")
   s5 = SizeMatters.new("VVVVV")

   s1 < s2                       #=> true
   s4.between?(s1, s3)           #=> false
   s4.between?(s3, s5)           #=> true
   [ s3, s2, s5, s4, s1 ].sort   #=> [Z, YY, XXX, WWWW, VVVVV]
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