1 files changed, 16 insertions, 3 deletions
diff --git a/lib/set.rb b/lib/set.rb
index cb07037e82..625046d37c 100644
--- a/lib/set.rb
+++ b/lib/set.rb
@@ -45,9 +45,9 @@
 # == Comparison
 #
 # The comparison operators <, >, <=, and >= are implemented as
-# shorthand for the {proper_,}{subset?,superset?} methods.  However,
-# the <=> operator is intentionally left out because not every pair of
-# sets is comparable ({x, y} vs. {x, z} for example).
+# shorthand for the {proper_,}{subset?,superset?} methods.
+# The <=> operator reflects this order, or return `nil` for
+# sets that both have distinct elements ({x, y} vs. {x, z} for example).
 #
 # == Example
 #
@@ -302,6 +302,19 @@ class Set
   end
   alias < proper_subset?
 
+  # Returns 0 if the set are equal,
+  # -1 / +1 if the set is a proper subset / superset of the given set,
+  # or nil if they both have unique elements.
+  def <=>(set)
+    return unless set.is_a?(Set)
+
+    case size <=> set.size
+    when -1 then -1 if proper_subset?(set)
+    when +1 then +1 if proper_superset?(set)
+    else 0 if self.==(set)
+    end
+  end
+
   # Returns true if the set and the given set have at least one
   # element in common.
   #