Add Check If An Object Includes A Module as a Ruby til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1170 TILs and counting..._
+_1171 TILs and counting..._
 
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@@ -896,6 +896,7 @@ _1170 TILs and counting..._
 - [Block Comments](ruby/block-comments.md)
 - [Build HTTP And HTTPS URLs](ruby/build-http-and-https-urls.md)
 - [Chaining Multiple RSpec Change Matchers](ruby/chaining-multiple-rspec-change-matchers.md)
+- [Check If An Object Includes A Module](ruby/check-if-an-object-includes-a-module.md)
 - [Check Return Status Of Running A Shell Command](ruby/check-return-status-of-running-a-shell-command.md)
 - [Click On Text With Capybara](ruby/click-on-text-with-capybara.md)
 - [Colorful Output With MiniTest](ruby/colorful-output-with-minitest.md)

ruby/check-if-an-object-includes-a-module.md

@@ -0,0 +1,25 @@
+# Check If An Object Includes A Module
+
+You may want to know if an object's class includes a module because that will
+tell you something about the object's behavior. It is another way of asking if
+an object responds to a method or set of methods, assuming you know what
+methods the module provides.
+
+This can be done with the [`Module#include?`
+method](https://ruby-doc.org/core-3.0.0/Module.html#method-i-include-3F).
+
+```ruby
+# assuming some object book of type Book that includes Rateable
+> book.class
+=> Book
+> book.class.include?(Rateable)
+=> true
+
+# assuming some object author of type Author that doesn't include Rateable
+> author.class
+=> Author
+> author.class.include?(Rateable)
+=> false
+```
+
+[source](https://stackoverflow.com/a/28667632/535590)