Add Turning Any Class Into An Enumerator as a ruby til

ruby/turning-any-class-into-an-enumerator.md

@@ -0,0 +1,60 @@
+# Turning Any Class Into An Enumerator
+
+At the core of any enumerator is the ability to respond to an `#each` call.
+With that in mind, we prepare any class for being turned into an enumerator.
+
+Consider this class `SquaresCollection` that allows you to turn an array of
+integers into an array of its squares.
+
+```ruby
+class SquaresCollection
+  def initialize(items)
+    @items = items
+  end
+
+  def run
+    @items.map { |item| item * item }
+  end
+end
+
+SquaresCollection.new([1,2,3]).run #=> [1,4,9]
+```
+
+We can work with this, but it opts out of Ruby's enumerator offerings.
+
+We can, instead, provide an `#each` method which allows instances of our
+`SquaresCollection` class to be turned into enumerators.
+
+```ruby
+class SquaresCollection
+  def initialize(items)
+    @items = items
+  end
+
+  def each
+    return enum_for(:each) unless block_given?
+
+    @items.each do |item|
+      yield item * item
+    end
+  end
+end
+```
+
+Here is how we can use it:
+
+```ruby
+sc = SquaresCollection.new([1,2,3])
+
+puts sc.to_enum
+#=> <Enumerator ...>
+
+puts sc.to_enum.map { |item| "* #{item} *" }
+#=> ["* 1 *", "* 4 *", "* 9 *"]
+```
+
+The [`#to_enum`](https://devdocs.io/ruby~2.5/object#method-i-to_enum) method
+looks for an `#each` method on the instance's class and uses that to create an
+enumerator.
+
+[source](https://blog.arkency.com/2014/01/ruby-to-enum-for-enumerator/)