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copied!<p>The answer is but a click away: the documentation for <a href="http://www.ruby-doc.org/core-1.9.3/Enumerator.html" rel="nofollow">Enumerator</a>:</p> <blockquote> <p>Most [<code>Enumerator</code>] methods [but presumably also <code>Kernel#to_enum</code> and <code>Kernel#enum_for</code>] have two forms: a block form where the contents are evaluated for each item in the enumeration, and a non-block form which returns a new <code>Enumerator</code> wrapping the iteration.</p> </blockquote> <p>It is the second that applies here:</p> <pre><code>enum = [4, 1, 2, 0].to_enum(:count) # => #<Enumerator: [4, 1, 2, 0]:count> enum.class # => Enumerator enum_ewi = enum.each_with_index # => #<Enumerator: #<Enumerator: [4, 1, 2, 0]:count>:each_with_index> enum_ewi.class # => Enumerator enum_ewi.each {|elem, index| elem == index} # => 2 </code></pre> <p>Note in particular irb's return from the third line. It goes on say, "This allows you to chain Enumerators together." and gives <code>map.with_index</code> as an example.</p> <p>Why stop here?</p> <pre><code> enum_ewi == enum_ewi.each.each.each # => true yet_another = enum_ewi.each_with_index # => #<Enumerator: #<Enumerator: #<Enumerator: [4, 1, 2, 0]:count>:each_with_index>:each_with_index> yet_another.each_with_index {|e,i| puts "e = #{e}, i = #{i}"} e = [4, 0], i = 0 e = [1, 1], i = 1 e = [2, 2], i = 2 e = [0, 3], i = 3 yet_another.each_with_index {|e,i| e.first.first == i} # => 2 </code></pre> <p>(Edit 1: replaced example from docs with one pertinent to the question. Edit 2: added "Why stop here?)</p>

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