Creating Ruby enumerators on the fly

When you treat your collections as enumerators, you get to use all your favorite functions like #map and #reduce without having to write any extra code. In this post I show you how easy it is to create enumerators on the fly, without defining extra classes or messing around with the Enumerable module.

When you treat your collections as enumerators, you get to use all your favorite functions like #map and #reduce without having to write any extra code. It's pretty awesome.

In the olden days, defining enumerators was a little cumbersome. You had to make a new class, include the Enumerable module, and define an #each function.

Ever since Ruby 1.9, though, we have a much more lightweight way to define enumerators on the fly. Let's take a look.

Introducing the Enumerator class

The Enumerator class lets you define a one-off enumerator, by using a block syntax. In the example below, we create an enumerator that returns an infinite series of random numbers.

e = do |y|
  loop do
    y << rand(10) # The << operator "yields" a value.

# Make the enumerator "yield" 10 values, then stop
puts e.first(10).inspect # => [6, 6, 7, 2, 2, 9, 6, 8, 2, 1]

You may have noticed that we're using the shift operator << in a strange way. This is a shortcut for the y.yield method. You will call it for each item in the enumerator. If this all seems a bit magical to you, don't worry. It is.

Collection size

Figuring out the size of a collection poses a problem for Ruby's lazy enumerators. To count the items in a collection, you have to load the entire collection - which is against the entire point of using lazy enumerators.

There is a work-around, kind of. If you happen to know the size of the collection at the time you create the Enumerator, you can provide it.

# You can pass the length as an argument to the constructor, if you have it
e = do |y|
  10.times { y << rand }

My real-world example

Just yesterday I was working on Honeybadger's new documentation site. It's built using Jekyll, and I was writing a plugin to create a table of contents based on the <h2> and <h3> tags in the documentation.

It's kind of awkward to figure out which <h3> tags belong to a section defined by <h2> tags. You have to parse the HTML using nokogiri, and then scan the resulting document. So I abstracted that bit of code out and made it an Enumerator. Here's what it looks like.

def subheadings(el) do |y|
    next_el = el.next_sibling
    while next_el && != "h2"
      if == "h3"
        y << next_el
      next_el = next_el.next_sibling
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    Starr Horne

    Starr Horne is a Rubyist and Chief JavaScripter at When she's not neck-deep in other people's bugs, she enjoys making furniture with traditional hand-tools, reading history and brewing beer in her garage in Seattle.

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