Ruby Enumerators are a really cool abstraction. They can be used in many ways. I particularly love that, in the standard library, most methods that take a block to iterate over something will return an enumerator when invoked without it. For instance, Enumerable#map or CSV.for_each use this convention.

I hadn’t found a practical use case for them until I needed to process several CSV files advancing through them at the same time. I could grab an enumerator for each CSV file and make them advance as I wanted. It worked like a charm, but I noticed it was very slow. I decided to profile and compare them with their internal iterator counterpart:

require 'benchmark/ips'

COUNT = 500000

Benchmark.ips do |x|'Enumerable') do
    total = 0
    COUNT.times do |i|
      total += i
  end'Enumerator') do
    total = 0
    enumerator = COUNT.times

    while true
        total +=
      rescue StopIteration

Results (Ruby 2.4.0):

Enumerable     37.073  (± 5.4%) i/s -    186.000  in   5.030412s
Enumerator      1.588  (± 0.0%) i/s -      8.000  in   5.040230s

As you see, Enumerators are much slower that the corresponding internal iterator. I can’t use them in my case since CSV processing speed was key to the global performance of the system, but I still love them.