Ruby's Lazy Enumerator
easily lounge around large collections with the lazy enumeratorEnumerator::Lazy, came to us with the release of Ruby 2.0 on December 25th, 2019, this allowed for the ability to process large or massive collections / files without 🧱’ing your console into ctrl c land. This allows for the processing chains of operations on an object without having them all execute simultaneously.
Let’s test this out: pre Ruby 2.0 behaviour:
list = (1..Float::INFINITY).select { |i| i%3 == 0 }.reject(&:even?)
# wait..
# 🧱'ed your console you'll ned to Ctrl+c 😿
Now travel_to post Ruby 2.0 time:
# we add `lazy` into the mix and we get actual
# lazy Object back.
lazy_list = (1..Float::INFINITY).lazy.select { |i| i%3 == 0 }.reject(&:even?)
=> #<Enumerator::Lazy: ...>
lazy_list.first(5)
=> [3, 9, 15, 21, 27]
well that was interesting.. but wait there’s more 🔪
Things are going quite well in lazy land, but you need to remember that the lovely Enumerator object that you called #lazy on is not in Kanas Anymore. If you go and call a usual array method on that object you’re going to get more 🧱’s. You need to call force or to_a on the object in question.
list = (1..30).lazy.select { |i| i%3 == 0 }.reject(&:even?)
# pass your list off to #map
result = list.map { |x| x if x <= 15 }
result.sample
# 🧱🧱
# undefined method `sample' for #<Enumerator::Lazy: #<Enumerator::Lazy: #<Enumerator::Lazy: #<Enumerator::Lazy: 1..30>:select>:reject>:map>
# what you need to do..
result.force.sample
=> 9
result.to_a.length
=> 3
But never fear, if above is not what you would like on you cornflakes read on.
no need to be over #eager
Using the Enumerable#eager method, a lazy enumerator can be converted to a normal enumerator object, so it can be passed to any method that takes a normal enumerator as parameter.
Likewise, other array methods can be called on the collection to get desired results.
eager_list = (1..30).lazy.select { |i| i%3 == 0 }.reject(&:even?).eager
=> #<Enumerator: #<Enumerator::Lazy: ... 1..30>:select>:reject>:each>
result = eager_list.select { |x| x if x <= 15 }
result.sample
=> 9
result.length
=> 3
Similarly, lazy enumerators can also be passed as arguments to any method that expects a normal enumerator.
list = (1..10).lazy.select { |i| i%3 == 0 }.reject(&:even?)
def display(enum)
enum.map { |x| p x }
end
display(list)
=> #<Enumerator::Lazy: #<Enumerator::Lazy: ... 1..30>:select>:reject>:map>
eager_list = (1..10).lazy.select { |i| i%3 == 0 }.reject(&:even?).eager
display(eager_list)
=> 3
=> 9
references: