class Enumerator::Lazy

Enumerator::Lazy is a special type of Enumerator, that allows constructing chains of operations without evaluating them immediately, and evaluating values on as-needed basis. In order to do so it redefines most of Enumerable methods so that they just construct another lazy enumerator.

Enumerator::Lazy can be constructed from any Enumerable with the Enumerable#lazy method.

lazy = (1..Float::INFINITY).lazy.select(&:odd?).drop(10).take_while { |i| i < 30 }
# => #<Enumerator::Lazy: #<Enumerator::Lazy: #<Enumerator::Lazy: #<Enumerator::Lazy: 1..Infinity>:select>:drop(10)>:take_while>

The real enumeration is performed when any non-redefined Enumerable method is called, like Enumerable#first or Enumerable#to_a (the latter is aliased as

#force for more semantic code):

lazy.first(2)
#=> [21, 23]

lazy.force
#=> [21, 23, 25, 27, 29]

Note that most Enumerable methods that could be called with or without a block, on Enumerator::Lazy will always require a block:

[1, 2, 3].map       #=> #<Enumerator: [1, 2, 3]:map>
[1, 2, 3].lazy.map  # ArgumentError: tried to call lazy map without a block

This class allows idiomatic calculations on long or infinite sequences, as well as chaining of calculations without constructing intermediate arrays.

Example for working with a slowly calculated sequence:

require 'open-uri'

# This will fetch all URLs before selecting
# necessary data
URLS.map { |u| JSON.parse(URI.open(u).read) }
  .select { |data| data.key?('stats') }
  .first(5)

# This will fetch URLs one-by-one, only till
# there is enough data to satisfy the condition
URLS.lazy.map { |u| JSON.parse(URI.open(u).read) }
  .select { |data| data.key?('stats') }
  .first(5)

Ending a chain with “.eager” generates a non-lazy enumerator, which is suitable for returning or passing to another method that expects a normal enumerator.

def active_items
  groups
    .lazy
    .flat_map(&:items)
    .reject(&:disabled)
    .eager
end

# This works lazily; if a checked item is found, it stops
# iteration and does not look into remaining groups.
first_checked = active_items.find(&:checked)

# This returns an array of items like a normal enumerator does.
all_checked = active_items.select(&:checked)