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(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(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)

Class Methods


Creates a new Lazy enumerator. When the enumerator is actually enumerated (e.g. by calling force), obj will be enumerated and each value passed to the given block. The block can yield values back using yielder. For example, to create a “filter+map” enumerator:

def filter_map(sequence)
  Lazy.new(sequence) do |yielder, *values|
    result = yield *values
    yielder << result if result
  end
end

filter_map(1..Float::INFINITY) {|i| i*i if i.even?}.first(5)
#=> [4, 16, 36, 64, 100]

Instance Methods


An alias for drop
An alias for filter
An alias for grep
An alias for map
An alias for reject
An alias for select
An alias for take
An alias for uniq

Like Enumerable#chunk, but chains operation to be lazy-evaluated.

Like Enumerable#chunk_while, but chains operation to be lazy-evaluated.

Like Enumerable#map, but chains operation to be lazy-evaluated.

(1..Float::INFINITY).lazy.map {|i| i**2 }
#=> #<Enumerator::Lazy: #<Enumerator::Lazy: 1..Infinity>:map>
(1..Float::INFINITY).lazy.map {|i| i**2 }.first(3)
#=> [1, 4, 9]

Returns a new lazy enumerator with the concatenated results of running block once for every element in the lazy enumerator.

["foo", "bar"].lazy.flat_map {|i| i.each_char.lazy}.force
#=> ["f", "o", "o", "b", "a", "r"]

A value x returned by block is decomposed if either of the following conditions is true:

  • x responds to both each and force, which means that x is a lazy enumerator.

  • x is an array or responds to to_ary.

Otherwise, x is contained as-is in the return value.

[{a:1}, {b:2}].lazy.flat_map {|i| i}.force
#=> [{:a=>1}, {:b=>2}]

Like Enumerable#drop, but chains operation to be lazy-evaluated.

Like Enumerable#drop_while, but chains operation to be lazy-evaluated.

Returns a non-lazy Enumerator converted from the lazy enumerator.

Similar to Object#to_enum, except it returns a lazy enumerator. This makes it easy to define Enumerable methods that will naturally remain lazy if called from a lazy enumerator.

For example, continuing from the example in Object#to_enum:

# See Object#to_enum for the definition of repeat
r = 1..Float::INFINITY
r.repeat(2).first(5) # => [1, 1, 2, 2, 3]
r.repeat(2).class # => Enumerator
r.repeat(2).map{|n| n ** 2}.first(5) # => endless loop!
# works naturally on lazy enumerator:
r.lazy.repeat(2).class # => Enumerator::Lazy
r.lazy.repeat(2).map{|n| n ** 2}.first(5) # => [1, 1, 4, 4, 9]

Like Enumerable#select, but chains operation to be lazy-evaluated.

Like Enumerable#filter_map, but chains operation to be lazy-evaluated.

(1..).lazy.filter_map { |i| i * 2 if i.even? }.first(5)
#=> [4, 8, 12, 16, 20]

Like Enumerable#select, but chains operation to be lazy-evaluated.

Returns a new lazy enumerator with the concatenated results of running block once for every element in the lazy enumerator.

["foo", "bar"].lazy.flat_map {|i| i.each_char.lazy}.force
#=> ["f", "o", "o", "b", "a", "r"]

A value x returned by block is decomposed if either of the following conditions is true:

  • x responds to both each and force, which means that x is a lazy enumerator.

  • x is an array or responds to to_ary.

Otherwise, x is contained as-is in the return value.

[{a:1}, {b:2}].lazy.flat_map {|i| i}.force
#=> [{:a=>1}, {:b=>2}]
An alias for to_a

Like Enumerable#grep, but chains operation to be lazy-evaluated.

Like Enumerable#grep_v, but chains operation to be lazy-evaluated.

Returns self.

Like Enumerable#map, but chains operation to be lazy-evaluated.

(1..Float::INFINITY).lazy.map {|i| i**2 }
#=> #<Enumerator::Lazy: #<Enumerator::Lazy: 1..Infinity>:map>
(1..Float::INFINITY).lazy.map {|i| i**2 }.first(3)
#=> [1, 4, 9]

Like Enumerable#reject, but chains operation to be lazy-evaluated.

Like Enumerable#select, but chains operation to be lazy-evaluated.

Like Enumerable#slice_after, but chains operation to be lazy-evaluated.

Like Enumerable#slice_before, but chains operation to be lazy-evaluated.

Like Enumerable#slice_when, but chains operation to be lazy-evaluated.

Like Enumerable#take, but chains operation to be lazy-evaluated.

Like Enumerable#take_while, but chains operation to be lazy-evaluated.

Expands lazy enumerator to an array. See Enumerable#to_a.

Similar to Object#to_enum, except it returns a lazy enumerator. This makes it easy to define Enumerable methods that will naturally remain lazy if called from a lazy enumerator.

For example, continuing from the example in Object#to_enum:

# See Object#to_enum for the definition of repeat
r = 1..Float::INFINITY
r.repeat(2).first(5) # => [1, 1, 2, 2, 3]
r.repeat(2).class # => Enumerator
r.repeat(2).map{|n| n ** 2}.first(5) # => endless loop!
# works naturally on lazy enumerator:
r.lazy.repeat(2).class # => Enumerator::Lazy
r.lazy.repeat(2).map{|n| n ** 2}.first(5) # => [1, 1, 4, 4, 9]

Like Enumerable#uniq, but chains operation to be lazy-evaluated.

If a block is given, iterates the given block for each element with an index, which starts from offset, and returns a lazy enumerator that yields the same values (without the index).

If a block is not given, returns a new lazy enumerator that includes the index, starting from offset.

offset

the starting index to use

See Enumerator#with_index.

Like Enumerable#zip, but chains operation to be lazy-evaluated. However, if a block is given to zip, values are enumerated immediately.