# Enumerators

An Enumerator is an object that implements iteration in a controlled fashion.

Instead of looping until some condition is satisfied, the object enumerates values as needed. Execution of the loop is paused until the next value is requested by the owner of the object.

Enumerators make infinite streams of values possible.

# Custom enumerators

Let's create an Enumerator for Fibonacci numbers.

fibonacci = Enumerator.new do |yielder|
  a = b = 1
  loop do
    yielder << a
    a, b = b, a + b
  end
end

We can now use any Enumerable method with fibonacci:

fibonacci.take 10
# => [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

# Existing methods

If an iteration method such as each is called without a block, an Enumerator should be returned.

This can be done using the enum_for method:

def each
  return enum_for :each unless block_given?

  yield :x
  yield :y
  yield :z
end

This enables the programmer to compose Enumerable operations:

each.drop(2).map(&:upcase).first
# => :Z

# Rewinding

Use rewind to restart the enumerator.

= Enumerator.new do |yielder|
  x = 0
  loop do
    yielder << x
    x += 1
  end
end.next
# => 0.next
# => 1.next
# => 2.rewind

ℕ.next
# => 0

# Parameters

Parameter Details
yield Responds to yield, which is aliased as <<. Yielding to this object implements iteration.