Are there something like Python generators in Ruby?

Question

I am new to Ruby, is there a way to yield values from Ruby functions? If yes, how? If not, what are my options to write lazy code?

Answer 1

Ruby's yield keyword is something very different from the Python keyword with the same name, so don't be confused by it. Ruby's yield keyword is syntactic sugar for calling a block associated with a method.

The closest equivalent is Ruby's Enumerator class. For example, the equivalent of the Python:

def eternal_sequence():
  i = 0
  while True:
    yield i
    i += 1

is this:

def eternal_sequence
  Enumerator.new do |enum|
    i = 0
    while true
      enum.yield i # <- Notice that this is the yield method of the enumerator, not the yield keyword
      i +=1
    end
  end
end

You can also create Enumerators for existing enumeration methods with enum_for. For example, ('a'..'z').enum_for(:each_with_index) gives you an enumerator of the lowercase letters along with their place in the alphabet. You get this for free with the standard Enumerable methods like each_with_index in 1.9, so you can just write ('a'..'z').each_with_index to get the enumerator.

Answer 2

I've seen Fibers used in that way, look at an example from this article:

fib = Fiber.new do  
  x, y = 0, 1 
  loop do  
    Fiber.yield y 
    x,y = y,x+y 
  end 
end 
20.times { puts fib.resume }

Answer 3

If you are looking to lazily generate values, @Chuck's answer is the correct one.

If you are looking to lazily iterate over a collection, Ruby 2.0 introduced the new .lazy enumerator.

range = 1..Float::INFINITY
puts range.map { |x| x+1 }.first(10) #  infinite loop
puts range.lazy.map { |x| x+1 }.first(10) #  [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]

Answer 4

Ruby supports generators out of the box using Enumerable::Generator:

require 'generator'

# Generator from an Enumerable object
g = Generator.new(['A', 'B', 'C', 'Z'])

while g.next?
  puts g.next
end

# Generator from a block
g = Generator.new { |g|
  for i in 'A'..'C'
    g.yield i
  end

  g.yield 'Z'
}

# The same result as above
while g.next?
  puts g.next
end

https://ruby-doc.org/stdlib-1.8.7/libdoc/generator/rdoc/Generator.html

Answer 5

Class Enumerator and its method next behave similar https://docs.ruby-lang.org/en/3.1/Enumerator.html#method-i-next

range = 1..Float::INFINITY
enumerator = range.each

puts enumerator.class # => Enumerator

puts enumerator.next  # => 1
puts enumerator.next  # => 2
puts enumerator.next  # => 3