Issue #14593 has been reported by skalee (Sebastian Skalacki).

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Feature #14593: Add `Enumerator#concat`
https://bugs.ruby-lang.org/issues/14593

* Author: skalee (Sebastian Skalacki)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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I propose introducing an `Enumerator#concat(other_enum1, other_enum2, ...)` which returns an enumerator being a concatenation of `self` and passed arguments.

Expectation
-----------

~~~ ruby
enum1 = [1, 2, 3].each
enum2 = %w[a b c].each
enum3 = %i[X Y Z].each
concatenation = enum1.concat(enum2, enum3)
concatenation.kind_of?(Enumerator)
#=> true
concatenation.to_a
#=> [1, 2, 3, "a", "b", "c", :X, :Y, :Z]
concatenation.size
#=> 9

enum_without_size = Enumerator.new {}
enum_without_size.size
#=> nil
concatenation2 = enum1.concat(enum2, enum_without_size, enum3)
concatenation2.kind_of?(Enumerator)
#=> true
concatenation2.to_a
#=> [1, 2, 3, "a", "b", "c", :X, :Y, :Z]
concatenation2.size
#=> nil
~~~


Reasoning
---------

Enumerators are generally useful.  They allow to iterate over some data set without loading them fully into memory.  They help separating data generation from its consumption.  If enumerators are desirable, then enumerator concatenation is desirable as well.

Reference implementation
------------------------

~~~ ruby
class Enumerator
  def concat(*enums)
    enumerators = [self, *enums]

    size = enumerators.reduce(0) do |acc, enum|
      s = enum.size
      break nil unless s
      acc + s
    end

    Enumerator.new(size) do |y|
      enumerators.each do |enum|
        enum.each { |item| y << item }
      end
    end
  end
end
~~~

Flat map one-liner
------------------

There's an answer on Stack Overflow suggesting a neat one-liner  https://stackoverflow.com/a/38962951/304175

~~~ ruby
enums.lazy.flat_map{|enum| enum.lazy }
~~~

It yields items correctly.  However, it is not very idiomatic.  Neither it implements `#size` method properly (see example below).  For these reasons, I think that implementing `Enumerator#concat` is a better option.

~~~ ruby
enums = [enum1, enum2, enum3]
#=> [#<Enumerator: [1, 2, 3]:each>, #<Enumerator: ["a", "b", "c"]:each>, #<Enumerator: [:X, :Y, :Z]:each>]
concatenation3 = enums.lazy.flat_map{|enum| enum.lazy }
#=> #<Enumerator::Lazy: #<Enumerator::Lazy: [#<Enumerator: [1, 2, 3]:each>, #<Enumerator: ["a", "b", "c"]:each>, #<Enumerator: [:X, :Y, :Z]:each>]>:flat_map>
concatenation3.to_a
#=> [1, 2, 3, "a", "b", "c", :X, :Y, :Z]
concatenation3.size
#=> nil
~~~

Example use cases
-----------------

Process 20 tweets/posts without fetching more than needed.  Generate some example posts if less than 20 is available

~~~ ruby
enum_tweets = lazy_fetch_tweets_from_twitter(count: 20)
#=> Enumerator
enum_fb_posts = lazy_fetch_posts_from_facebook(count: 20)
#=> Enumerator
enum_example_posts = Enumerator.new { |y| loop { y << generate_random_post } }
#=> Enumerator
posts = enum_tweets.concat(enum_fb_posts).concat(enum_example_posts).take(20)
process(posts)
~~~

Perform a table union on large CSV files

~~~ ruby
csv1_enum = CSV.foreach("path/to/1.csv")
csv2_enum = CSV.foreach("path/to/2.csv")

csv1_enum.concat(csv2_enum).detect { |row| is_what_we_are_looking_for?(row) }
~~~




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