Enumerator::IO::Reader

Ever had a IO object you wanted to enumerate while not reading the whole stream into memory? no problems just use http://ruby-doc.org/core-2.2.2/IO.html#method-i-each_line and specify separator or any of it's siblings #each_byte, #each_char etc.

But what if the situation was the opposite, you had your enumerable, in this case a database response that was to big for your wallet to fit in memory and the interface required a IO object to read from.

Like #put_object in the legacy aws sdk http://www.rubydoc.info/gems/aws-sdk-euca/1.8.5/AWS/S3/Client:put_object

gem install enumerator_io_reader

A simple case first

require 'enumerator/io/reader'
range = "abc".."abz"
io = Enumerator::IO::Reader.new(range.to_enum)
io.read(20) # => "abcabdabeabfabgabhab"
io.read # => "iabjabkablabmabnaboabpabqabrabsabtabuabvabwabxabyabz"
io.eof? # => true

#to_s is called for every entry in the enumerable but pass a block to the constructor and you can serialize the object in a lazy manner after it has been yielded. The same lazy mapping can of course be accomplished by something like Enumerator::Lazy.

require 'enumerator/io/reader'
require 'csv'

io = Enumerator::IO::Reader.new(Post.each.to_enum) do |post|
  post.attributes.values.to_csv
end

s3.put_object(
  bucket_name: 'foo',
  key: 'key',
  data: io)

If you use this gem alot through out your project you might consider the following monkey patches

module Enumerable
  def to_io(method = :each)
    Enumerator::IO::Reader.new(enum_for(method))
  end
end

range = "abc".."abz"
io = range.to_io
io.read(20) # => "abcabdabeabfabgabhab"

class Enumerator
  def to_io
    Enumerator::IO::Reader.new(self)
  end
end

enumerator = Enumerator.new do |y|
  y.yield(1)
  sleep 1
  y.yield(2)
end
enumerator.to_io.read # => "12"

(The MIT License)

Copyright (c) 2015 Erik Fonselius

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