Ruby 1.9 enumeration changes

Question

So, I don't quite get Ruby 1.9 's changes on enumerators, but what I'd like is a lazy filtering/mapping on a collection, so for example how can I create a lazy enumerator that would be the equivalent of this ...

(1..100).find_all{|x| x % 2 == 1}.map{|x| x * 2}.find_all{|x| x % 3 == 0}

I tried passing a lambda to #enum_for, but it doesn't work on my machine.

Answer 1

You can find implementations of lazy_select and lazy_map in this blog. You will just have to extend the Enumerator module by those two methods. Then you should be able to use

 (1..100).lazy_select{|x| x % 2 == 1}.lazy_map{|x| x * 2}.lazy_select{|x| x % 3 == 0}

Answer 2

Sadly, you can't do this with enum_for. You have to use something like this for lazy enumerables:

class LazyEnum
  include Enumerable

  attr_accessor :enum, :operations

  def initialize enum
    @enum = enum
    @operations = []
  end

  def each
    enum.each do |x|
      filtered = false
      operations.each do |type, op|
        if type == :filter
          unless op[x]
            filtered = true
            break
          end
        else
          x = op[x]
        end
      end
      yield x unless filtered
    end
  end

  def map! &blk
    @operations << [:transform, blk]
    self
  end

  def select! &blk
    @operations << [:filter, blk]
    self
  end

  def reject!
    select! {|x| !(yield x)}
  end

  def dup
    LazyEnum.new self
  end

  def map &blk
    dup.map! &blk
  end

  def select &blk
    dup.select! &blk
  end

  def reject &blk
    dup.reject! &blk
  end
end

LazyEnum.new(1..100).select{|x| x % 2 == 1}.map{|x| x * 2}.select{|x| x % 3 == 0}
#=> #<LazyEnum:0x7f7e11582000 @enum=#<LazyEnum:0x7f7e115820a0 @enum=#<LazyEnum:0x7f7e11582140 @enum=#<LazyEnum:0x7f7e115822d0 @enum=1..100, @operations=[]>, @operations=[[:filter, #<Proc:0x00007f7e11584058@(irb):348>]]>, @operations=[[:transform, #<Proc:0x00007f7e11583a90@(irb):348>]]>, @operations=[[:filter, #<Proc:0x00007f7e115823c0@(irb):348>]]>
irb(main):349:0> LazyEnum.new(1..100).select{|x| x % 2 == 1}.map{|x| x * 2}.select{|x| x % 3 == 0}.to_a
#=> [6, 18, 30, 42, 54, 66, 78, 90, 102, 114, 126, 138, 150, 162, 174, 186, 198]