crowflower_question.rb

require "rspec"

class Array

  def amap
    return to_enum(:amap) unless block_given?

    self.class.new.tap do |arr|
      each { |i| arr << yield(i) }
    end
  end

end

describe Array do
  describe '#amap' do

    let(:array)     { [1,2,3,4,5] }
    let(:add_one)   { array.amap { |i| i + 1 } }

    it 'returns a new array instance' do
      add_one.object_id.should_not == array.object_id
    end

    it 'yields each element to the block' do
      count = 0
      array.amap { |i| count += 1 }
      count.should == array.length
    end

    it 'returns a new array made by results of calling each block' do
      add_one.should == [2,3,4,5,6]
    end

    it 'works on an empty array' do
      [].amap { |i| i + 5 }.should == []
    end

    it 'returns an Enumerator if no block is given' do
      array.amap.should be_an(Enumerator)
    end

    it 'allows itself to be chained with other collection methods' do
      combo = array.amap.with_index { |val, i| "#{i}-#{val}" }
      combo.should == %w[0-1 1-2 2-3 3-4 4-5]
    end

  end
end

__END__

Sample IRB session

1.9.2p290> require 'crowflower_question'
0.6900 => true
1.9.2p290> [1,2,3,4].amap { |i| i + 1 }
0.0002 => [2, 3, 4, 5]