flat_me.rb

# Simple and straight variant - only using if the task will never
# be extended (almost write-only-code)
def flat_me_rube(what)
  if what.is_a?(Array)
    what.map {|o| flat_me_rube o }  # convert all non-array element of array to array too
        .reduce &:concat            # and then concat them all together
  else
    [what]                          # converting itself
  end
end

puts flat_me_rube([1, 2, [3], [4, 11], 5, nil]).inspect


# The same function, but split into modules and included to native classes if
# needed, mean logic will be more difficult later
module Flattener

  module Element
    def flat_me_tender
      [self]
    end
  end

  module Array
    def flat_me_tender
      map(&:flat_me_tender).reduce(&:concat)
    end
  end

end

#The Int class should be set here if the ONLY int values should be converted
Object.include Flattener::Element
Array.include Flattener::Array

puts [1, 2, [3], [4, 11], 5, nil].flat_me_tender.inspect