some playing with functional programming and ruby

hash_manipulation.rb

# A HashManipulation is a helper object.
# The idea is to aid when modifying hashes.
# changes = HashManipulation.new
# changes.key { |key| key.upcase }
# changes.value { |value| value.length }
# changes.("children" => [1, 2, 3]) # {"CHILDREN" => 3}
module Reports
class HashManipulation
  def initialize
    @key = -> (obj) { obj }
    @value = -> (obj) { obj }
  end

  def key(&block)
    @key = block
    self
  end

  def value(&block)
    @value = block
    self
  end

  def call(obj)
    obj.map do |key, value|
      [@key.call(key), @value.call(value)]
    end.to_h
  end

  def self.value(&block)
    self.new.value(&block)
  end

  def self.key(&block)
    self.new.key(&block)
  end
end
end

pipe.rb

# A Pipe is intended to be simple way of doing function composition (?)
module Reports
class Pipe
  def initialize(procs = [])
    @procs = procs
  end

  delegate :<<, to: :@procs

  def call(obj)
    @procs.inject(obj) { |memo, func| func.(memo) }
  end
end
end

repeat_switches.rb

module Reports
class RepeatSwitches
  def initialize
    @contract_data = ContractDatum.all.ordered.includes(:user)
  end

  def report
    group_by_first_or_second_switch = procerize(:group_by) do |contract|
      first_switch?(contract) ? "First Switch" : "Repeat Switch"
    end
    get_counts = HashManipulation.value {|values| values.length }
    get_switches_count = HashManipulation.value do |contracts|
      get_counts.(group_by_first_or_second_switch.(contracts))
    end

    add_totals = HashManipulation.value do |hash|
      hash.dup.tap do |res|
        res["Total"] = hash.values.inject(:+)
      end
    end

    pipe = Pipe.new
    pipe << get_switches_count
    pipe << add_totals

    final_function = group_by_month(pipe)
    final_function.(@contract_data)
  end

  def group_by_month(action)
    group_by_month = procerize(:group_by) do |contract_datum|
      ordered_at = contract_datum.ordered_at
      [ordered_at.year, ordered_at.mon]
    end
    sort_by_first = -> (hash) { hash.sort_by(&:first).to_h }
    humanize_month = HashManipulation.key{|(year, month)| "#{year}/#{month}" }

    pipe = Pipe.new
    pipe << group_by_month
    pipe << action
    pipe << sort_by_first
    pipe << humanize_month
    pipe
  end

  private
  def first_switch?(contract_datum)
    ordered_at = ContractDatum.arel_table[:ordered_at]
    user_contracts = contract_datum.user.contract_data.ordered
    user_contracts.where(ordered_at.lt(contract_datum.ordered_at)).empty?
  end

  def procerize(method, &block)
    -> (obj) { obj.send(method, &block) }
  end
end
end

@bitboxer the intention still counts :)