FuncTional Programming

ON RAILS

Functional / OO

OOP IS GREAT

• Colocation of "material" and "tools"
• Clear boundaries between entities 
• Powerful mental model of noun → class
• Built-in, easy state management

FP IS GREAT

• Immutable values are hard to screw up 
• Pure functions are hard to screw up

I Want IT all

• From OOP: 
  • Code organization 
  • Colocation of "material" and "tools"
  • Straightforward state management
• From FP:
  • Immutable values 
  • Pure Functions

VALUE OBject

# Given some event times, 
# Find the check-ins for those event times
class Event::AttendanceChart::ChartBar
  include HasCheckInCounts 

  def initialize(event_times)
    @event_times = event_times
  end

  def check_ins
    @check_ins ||= CheckIn
      .includes(:check_in_times)
      .where("check_in_times.event_time_id" => @event_times.pluck(:id))
  end
end

Public API is immutable data

Function OBject

class Location::FilterSentence::MaxMinFilterString
  def initialize(stringify_value:)
    @stringify_value = stringify_value
  end

  def comparison_string(min, max)
    if min.present? && max.present?
      if min == max
        value_string(min)
      else
        "#{value_string(min)} to #{value_string(max)}"
      end
    elsif min.present?
      "#{value_string(min)} and Up"
    elsif max.present?
      "Up to #{value_string(max)}"
    else
      nil
    end
  end

  private

  def value_string(val)
    @stringify_value.call(val)
  end
end

Public API is pure functions

Object OBject

class Event::AttendanceChart::ChartRange
  attr_reader :ends_at, :starts_at, :dates
  
  def initialize(ends_at:, range:, period:)
    @ends_at = ends_at
    @starts_at = send("starts_at_#{range}", @ends_at) 
    date_range = (starts_at.to_date..@ends_at.to_date)
    @dates = send("group_by_#{period}", date_range)
  end

  private

  def starts_at_this_month(ends_at)
    # ...
  end

  def starts_at_this_year(ends_at)
    # ...
  end

  def group_by_day(range)
    # ...
  end

  def group_by_week(range)
    # ...
  end
end

Public API is data, 

methods are pure functions

Sources

"Using a Ruby Class To Write Functional Code", Pat Shaunessy

"A Functional Pattern System for Object-Oriented Design", 

    Thomas Kuhne

"Commanding Objects Toward Immutability", Jim Gay