smokers.rb

class MarketResearch

  # Assume we parsed some file or fetched an HTTP endpoint and
  # got this JSON
  DATA = [
    {age: 19, smoker: false, income: 10_000, education: :high_school},
    {age: 49, smoker: true, income: 120_000, education: :bachelors},
    {age: 55, smoker: false, income: 400_000, education: :masters},
    {age: 23, smoker: true, income: 10_000, education: :bachelors},
    {age: 70, smoker: false, income: 70_000, education: :phd },
    {age: 34, smoker: false, income: 90_000, education: :masters},
    {age: 90, smoker: true, income: 0, education: :high_school},
  ]

  # Since we're looking at people, we need a type to know what we are
  # operating on
  class Person
    def self.to_proc(hash)
      Person.new(
        hash.fetch(:income),
        hash.fetch(:smoker))
    end

    attr_reader :income

    # Just care about income and is a smoker, but could add more here if needed
    def initialize(income,smoker)
      # Coerce types here using protocols
      @income =   income.to_i
      @smoker = !!smoker
    end

    def smoker?
      @smoker
    end
  end

  def average_income_by_smoking(data)
    # name the operations, but no need to pollute anything other
    # than local namespace with this stuff for now
    income_under_10_000 = ->(person)   { person.income < 10_000 }
    averge_hash_values  = ->(_,values) { values.map(&:income).reduce(&:+).to_f / values.size }

    data.map(&Person).                 # Turn our data into a typed object
      reject(&income_under_10_000)).   # no need to parameterize this - YAGNI
      group_by(&:smoker?).             # group by an attribute of our person.  Use of "?" method 
                                       #  should make it clear we get a Map<Boolean,Array<Person>>
      map(&averge_hash_values).        # name the operation, doesn't need to be a global feature of ActiveSupport :)
      to_h                             # Type conversion using a protocol
  end
end