Play around with dry monads.

dry-monads.rb

require 'dry-monads'

class Address
  attr_reader :street

  def initialize(street = nil)
    @street = street
  end
end

class User
  attr_reader :address

  def initialize(address =  nil)
    @address = address
  end
end

M = Dry::Monads

user = User.new(Address.new('some address'))
maybe_user = M.Maybe(user).bind do |u|
  M.Maybe(u.address).bind do |a|
    M.Maybe(a.street)
  end
end

p maybe_user

# pass a proc
add_two = -> (x) { M.Maybe(x + 2) }

p M.Maybe(38).bind(add_two).bind(add_two)
p M.Maybe(nil).bind(add_two)

p M.Maybe(user).fmap(&:address).fmap(&:street)

p M.Some(5).fmap(&:succ).value
p M.None.fmap(&:succ).value

p M.Some(32).bind(add_two).bind(add_two).or(M.Some(1))
p M.None.bind(add_two).bind(add_two).or(M.Some(1))

class EitherCalculator
  include Dry::Monads::Either::Mixin

  attr_accessor :input

  def calculate
    i = Integer(input)

    Right(i).bind do |value|
      if value > 1
        Right(value + 3)
      else
        Left('Value is less then 1')
      end
    end.bind do |value|
      if value % 2 == 0
        Right(value * 2)
      else
        Left('Value was not even')
      end
    end
  end
end

calculator = EitherCalculator.new

calculator.input = 3
p calculator.calculate
p calculator.calculate.value

calculator.input = 0
p calculator.calculate
p calculator.calculate.value

calculator.input = 2
p calculator.calculate
p calculator.calculate.value

foo = 1
bar = 2
result = if foo > bar
           M.Right(10)
         else
           M.Left('Wrong values')
         end.fmap { |x| x * 2 }

p result
p result.left?
p result.right?