1_method_restriction.cr

class Developer
  def salery
    5000
  end
end

class Manager
  def salery
    8000
  end
end

class Firm
  def initialize
    @employees = [] of Object
  end

  def add_employee(emp : Object)
    if emp.responds_to?(:salery)
      @employees << emp
    end
  end

  def total_salery
    @employees.sum(&.salery)
  end
end

firm = Firm.new
firm.add_employee Developer.new
firm.add_employee Manager.new
puts firm.total_salery

2_output.txt

Error in /Users/senny/Projects/playground/crystal/first_steps/objects.cr:36: instantiating 'Firm#total_salery()'

puts firm.total_salery
          ^~~~~~~~~~~~

in /Users/senny/Projects/playground/crystal/first_steps/objects.cr:29: instantiating 'Array(Object+)#sum()'

    @employees.sum(&.salery)
               ^~~

in /usr/local/Cellar/crystal/0.5.3/src/enumerable.cr:291: instantiating 'inject(Int32)'

    inject(initial) { |memo, e| memo + (yield e) }
    ^~~~~~

in /usr/local/Cellar/crystal/0.5.3/src/enumerable.cr:53: instantiating 'each()'

    each do |elem|
    ^~~~

in /usr/local/Cellar/crystal/0.5.3/src/enumerable.cr:53: instantiating 'each()'

    each do |elem|
    ^~~~

in /usr/local/Cellar/crystal/0.5.3/src/enumerable.cr:291: instantiating 'inject(Int32)'

    inject(initial) { |memo, e| memo + (yield e) }
    ^~~~~~

in /Users/senny/Projects/playground/crystal/first_steps/objects.cr:29: instantiating 'Array(Object+)#sum()'

    @employees.sum(&.salery)
               ^~~

in /Users/senny/Projects/playground/crystal/first_steps/objects.cr:29: undefined method 'salery' for Reference

    @employees.sum(&.salery)
                     ^~~~~~

You can define an abstract super class.

abstract class Employee; end

class Developer < Employee
  def salery
    5000
  end
end

class Manager < Employee
  def salery
    8000
  end
end

class Firm
  def initialize
    @employees = [] of Employee
  end

  def add_employee(emp : Object)
    if emp.responds_to?(:salery)
      @employees << emp
    end
  end

  def total_salery
    @employees.sum(&.salery)
  end
end

firm = Firm.new
firm.add_employee Developer.new
firm.add_employee Manager.new
puts firm.total_salery