inteface_error.cr

abstract class Baz
  abstract def call(args : String) : String
end

class Bar < Baz
  def call(args : String) : String
    "#{args} + bar"
  end
end

class Bur < Baz
  def call(args : String) : String
    "#{args} + bur"
  end
end


class Foo
  @foos = Array(Baz)

  def initialize(args : Array(Baz))
    @foos = args
  end

  def add(arg : Baz)
    @foos.append(arg)
  end
end

foo = Foo.new([Bar.new, Bur.new])
foo.add(Bar.new)