text.cr

# Code

abstract class Interface

  def initialize
  end

  property! application : Application

  abstract def call

end

class Interface::CommandLine < Interface

  def call
    puts "Running application through the command-line interface." if @application.verbose
  end

end

class Interface::Graphical < Interface

  def call
    puts "Running application through the graphical interface." if @application.verbose
  end

end

class Application

  def self.call(interface : Interface)
    new(interface).call
  end

  def initialize(@interface : Interface)
    @interface.application = self
  end

  getter verbose : Bool = true
  getter interface : Interface

  delegate call, to: @interface

end

Application.call(Interface::CommandLine.new)


# Error:
#
#     Error in examples/app_problem.cr:48: instantiating 'Application:Class#call(Interface::CommandLine)'
#
#     Application.call(Interface::CommandLine.new)
#                 ^~~~
#
#     in examples/app_problem.cr:33: instantiating 'Application#call()'
#
#         new(interface).call
#                        ^~~~
#
#     in examples/app_problem.cr:43: expanding macro
#
#       delegate call, to: @interface
#       ^
#
#     in macro 'delegate' /usr/local/Cellar/crystal-lang/0.23.1_3/src/object.cr:1067, line 3:
#
#        1.
#        2.       def call(*args, **options)
#     >  3.         @interface.call(*args, **options)
#        4.       end
#        5.
#        6.       def call(*args, **options)
#        7.         @interface.call(*args, **options) do |*yield_args|
#        8.           yield *yield_args
#        9.         end
#       10.       end
#       11.
#       12.
#
#     instantiating 'Interface+#call(Tuple(), NamedTuple())'
#     in examples/app_problem.cr:25: undefined method 'verbose' for Nil (compile-time type is (Application | Nil))
#
#         puts "Running application through the graphical interface." if @application.verbose
#                                                                                     ^~~~~~~
#
#     Rerun with --error-trace to show a complete error trace.