command.rb

module MyConcept
  class DoSomethingCommand
    incldue Fancy::Command
    
    arguments do
      key(:user_id).required
      key(:name).required
      key(:email).required(format?: EMAIL_REGEX)
    end
    
    dependencies do
      # arguments available, resolve dependencies needed to execute this action
      # maybe even row-lock certain dependencies for the time being?
      resolve(:user).from(Repository).by(:id => arguments[:user_id]).required
    end
    
    policy do 
      # user, dependencies, arguments available, check permissions
      user.can_execute_xy?
    end
    
    preconditions do
      # user, dependencies, arguments available, check whether state context is OK
    end
    
    def call
      # the action is supposed to execute fine
      context.success!(result)
      context.fail!(error)
    end
  end
end