Class descendant syntax in ruby

Question

I am doing 'ruby bits' course on codeschool, and starting to hate it a bit. Assignments require knowledge which was not covered in their short lectures, which forces me to google. Sometimes I don't even know the search terms needed. Can anyone help me to understand what's going on in the code below?

class InvalidGameError < StandardError; end

def new_game(name, options={})
  raise InvalidGameError, "You must provide a name for this game." unless name
  {
    name: name,
    year: options[:year],
    system: options[:system]
  }
end

begin
  game = new_game(nil)
rescue InvalidGameError => e
  puts "There was a problem creating your new game: #{e.message}"
end

I don't quite understand what's happening in the first line. Also why the begin statement needed? Isn't the fourth line enough?

Answer 1

The first line is equivalent to

class InvalidGameError < StandardError
end

which is the common Ruby way to define a new exception. In this case, you define an InvalidGameError exception that inherits from StandardError .

The begin/rescue/end block is the Ruby exception handling mechanism .

If any InvalidGameError will be raised during the execution of the code between the begin/rescue , Ruby will execute whatever code is after the rescue .

begin
  # do something
rescue StandardError
  # do something if the error occurs
end

Answer 2

The first line is defining a new Exception which is how object oriented programming languages handle runtime errors. In this case the only reason your instructor is defining a new exceptions is so that when you read the code you know exactly what error is being handled. You could have just used StandardError directly but InvalidGameError is a better name for the error the code is handling. So let's take the code you posted for an example: The method new_game requires the user to provide a name, and if you try to call it with a name that's set to nil or false (new_game(nil, {year: 2015, system: "xbox"})) your code will raise an exceptions which will stop and exit the program otherwise the method returns a hash that has three pairs: a name key with its value set to the name you provided as a parameter, a year key with its value set to the year key of the options hash and a system key with its value set to the system key of the options hash.

I know, its confusing but this code uses some concept that your really need to know before you can understand it, namely: Exception Handling, Hashes and Conditionals and Ruby objects truth values