Exceptions

Execptions are a way to end the current execution of your program. They are basically a different kind of control flow.

More specifically you can use them to "skip up" several layers of your application and handle errors in a centralized place.

This is an important step in creating a more "professional" kind of app that gracefully handles all possible error states.

Basic exception:

begin
  puts 'I am before the raise.'
  raise 'An error has occured.'
  puts 'I am after the raise.'
rescue
  puts 'I am rescued.'
end

Exception with built in error classes:

begin
  File.open('p014constructs.rb', 'r') do |f1|
    while line = f1.gets
      puts line
    end
  end

  # Create a new file and write to it
  File.open('test.rb', 'w') do |f2|
    # use "" for two lines of text
    f2.puts "Created by Satish\nThank God!"
  end
rescue StandardError => msg
  # display the system generated error message
  puts msg
end

Exception with custom error classes:

  # When someone tries to set a last name, enforce rules about it.
  def last=(last)
    if last == nil or last.size == 0
      raise ArgumentError.new('Everyone must have a last name.')
    end
    @last = last
  end
end

The class itself:

class ArgumentError < StandardError
    def initialize(msg="Argument Banana!")
        super
    end
end

Rescue a custom exception:

begin
  User.last
rescue ArgumentError => msg
  # display the system generated error message
  puts msg
end

Warning: you are able to rescue any exception:

begin
  # stuff
rescue Exception
  # generic exception rescue
end

But DON'T DO IT.

Pairing Exercise

Create a class for transactions.

This class will have a method that does addition.

class Transaction
  def add( a,b )
    a + b
  end
end

In the add function, create the logic that if a string is passed in as an argument, throw an error.

Write a function that creates a new transaction and adds 2 numbers together.

Call the function and pass in numbers, and then call it on another line with a string and see what happens.

Write the begin / rescue code that rescues the exception.

When you get an exception do puts that the number couldn't be added.