Show that rescue ordering determines exception handling

ruby-rescue-ordering-test-run.out

❯ ./ruby-rescue-ordering-test.rb
Next? [eis] e
StandardError handler       # Encoding is a subclass of StandardError
this is an encoding error
Next? [eis] i
StandardError handler       # IOError is also a subclass of StandardError
this is an IO error
Next? [eis] s
ScriptError handler         # ScriptError is NOT a subclass of StandardError
this is a script error
Next? [eis] %

# conclusion: rescues are attempted in the order given.  First one that matches, wins.

ruby-rescue-ordering-test.rb

#!/usr/bin/env ruby

# Test class
class Test
  def failer(what = nil)
    case what
    when 'e', 'enc', 'encoding'
      raise EncodingError, 'this is an encoding error'
    when 's', 'script', 'scr'
      raise ScriptError, 'this is a script error'
    when 'i', 'ioerr', 'ioerror'
      raise IOError, 'this is an IO error'
    else
      raise 'ArgumentError', 'no argument'
    end
  rescue StandardError => e
    puts 'StandardError handler'
    puts e.message
  rescue ScriptError => e
    puts 'ScriptError handler'
    puts e.message
  rescue EncodingError => e
    puts 'EncodingError handler'
    puts e.message
  rescue IOError => e
    puts 'IOError handler'
    puts e.message
  end

  def prompt(msg)
    print msg
    gets
  end

  def run
    while (ans = prompt('Next? [eis] '))
      failer(ans.strip)
    end
  end
end

Test.new.run