Exceptional Ruby

A tribute to Avdi Grimm's excellent book on error handling for the YYC Ruby group.

Transcript

1. Exceptional Ruby
2. Tim Uruski timuruski.net
3. Avdi Grimm virtuouscode.com
4. • What is a failure? • How does Ruby handle errors? • What to do when code fails?
5. What is a failure?
9. Human Fails Computer Fails Human Fails
10. I realized that when a computer program had a fault, the machine could turn out errors millions of times faster than any human or group of humans. [...] I could actually program a machine to make more errors in a day than all human beings had made in the last 10,000 years. – Gerald M. Weinberg. “Errors.”
11. begin do_work rescue handle_error end
12. begin do_work rescue puts $!.message end begin do_work rescue => my_error puts my_error.message end
13. begin do_work rescue # Handle StandardError end begin do_work rescue ArgumentError # Handle ArgumentError end
14. begin do_work rescue ErrorA, ErrorB # Handle ErrorA or ErrorB end begin do_work rescue ErrorA # Handle ErrorA rescue ErrorB # Handle ErrorB end
15. begin do_work rescue ErrorA, ErrorB # Handle ErrorA or ErrorB end begin do_work rescue ErrorA, ErrorB => error # Handle ErrorA or ErrorB as error end
16. begin do_work rescue ErrorA # Handle ErrorA rescue ErrorB # Handle ErrorB end
17. parse(str) rescue 'parse failed'
18. def example do_work rescue => error puts error.message end def example begin do_work rescue => error puts error.message end end
19. begin do_work ensure cleanup end
20. begin file = File.new('greeting.txt', 'w') file << 'Hello, world!' ensure file.close end
21. def write_to(path) file = File.new(path, 'w') yield file ensure file.close end write_to('greeting.txt') do |file| file << 'Hello, world!' end
22. File.open('greeting.txt', 'w') do |f| f << 'Hello, world!' end
23. begin do_work rescue => error warn error.message ensure cleanup end
24. begin raise 'Silent failure' ensure return end
25. begin do_work raise end
26. begin fail ErrorA rescue raise ErrorB end
27. raise #=> <RuntimeError> raise RuntimeError #=> <RuntimeError> raise RuntimeError.new #=> <RuntimeError>
28. raise "boom" #=> <RuntimeError "boom"> raise RuntimeError, "boom" #=> <RuntimeError "boom"> raise RuntimeError.new("boom") #=> <RuntimeError "boom"> raise RuntimeError.new("bada"), "boom" #=> <RuntimeError "boom">
29. raise RuntimeError, "boom", ['./boom.rb:1'] raise RuntimeError, "boom", caller(1..10) raise RuntimeError, "boom", Thread.current.backtrace
30. Raise Internals
31. 1. Get the exception 2. Set the backtrace and cause 3. Set global exception variable 4. Unwind the call stack
32. RuntimeError.exception #=> <RuntimeError:0x123> <RuntimeError:0x123>.exception #=> <RuntimeError:0x123> RuntimeError.exception('boom') #=> <RuntimeError:0x123 'boom'> <RuntimeError:0x123 'boom'>.exception('pow') #=> <RuntimeError:0x789 'pow'>
33. exception.set_backtrace exception.cause
34. $! = exception require 'english' $ERROR_INFO = exception
35. t1 = Thread.new { raise 'boom' rescue $! } t2 = Thread.new { raise 'pow' rescue $! } puts "main: #{$!}" puts "t1: #{t1.value.inspect}" puts "t2: #{t2.value.inspect}" #=> main: #=> t1: #<RuntimeError: boom> #=> t2: #<RuntimeError: pow>
36. uncaught.rb:2:in `boom': unhandled exception from uncaught.rb:5:in `<main>'
37. Kernel.raise
38. module RaiseExit
 def raise(msg_or_exc, msg=msg_or_exc, trace=caller) warn msg.to_s warn caller exit! end end module Kernel include RaiseExit if ENV['DEBUG'] end
39. raise inside ensure? raise inside rescue?
40. begin do_work rescue Timeout handle_error retry end
41. tries = 0 begin do_work rescue TimeoutError tries += 1 retry if tries < 3 end
42. tries = 1 begin do_work rescue TimeoutError retry if tries < 3 tries += 1 # Never called... end
43. begin do_work rescue handle_failure else handle_success end
44. begin do_work puts 'success' rescue puts 'failure' end begin do_work rescue puts 'failure' else puts 'success' end
45. begin step_1 step_2 rescue revert_step_1 # Maybe? end
46. begin step_1 revert = false rescue revert = true end if revert revert_step_1 else step_2 end
47. begin step_1 revert = false rescue revert = true end if revert revert_step_1 else step_2 end begin step_1 rescue revert_step_1 else step_2 end
48. begin do_work rescue Timeout retry rescue Fatal log_failure else log_success ensure cleanup end
49. Exception
50. Exception NoMemoryError ScriptError LoadError NotImplementedError SyntaxError SecurityError SignalException Interrupt StandardError ArgumentError UncaughtThrowError EncodingError Encoding::CompatibilityError Encoding::ConverterNotFoundError Encoding::InvalidByteSequenceError Encoding::UndefinedConversionError FiberError IOError EOFError IndexError KeyError StopIteration ClosedQueueError LocalJumpError Math::DomainError NameError DidYouMean::Correctable NoMethodError RangeError FloatDomainError RegexpError RuntimeError SystemCallError Errno::EAGAIN IO::WaitReadable IO::EAGAINWaitReadable IO::WaitWritable IO::EAGAINWaitWritable Errno::EINPROGRESS IO::WaitReadable IO::EINPROGRESSWaitReadable IO::WaitWritable IO::EINPROGRESSWaitWritable ThreadError TypeError ZeroDivisionError SystemExit SystemStackError fatal Exception NoMemoryError ScriptError LoadError NotImplementedError SignalException StandardError ArgumentError LocalJumpError NameError NoMethodError RuntimeError SystemExit ...
51. class MyError < Exception # ... end
52. begin do_work rescue Exception handle_error end
53. Responding to failures
54. exit
55. $ ruby -e "sleep" ^C -e:1:in `sleep': Interrupt from -e:1:in `<main>'
56. begin sleep rescue Interrupt exit end
57. begin do_work rescue exit 1 end Code = 0 -> Success Code = 1 -> Failure Code > 1 -> Custom Failure
58. Output an error message
59. begin do_work rescue => error puts error.message exit 1 end
60. begin do_work rescue => error warn error.message exit 1 end
61. begin do_work rescue => error abort error.message end
62. Exception Logging Service
63. class LogExceptions < Middlware def call(env) @app.call(env) rescue => error log_exception(error) end def log_exception(error) # ... end end
64. Errbit
65. exceptionalruby.com $15 USD $20 CAD