Ruby Exceptions

Something about Ruby Exceptions

Transcript

1. RUBY EXCEPTIONS ©Martin 2014-09-26
2. OVERVIEW 1. An Simple Example 2. Why Exceptions 3. How To Raise Exception 4. How To Rescue Exception 5. Real Cases 6. Talk With Our Projects 7. Conclusions 8. Some Tips
3. AN SIMPLE EXAMPLE • Exception class inheritances: http://ruby-doc.org/core-2.0.0/ Exception.html
4. WHY EXCEPTIONS • More clear error messages
 AuthenticationError: ‘token invalide’ v.s. ‘your token is invalid’ • Can be used to block unnecessary further more operations • Forced
 A developer can ignore (or not be aware of) your return status and go on blissfully unaware that something failed.
 Exception can't be silently ignored.
5. HOW TO RAISE AN ERROR • Simpliest - just use `raise` (RuntimeError) • Raise with specified type of exception
 e.g. `raise App::AuthenticationFailedError, ‘token is invalid’`
 
 Note: Remember to define your own errors
6. HOW TO RESCUE FROM ERROR(S) • Simpliest - just use `rescue` (StandardError, almost all usual exceptions) • Rescue specified type of exception • Rescue a exception • Rescue multiple exceptions parallelism • Use `retry` and `ensure`
7. DETECT USEFUL DEBUG MESSAGES • error.message • error.backtrace
8. REAL CASES • Exception Notification
 rubygem: exception_notification • Provide a complete exceptions mechanism • Validate user input (ArgumentError) • Detect other errors: Network, system environment, etc...
9. FROM OUR PROJECTS • StateMachine: use `object.event!`, instead of `object.event`
 the former will raise error if transition failed, so we can handle AEAP • Communication between different layers(M v.s. C)
 If you need to display error messages to users, you should use flash, but flash is existed at controller layer.
10. CONCLUSIONS • Exception is not evil • Remember to rescue exceptions • Provide exceptions mechanism if you are developing a gem
11. SOME TIPS • It is to be noted that the body of a method definition is an implicit begin-end block • Be careful to rescue `Exception` • A library should have one subclass of StandardError or RuntimeError and have specific exception types inherit from it
12. WHY NOT RESCUE `EXCEPTION` • Exception is the root of Ruby's exception hierarchy, so when you rescue Exception you rescue from everything • Exceptions to events map: • Interrupt => Ctrl + C • SignalException => kill -9 • SyntaxError
13. WHY RESCUE `EXCEPTION` • One of the few common cases where it’s sane to rescue from Exception is for logging/reporting purposes, in which case you should immediately re-raise the exception