Exceptional Ruby 

Tim Uruski timuruski.net 

Avdi Grimm virtuouscode.com 

• What is a failure? • How does Ruby handle errors? • What to do when code fails? 

What is a failure? 

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Human Fails Computer Fails Human Fails 

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.” 

begin do_work rescue handle_error end 

begin do_work rescue puts $!.message end begin do_work rescue => my_error puts my_error.message end 

begin do_work rescue # Handle StandardError end begin do_work rescue ArgumentError # Handle ArgumentError end 

begin do_work rescue ErrorA, ErrorB # Handle ErrorA or ErrorB end begin do_work rescue ErrorA # Handle ErrorA rescue ErrorB # Handle ErrorB end 

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 

begin do_work rescue ErrorA # Handle ErrorA rescue ErrorB # Handle ErrorB end 

parse(str) rescue 'parse failed' 

def example do_work rescue => error puts error.message end def example begin do_work rescue => error puts error.message end end 

begin do_work ensure cleanup end 

begin file = File.new('greeting.txt', 'w') file << 'Hello, world!' ensure file.close end 

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 

File.open('greeting.txt', 'w') do |f| f << 'Hello, world!' end 

begin do_work rescue => error warn error.message ensure cleanup end 

begin raise 'Silent failure' ensure return end 

begin do_work raise end 

begin fail ErrorA rescue raise ErrorB end 

raise #=> raise RuntimeError #=> raise RuntimeError.new #=> 

raise "boom" #=> raise RuntimeError, "boom" #=> raise RuntimeError.new("boom") #=> raise RuntimeError.new("bada"), "boom" #=> 

raise RuntimeError, "boom", ['./boom.rb:1'] raise RuntimeError, "boom", caller(1..10) raise RuntimeError, "boom", Thread.current.backtrace 

Raise Internals 

1. Get the exception 2. Set the backtrace and cause 3. Set global exception variable 4. Unwind the call stack 

RuntimeError.exception #=> .exception #=> RuntimeError.exception('boom') #=> .exception('pow') #=> 

exception.set_backtrace exception.cause 

$! = exception require 'english' $ERROR_INFO = exception 

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: # #=> t2: # 

uncaught.rb:2:in `boom': unhandled exception from uncaught.rb:5:in `' 

Kernel.raise 

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 

raise inside ensure? raise inside rescue? 

begin do_work rescue Timeout handle_error retry end 

tries = 0 begin do_work rescue TimeoutError tries += 1 retry if tries < 3 end 

tries = 1 begin do_work rescue TimeoutError retry if tries < 3 tries += 1 # Never called... end 

begin do_work rescue handle_failure else handle_success end 

begin do_work puts 'success' rescue puts 'failure' end begin do_work rescue puts 'failure' else puts 'success' end 

begin step_1 step_2 rescue revert_step_1 # Maybe? end 

begin step_1 revert = false rescue revert = true end if revert revert_step_1 else step_2 end 

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 

begin do_work rescue Timeout retry rescue Fatal log_failure else log_success ensure cleanup end 

Exception 

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 ... 

class MyError < Exception # ... end 

begin do_work rescue Exception handle_error end 

Responding to failures 

exit 

$ ruby -e "sleep" ^C -e:1:in `sleep': Interrupt from -e:1:in `' 

begin sleep rescue Interrupt exit end 

begin do_work rescue exit 1 end Code = 0 -> Success Code = 1 -> Failure Code > 1 -> Custom Failure 

Output an error message 

begin do_work rescue => error puts error.message exit 1 end 

begin do_work rescue => error warn error.message exit 1 end 

begin do_work rescue => error abort error.message end 

Exception Logging Service 

class LogExceptions < Middlware def call(env) @app.call(env) rescue => error log_exception(error) end def log_exception(error) # ... end end 

Errbit 

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