Ruby Exception Handling

Why

Exception packages error information, which is propagated back up the calling stack until a code block declaring how to handle that type of exception is found.

What

The exception hierarchy is like:

	Exception
           NoMemoryError
           ScriptError
               LoadError
                   Gem::LoadError
               NotImplementedError
               SyntaxError
           SecurityError
           SignalException
               Interrupt
           StandardError
               ArgumentError
                   Gem::Requirement::BadRequirementError
               EncodingError
                   Encoding::CompatibilityError
                   Encoding::ConverterNotFoundError
                   Encoding::InvalidByteSequenceError
                   Encoding::UndefinedConversionError
               FiberError
               IndexError
				   KeyError
                   StopIteration
               IOError
                   EOFError
               LocalJumpError
               Math::DomainError
               NameError
                   NoMethodError
               RangeError
                   FloatDomainError
               RegexpError
               RuntimeError
                   Gem::Exception
               SystemCallError
               ThreadError
               TypeError
               ZeroDivisionError
           SystemExit
               Gem::SystemExitException
           SystemStackError

How

The error handling structure is like:

begin 
	# Code that might throw exception
rescue ExceptionType1, ExceptionType2... => exception
	# handle error
rescue exception
	# Default to StandardError
	retry # Reexecute "begin" block
ensure
	# Code that always execute no matter error happens
end

With Kernel#raise, you can either raise a RuntimeError or any other types of exception, along with message and stack-trace.

Where

When

Who

The catch & throw structure allows us to unwind the stack and continue to execute thereafter. Technically speaking, It’s just like the goto statement in C.

Here is an example:

word_list = File.open("wordlist") catch (:done) do
	result = []  
	while line = word_list.gets
		word = line.chomp  
		throw :done unless word =~ /^\w+$/ result << word
	end
	
	 puts result.reverse 
end

Written on May 22, 2024