What is the difference between Raising Exceptions vs Throwing Exceptions in Ruby?

Question

Ruby has two different exceptions mechanisms: Throw/Catch and Raise/Rescue.

Why do we have two?

When should you use one and not the other?

Answer 1

• raise, fail, rescue, and ensure handle errors, also known as exceptions
• throw and catch are control flow

Unlike in other languages, Ruby’s throw and catch are not used for exceptions. Instead, they provide a way to terminate execution early when no further work is needed. (Grimm, 2011)

Terminating a single level of control flow, like a while loop, can be done with a simple return. Terminating many levels of control flow, like a nested loop, can be done with throw.

While the exception mechanism of raise and rescue is great for abandoning execution when things go wrong, it's sometimes nice to be able to jump out of some deeply nested construct during normal processing. This is where catch and throw come in handy. (Thomas and Hunt, 2001)

References

1. Grimm, Avdi. "Throw, Catch, Raise, Rescue… I’m so Confused!" RubyLearning Blog. N.p., 11 July 2011. Web. 1 Jan. 2012. http://rubylearning.com/blog/2011/07/12/throw-catch-raise-rescue--im-so-confused/.
2. Thomas, Dave, and Andrew Hunt. "Programming Ruby." : The Pragmatic Programmer's Guide. N.p., 2001. Web. 29 Sept. 2015. http://ruby-doc.com/docs/ProgrammingRuby/html/tut_exceptions.html.

Answer 2

I think http://hasno.info/ruby-gotchas-and-caveats has a decent explanation of the difference:

catch/throw are not the same as raise/rescue. catch/throw allows you to quickly exit blocks back to a point where a catch is defined for a specific symbol, raise rescue is the real exception handling stuff involving the Exception object.

Answer 3

https://coderwall.com/p/lhkkug/don-t-confuse-ruby-s-throw-statement-with-raise offers an excellent explanation that I doubt I can improve on. To summarize, nicking some code samples from the blog post as I go:

1. raise/rescue are the closest analogues to the throw/catch construct you're familiar with from other languages (or to Python's raise/except). If you've encountered an error condition and you would throw over it in another language, you should raise in Ruby.

2. Ruby's throw/catch lets you break execution and climb up the stack looking for a catch (like raise/rescue does), but isn't really meant for error conditions. It should be used rarely, and is there just for when the "walk up the stack until you find a corresponding catch" behaviour makes sense for an algorithm you're writing but it wouldn't make sense to think of the throw as corresponding to an error condition.

   What is catch and throw used for in Ruby? offers some suggestions on nice uses of the throw/catch construct.

The concrete behavioural differences between them include:

• rescue Foo will rescue instances of Foo including subclasses of Foo. catch(foo) will only catch the same object, Foo. Not only can you not pass catch a class name to catch instances of it, but it won't even do equality comparisons. For instance

  catch("foo") do
    throw "foo"
  end
  

  will give you an UncaughtThrowError: uncaught throw "foo" (or an ArgumentError in versions of Ruby prior to 2.2)

• Multiple rescue clauses can be listed...

  begin
    do_something_error_prone
  rescue AParticularKindOfError
    # Insert heroism here.
  rescue
    write_to_error_log
    raise
  end
  

  while multiple catches need to be nested...

  catch :foo do
    catch :bar do
      do_something_that_can_throw_foo_or_bar
    end
  end
  

• A bare rescue is equivalent to rescue StandardError and is an idiomatic construct. A "bare catch", like catch() {throw :foo}, will never catch anything and shouldn't be used.