What is the difference between Raising Exceptions vs Throwing Exceptions in Ruby?
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?
Solution 1:
-
raise
,fail
,rescue
, andensure
handle errors, also known as exceptions -
throw
andcatch
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
- 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/.
- 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.
Solution 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.
Solution 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:
raise
/rescue
are the closest analogues to thethrow
/catch
construct you're familiar with from other languages (or to Python'sraise
/except
). If you've encountered an error condition and you wouldthrow
over it in another language, you shouldraise
in Ruby.-
Ruby's
throw
/catch
lets you break execution and climb up the stack looking for acatch
(likeraise
/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 correspondingcatch
" behaviour makes sense for an algorithm you're writing but it wouldn't make sense to think of thethrow
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 ofFoo
including subclasses ofFoo
.catch(foo)
will only catch the same object,Foo
. Not only can you not passcatch
a class name to catch instances of it, but it won't even do equality comparisons. For instancecatch("foo") do throw "foo" end
will give you an
UncaughtThrowError: uncaught throw "foo"
(or anArgumentError
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
catch
es 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 torescue StandardError
and is an idiomatic construct. A "barecatch
", likecatch() {throw :foo}
, will never catch anything and shouldn't be used.