Why "else"?
I'm trying to understand exceptions in Ruby but I'm a little confused. The tutorial I'm using says that if an exception occurs that does not match any of the exceptions identified by the rescue statements, you can use an "else" to catch it:
begin
# -
rescue OneTypeOfException
# -
rescue AnotherTypeOfException
# -
else
# Other exceptions
ensure
# Always will be executed
end
However, I also saw later in the tutorial "rescue" being used without an exception specified:
begin
file = open("/unexistant_file")
if file
puts "File opened successfully"
end
rescue
file = STDIN
end
print file, "==", STDIN, "n"
If you can do this, then do I ever need to use else? Or can I just use a generic rescue at the end like this?
begin
# -
rescue OneTypeOfException
# -
rescue AnotherTypeOfException
# -
rescue
# Other exceptions
ensure
# Always will be executed
end
The else is for when the block completes without an exception thrown. The ensure is run whether the block completes successfully or not. Example:
begin
puts "Hello, world!"
rescue
puts "rescue"
else
puts "else"
ensure
puts "ensure"
end
This will print Hello, world! , then else , then ensure .
Here's a concrete use-case for else in a begin expression. Suppose you're writing automated tests, and you want to write a method that returns the error raised by a block. But you also want the test to fail if the block doesn't raise an error. You can do this:
def get_error_from(&block)
begin
block.call
rescue => err
err # we want to return this
else
raise "No error was raised"
end
end
Note that you can't move the raise inside the begin block, because it'll get rescue d. Of course, there are other ways without using else , like checking whether err is nil after the end , but that's not as succinct.
Personally, I rarely use else in this way because I think it's rarely needed, but it does come in handy in those rare cases.
The else block in a begin rescue end block is used when you are perhaps expecting an exception of some sort to occur. If you run through all of your expected exceptions but still have nothing raised, then in your else block you can do whatever's needed now that you know that your original code ran error free.
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