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How to catch an exception in Perl 6

In Perl 6 every block acts as a try exception if there is a CATCH block in it. Inside the CATCH block one can use a default block to catch all the exceptions.

• CATCH

use v6;

my $x = 10;
my $y = 0;

my $z = $x / $y;

my $exception;
{
	say $z;
	CATCH {
		default {
			$exception = $_;
		}
	}
}

if ($exception) {
	say "There was an exception: $exception";
}

say "still running";

Output:

There was an exception: Divide by zero
still running

One very important thing to note here. The actual exception was caused bye the division: my $z = $x / $y;.

That did not throw an exception yet. The exception was thrown only when we tried to use the variable that had an invalid value.

The actual exception object is in $_, but we copied it to a variable just so we can have it outside the block. In most cases this is not really necessary.

What is important though, is that it is not just a simple string as the above output would suggest.

It is actually an instance of the X::Numeric::DivideByZero class. It has methods such as backtrace that returns a list of Backtrace::Frame instances. Each Backtrace::Frame instance has several attributes including file and line that can help identify where the exception was created and where was it thrown.

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Written by Gabor Szabo

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Published on 2013-12-08

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