Course of Raku / Objects, I/O, and exceptions / Exceptions / Catching exceptions with try

Not every error is immediate

By default a thrown exception stops the program at once, and try is how you catch it. But not everything that looks like a fatal error actually halts your program the moment it happens. Raku has failures that stay quiet — postponing their reaction until you use the result, or letting you inspect them and carry on. A couple of these have already appeared in this part, and it is worth collecting them in one place.

A division by zero. Dividing by zero does not blow up where it is written. The bad value is produced and held quietly; it only throws when you use it — by printing or coercing it, for example:

my $x = 1 / 0;    # no error here
say 'still running';
say $x;           # only now does it throw

This deferred behaviour, and the Failure objects behind it, are the subject of Soft failures.

A failing external program. A Proc returned by run or shell that exited unsuccessfully does not throw when it is produced. It throws only if you discard it unhandled; store it and read .exitcode (or test it as a Boolean) and the failure stays a plain value you can react to. You saw this in The Proc object.

The common thread: a failure can be deferred — turned into a value you examine when you are ready, rather than an immediate crash. try and the CATCH phaser deal with the errors that are thrown right away; these mechanisms deal with the ones that are not.

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Not every error is immediate

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