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10.10. Returning Failure
10.10.1. Problem
You want to return a
value indicating that your function failed.
10.10.2. Solution
Use a bare
return statement without any argument, which
returns undef in scalar context and the empty list
( ) in list context.return;
10.10.3. Discussion
A return without an argument means:sub empty_retval {
return ( wantarray ? ( ) : undef );
}
You can't use just return
undef, because in list context you will get a list
of one value: undef. If your caller says:if (@a = yourfunc( )) { ... }
then the "error" condition will be perceived as true because
@a will be assigned (undef) and
then evaluated in scalar context. This yields 1,
the number of elements assigned to @a, which is
true. You could use the wantarray function to see
what context you were called in, but a bare return
is a clear and tidy solution that always works:unless ($a = sfunc( )) { die "sfunc failed" }
unless (@a = afunc( )) { die "afunc failed" }
unless (%a = hfunc( )) { die "hfunc failed" }
Some of Perl's built-in functions have
a peculiar return value. Both fcntl and
ioctl have the curious habit of returning the
string "0 but
true" in some circumstances. (This magic string is
conveniently exempt from nagging warnings about improper numerical
conversions.) This has the advantage of letting you write code like
this:ioctl(....) or die "can't ioctl: $!";
That way, code doesn't have to check for a defined zero as distinct
from the undefined value, as it would for the read
or glob functions. "0
but true" is zero when used
numerically. It's rare that this kind of return value is needed. A
more common (and spectacular) way to indicate failure in a function
is to raise an exception, as described in Recipe 10.12.
10.10.4. See Also
The undef, wantarray, and
return functions in Chapter 29 of
Programming Perl and in
perlfunc(1); Recipe 10.12
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| 10.9. Returning More Than One Array or Hash |  | 10.11. Prototyping Functions |
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