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How to deal with undefined returned variables.

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Contributed by Pug on Jan 19, 2001 at 21:47 UTC
Q&A  > subroutines


Description:

I'm using IniConf which returns undefined if the section does not exist. I have found that this works but it is painful to read:
if ( ! my $temp = $cfg->val("MISC","HASHTYPE") ) { $CONFIG{HASHTYPE}="sha1"; } else { $CONFIG{HASHTYPE}=$temp; }
Is there a better way?

Answer: How to deal with undefined returned variables.
contributed by knight

davorg presents the common idiom, but note that it can lose if the actual value returned is "defined but false", e.g. 0 or "0". To handle this robustly can be done in two lines:

$CONFIG{HASHTYPE} = $cfg->val("MISC","HASHTYPE"); $CONFIG{HASHTYPE} = "shal" if ! defined $CONFIG{HASHTYPE};
Answer: How to deal with undefined returned variables.
contributed by davorg

If zero or the empty string aren't valid values then you could use:

$CONFIG{HASHTYPE} = $cfg->val("MISC","HASHTYPE") || 'sha1';

Otherwise, you'd need:

$CONFIG{HASHTYPE} = defined $cfg->val("MISC","HASHTYPE") ? $cfg->val("MISC","HASHTYPE") : 'sha1';

which isn't much different to what you've got.

Answer: How to deal with undefined returned variables.
contributed by Russ

knight makes a good point. Fortunately, in Perl 6, there will be a defaulting operator to simplify this very problem.

A binary // operator is the defaulting operator.
That is:
$a // $b
is short for:
defined($a) ?? $a :: $b
except that the left side is evaluated only once. It will work on arrays and hashes as well as scalars. It also has a corresponding assignment operator, which only does the assignment if the left side is undefined:
$pi //= 3;
Perl 6 should be such fun!
Answer: How to deal with undefined returned variables.
contributed by Boldra

Russ mentioned that the // operator is in Perl 6, but in fact it is available in Perl 5.10:

use 5.010; $CONFIG{HASHTYPE} = $cfg->val("MISC","HASHTYPE") // 'sha1';
This only puts 'sha1' in your config value if your $cfg->val returns undef or nothing.

Definitely a valuable addition to the language.

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