How to deal with undefined returned variables.

Pug has asked for the wisdom of the Perl Monks concerning the following question: ⭐ (subroutines)

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Re: How to deal with undefined returned variables.⭐ by knight (Friar) on Jan 19, 2001 at 21:55 UTC
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 $CONFIG{HASHTYPE};` [download]	[reply] [d/l]
Re: How to deal with undefined returned variables.⭐ by davorg (Chancellor) on Jan 19, 2001 at 21:54 UTC
If zero or the empty string aren't valid values then you could use: `$CONFIG{HASHTYPE} = $cfg->val("MISC","HASHTYPE") \|\| 'sha1';` [download] Otherwise, you'd need: `$CONFIG{HASHTYPE} = defined $cfg->val("MISC","HASHTYPE") ? $cfg->val("MISC","HASHTYPE") : 'sha1';` [download] which isn't much different to what you've got.	[reply] [d/l] [select]
Re: How to deal with undefined returned variables. by Russ (Deacon) on Mar 05, 2002 at 19:51 UTC
knight makes a good point. Fortunately, in Perl 6, there will be a defaulting operator to simplify this very problem. A binary // operator is the defaulting operator. That is: `$a // $b` [download] is short for: `defined($a) ?? $a :: $b` [download] except that the left side is evaluated only once. It will work on arrays and hashes as well as scalars. It also has a corresponding assignment operator, which only does the assignment if the left side is undefined: `$pi //= 3;` [download] Perl 6 should be such fun!	[reply] [d/l] [select]
Re: Answer: How to deal with undefined returned variables. by rinceWind (Monsignor) on Mar 06, 2002 at 17:00 UTC
I take it that the difference between // and \|\| is the difference between testing truthfulness and testing definedness. Hence there is not difference between $a // $b and $a \|\| $b unless $a is 0 or "". It doesn't sound that useful to me.	[reply]
Re: How to deal with undefined returned variables. by Boldra (Curate) on Dec 09, 2008 at 10:23 UTC
Russ mentioned that the // operator is in Perl 6, but in fact it is available in Perl 5.10: `use 5.010; $CONFIG{HASHTYPE} = $cfg->val("MISC","HASHTYPE") // 'sha1';` [download] This only puts `'sha1'` in your config value if your `$cfg->val` returns undef or nothing. Definitely a valuable addition to the language.	[reply] [d/l] [select]