Re: Possible useless use of map

Could this bug have been picked up by some sibling of use warnings ?

Based on what? Lets see

you're using map in void context, but that has been silenced since 5.9.0

you're treating $hr as a hashref, but you're discarding the value, hmmm, lets check

$ perl -Mstrict -Mwarnings -le " my $hr; $hr->{stuff}"
Useless use of hash element in void context at -e line 1.

$ perl -Mstrict -Mwarnings -le " my $hr; map { $hr->{stuff} } 1"

$ perl -Mstrict -Mwarnings -le " my $hr;  $hr->{stuff} for 1"
Useless use of hash element in void context at -e line 1.
[download]

Congratulations andreas1234567, you have found a bug: map in void context silences "Useless use of hash element in void context".

Please use perlbug and report this bug :)

Comment on Re: Possible useless use of map Download Code

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Re^2: Possible useless use of map by almut (Canon) on Mar 18, 2010 at 10:27 UTC
Actually, I'm a bit surprised that I do get the same warning with a slightly modified variant: `$ perl -Mstrict -Mwarnings -MData::Dumper -le 'my $hr; $hr->{stuff}{fo +o} for 1; print Dumper $hr' Useless use of hash element in void context at -e line 1. $VAR1 = { 'stuff' => {} };` [download] As it autovivifies, it's not "useless", strictly speaking... Also, what about (intended) side effects with tied hashes?	[reply] [d/l]
Re^3: Possible useless use of map by ikegami (Patriarch) on Mar 18, 2010 at 20:40 UTC
As it autovivifies, it's not "useless", strictly speaking.. By that logic, the following shouldn't warn since calling f() isn't useless: `$ perl -c -we'f()+123' Useless use of addition (+) in void context at -e line 1. -e syntax OK` [download] You're mistaken about what gives the warning. The op that does the autovivification is not the one issuing the warning. It's the access of the foo element that issues the warning, and it's indeed useless to access the foo element. `$ perl -we'my $hr; $hr->{stuff}{foo}' Useless use of hash element in void context at -e line 1.` [download] is basically equivalent to `$ perl -we'my $hr; $_ = \%{ $hr }; $_ = \%{ $_->{stuff} }; $_->{foo};' Useless use of hash element in void context at -e line 1.` [download] The last bit is clearly useless. If you prefer a diagram, `___________________ scalar context pad fetch. autovivifies / ________________ deref \| / ____________ scalar context hash fetch. autovivifies \| \| / _______ deref \| \| \| / ____ void context hash fetch. warns. \| \| \| \| / \|--\|-\|------\|-\|----\| $hr->{stuff}->{foo};` [download] Similar, without useless bits: `$hr->{stuff} //= {};` [download] Also, what about (intended) side effects with tied hashes? Warnings aren't always accurate. That's why you can turn them off. Update: Additions for the sake of clarity.	[reply] [d/l] [select]
Re^3: Possible useless use of map by Anonymous Monk on Mar 18, 2010 at 10:31 UTC
Well the first version also auotovivifies.... i'm not sure what you mean about tied hashes.	[reply]
Re^4: Possible useless use of map by almut (Canon) on Mar 18, 2010 at 10:39 UTC
No, `$hr->{stuff}` alone doesn't autovivify the element. It's the dereferencing operation `$hr->{stuff}->{foo} ^^` [download] that autovivifies, not merely accessing the hash element. As for side effects with tied hashes, I mean that arbitrary code could be associated with the tied read operation.	[reply] [d/l] [select]
Re^5: Possible useless use of map by Anonymous Monk on Mar 18, 2010 at 10:57 UTC
Re^6: Possible useless use of map by almut (Canon) on Mar 18, 2010 at 11:23 UTC
Some notes below your chosen depth have not been shown here
Re^2: Possible useless use of map by JavaFan (Canon) on Mar 18, 2010 at 14:39 UTC
What bug? In: `map {$hr->{stuff}} 1` [download] there is no hash element in void context. It's in list context.	[reply] [d/l]
Re^3: Possible useless use of map by Anonymous Monk on Mar 18, 2010 at 14:48 UTC
perl590delta says map in void context is no longer expensive. map is now context aware, and will not construct a list if called in void context. if there is no list, why is there list context?	[reply]
Re^4: Possible useless use of map by JavaFan (Canon) on Mar 18, 2010 at 15:04 UTC
Just like about any other operator in Perl, `map` imposes a certain context on its operands, regardless of the context it itself is in. In: `map {$hash->{foo}} 1;` [download] there are actually two lists, both containing a single element: the first list consists of `$hash->{foo}`, the second of `1`. Think of it this way, if `map` would not provide list context to its inner block, the following wouldn't do what it does now: `%h = map {$_ => 1} qw[foo bar baz];` [download]	[reply] [d/l] [select]


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