Re^4: Why do I get a "used only once" warning here?

erl has no way of knowing (as I should have from the name of the function, "log") that your function has a side-effect.

This is correct, but if we use an operator like ||=, &&=, .= and so on, implies that at least at that statement, the value of the variable is read AND written, so one could argue that this means that the variable is used at least twice. This has nothing to do with whether or not the expression on the right-hand side has side effects. In other words:

$P::x ||= (f(),1);
[download]

produces a warning, while the equivalent

$P::x = $P::x || (f(),1);
[download]

does not.

--
Ronald Fischer <ynnor@mm.st>

Comment on Re^4: Why do I get a "used only once" warning here? Select or Download Code

Replies are listed 'Best First'.
Re^5: Why do I get a "used only once" warning here? by chromatic (Archbishop) on Mar 16, 2009 at 09:19 UTC
... one could argue that this means that the variable is used at least twice. The warning is about the human-visible name, not the storage location.	[reply]
Re^5: Why do I get a "used only once" warning here? by bellaire (Hermit) on Mar 16, 2009 at 12:04 UTC
I understand, and you're right of course. This doesn't have to do with the semantics of the statement. It's an interesting case, and I think it illustrates that the logic for generating this warning is simplistic (as chromatic points out). In fairness, I think that's the intent of warnings: They aren't errors, they simply bring your attention to something that, in the many cases, may indicate a potential error. In this particular case, you know better, so the warning doesn't make sense. If you do decide to use an alternative syntax, FWIW I'd probably use an alternative syntax like: `if (not $P::x) { $P::x = 1; f(); }` [download] Not only is this considerably more understandable, IMO, but I ran some benchmarks, and the speed difference between this and the form using `\|\|=` is negligible. On the other hand, using the expanded syntax with `\|\|` is (surprisingly!) slower. `use strict; use warnings; use Benchmark qw( timethese cmpthese ); my $results = timethese( 5000, { orig => sub { delete $P::x{some_key}; for ( 1 .. 1000 ) { $P::x{some_key} \|\|= ( f(), 1 ); } }, alt_1 => sub { delete $P::x{some_key}; for ( 1 .. 1000 ) { $P::x{some_key} = $P::x{some_key} \|\| ( f(), 1 ); } }, alt_2 => sub { delete $P::x{some_key}; for ( 1 .. 1000 ) { if ( not $P::x{some_key} ) { $P::x{some_key} = 1; f(); } } }, } ); cmpthese($results); sub f() { #No op. }` [download] `Benchmark: timing 5000 iterations of alt_1, alt_2, orig... alt_1: 3 wallclock secs ( 3.08 usr + 0.00 sys = 3.08 CPU) @ 16 +23.38/s (n=5000) alt_2: 2 wallclock secs ( 1.29 usr + 0.00 sys = 1.29 CPU) @ 38 +75.97/s (n=5000) orig: 1 wallclock secs ( 1.25 usr + 0.00 sys = 1.25 CPU) @ 40 +00.00/s (n=5000) Rate alt_1 alt_2 orig alt_1 1623/s -- -58% -59% alt_2 3876/s 139% -- -3% orig 4000/s 146% 3% --` [download]	[reply] [d/l] [select]