a declaring

I'm not sure, but why this can work?

use strict;
use warnings;
use Data::Dumper;

my $y=0;
my @x =(1,2,3) if $y;
print Dumper \@x;


Since $y is false, it seems @x shouldn't be declared.
But why the last print can work?

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Re: a declaring

J. Peng wrote:
> I'm not sure, but why this can work?
>
> use strict;
> use warnings;
> use Data::Dumper;
>
> my $y=0;
> my @x =(1,2,3) if $y;
> print Dumper \@x;
>
>
> Since $y is false, it seems @x shouldn't be declared.

Please read the last para in the "Statement Modifiers" section in
"perldoc perlrun".

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Re: a declaring

Gunnar Hjalmarsson wrote:
> J. Peng wrote:
>> I'm not sure, but why this can work?
>>
>> use strict;
>> use warnings;
>> use Data::Dumper;
>>
>> my $y=0;
>> my @x =(1,2,3) if $y;
>> print Dumper \@x;
>>
>> Since $y is false, it seems @x shouldn't be declared.
>
> Please read the last para in the "Statement Modifiers" section in
> "perldoc perlrun".

I should add that variable declaration happens at compile time, while
assigning happens at run time.

C:\home>type test.pl
my $var = 123;
BEGIN { print "\$var is $var at compile time.\n" }
print "\$var is $var at run time.\n";

C:\home>test.pl
$var is at compile time.
$var is 123 at run time.

C:\home>

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Re: a declaring

On Mon, Apr 21, 2008 at 11:37 AM, Gunnar Hjalmarsson wrote:
snip
> Please read the last para in the "Statement Modifiers" section in "perldoc
> perlrun".
snip

I think you mean perlsyn, not perlrun:
http://perldoc.perl.org/perlsyn.html#Statement-Modifiers

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Re: a declaring

On Mon, Apr 21, 2008 at 10:40 AM, J. Peng wrote:
> I'm not sure, but why this can work?
>
> use strict;
> use warnings;
> use Data::Dumper;
>
> my $y=0;
> my @x =(1,2,3) if $y;
> print Dumper \@x;
>
>
> Since $y is false, it seems @x shouldn't be declared.
> But why the last print can work?
snip

Because of a quirk in how the current and past versions of perl parsed
and handled the statement. It is a mis-feature according to Larry.
Some people used to use it to create state variables. The correct way
to create a state variable prior to 5.10 is with a closure:

{
my @x = (1, 2, 3);
sub function_that_uses_x {
...
}
}

In Perl 5.10 we got the state* function that works like the my
function but does not reinitialize each time through:

sub function_that_uses_x {
state @x = (1, 2, 3);
...
}

Of course, the old way is still handy if you need to share a state
value between functions:

{
my $shared = 0;
sub f1 {
...
}
sub f2 {
...
}
}

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Re: a declaring

Chas. Owens wrote:
> On Mon, Apr 21, 2008 at 11:37 AM, Gunnar Hjalmarsson wrote:
> snip
>> Please read the last para in the "Statement Modifiers" section in "perldoc
>> perlrun".
> snip
>
> I think you mean perlsyn, not perlrun:
> http://perldoc.perl.org/perlsyn.html#Statement-Modifiers

Yes I did; thanks for the correction.

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Re: a declaring

On Tue, Apr 22, 2008 at 1:58 AM, Chas. Owens wrote:

> Because of a quirk in how the current and past versions of perl parsed
> and handled the statement. It is a mis-feature according to Larry.

Chas, do you mean this is a bad style to declare and assign a variable
like below?

my $y=0;
my @x =(1,2,3) if $y;
print Dumper \@x;


Thanks!

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Re: a declaring

On Mon, Apr 21, 2008 at 9:25 PM, J. Peng wrote:
> On Tue, Apr 22, 2008 at 1:58 AM, Chas. Owens wrote:
>
> > Because of a quirk in how the current and past versions of perl parsed
> > and handled the statement. It is a mis-feature according to Larry.
>
> Chas, do you mean this is a bad style to declare and assign a variable
> like below?
>
>
> my $y=0;
> my @x =(1,2,3) if $y;
> print Dumper \@x;
snip

I mean it is an abuse of a bug. The proper way to achieve that
behavior is to either use the state function in 5.10 or closures in
earlier versions.

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The most important skill a programmer can have is the ability to read.

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