Re: [MacPerl] defining variables if not already defined

[Bruce Van Allen <bva@cruzio.com>]

At 2:24 PM 7/28/00, Stewart Leicester wrote:
>Now I'm confused. I thought that "my $test = 1 if 0;" would 
>lexically scope $test to foo alone but it appears that only happens 
>if "my $test = 1" is executed.
>
>What am I missing?
>
>Stewart

At 6:02 PM 7/28/00, Keary Suska wrote:
>This is a rather bizarre side effect, but I think what Bart is trying to
>[snip]
>"$test" as a lexically scoped variable, and ends up creating a weird ad-hoc
>static scope (vis a vis C). An interesting exploit (if you want a static

I'm confused by your confusion :-) Study Perl precedence and control 
flow. The statement
   my $test = 1 if 0

is equivalent, control-flow-wise, to:

if 0 {
   my $test = 1
}

In Bart's example, this condition is never reached*. The third time 
foo is called, $test still has the value from the second time. If you 
ran this code with -w, you'd get a complaint about $test being 
unititialized.

*By the way, what's being tested for a zero (or false, perhaps) value 
here, anyway? @_ in a sub isn't the same as implicit $_ in a foreach 
loop.

>
>>On Fri, 28 Jul 00 12:28:59 +0900, Joel Rees wrote:
>>
>>   >Does my() always operate at compile time? Does that mean that my hopes
>>   >for easy run-time allocation must be dashed?
>>

>Bart Lateur replied:
>>my() has both a compile time, and a runtime effect.
>>
>>At compile time, it takes care of scoping issues. So, you cannot
>>possibly use a variable in fixed code, that is allocated at runtime in
>>eval STRING. The mere fact that a variable is accessed, makes sure that
>>it is allocated, so these would refer to different variables.
>>
>>At runtime, it takes care of storage allocation.
>>
>>An example of the confusing side effects can be seen in:
>>
>>	$\ = "\n";
>>	print foo(10);
>>	print foo(20);
>>	print foo(10);
>>
>>	sub foo {
>>	    my ($comp) = @_;
>>	    my $test = 1 if 0;
>>	    $test = 2 if $comp == 20;
>>	    $test || 3;
>>	}
>>
>>This prints:
>>
>>	3
>>	2
>>	2
>>
>>So, don't do this.
>>
>>In summary: you needn't worry about compile time allocation. You need
>>the declaration at compile time, to take proper care of scoping. But
[snip]
>>	Bart.


1;
-- 

- Bruce

__Bruce_Van_Allen___bva@cruzio.com__Santa_Cruz_CA__

# ===== Want to unsubscribe from this list?
# ===== Send mail with body "unsubscribe" to macperl-request@macperl.org