Jerald Sheets wrote:
On Mar 8, 2009, at 1:29 PM, Ron Bergin wrote:
On Mar 4, 4:46 am, que...@gmail.com (Jerald Sheets) wrote:
#!/usr/bin/perl -w
It's better to use the warnings pragma, instead of the -w switch
<snip>
... on the -w switch, here are Damian's words:
<quote>
18.8. Warnings
Always turn on warnings explicitly.
If you're developing under Perl 5.6 or later, always use warnings at
the start of each file. Under earlier versions of Perl, always use
the -w command-line flag, or set the $WARNING variable (available
fromuse English) to a true value.
Perl's warning system is invaluable. It can detect more than 200
different questionable programming practices, including common errors
like using the wrong sigil on an array access; trying to read an
output stream (or vice versa); leaving parentheses off ambiguous
declarations; runaway strings with no closing delimiter; dyslexic
assignment operators (=-, =+, etc.); using non-numbers as numbers;
using | instead of ||, or || instead of or; misspelling a package or
class name; mixing up \1 and $1 in a regex; ambiguous
subroutine/function calls; and improbable control flow (e.g.,
returning from a subroutine via a call to next).
</quote>
He appears to insinuate they are interchangeable with the phrase:
"always use the -w command-line flag, or set the $WARNING variable
(available fromuse English) to a true value."
No he does not. You failed to re-quote the "Under earlier versions of Perl".
It's true that for a program to be compatible with pre 5.6 Perl
versions, the dynamically scoped warnings feature is your only choice.
Otherwise you can choose, and as long as you know the differences, and
make your choice based on what you want to achieve, all is fine IMO. As
long as you do not know all the implications, you'd better believe the
experienced Perlers when they say that "use warnings;" is typically the
best choice.
Another note on this... I just perldoc'ed it to see what it had to
say:
DESCRIPTION
The "warnings" pragma is a replacement for the command line flag
"−w", but the pragma is limited to the enclosing block, while the
flag is global. See perllexwarn for more information.
If the pragma is limited to the enclosing block but the flag is
global *AND* it is considered best practice to remove the pragma when
distributing your program:
<quote>
ote that it may still be appropriate to comment out the use warnings
line when your application or module is deployed, especially if
non-technical users will interact with it, or if it will run in a CGI
or other embedded environment. Issuing warnings in these contexts can
needlessly alarm users, or cause server errors.
Don't remove the use warnings completely, though; when something goes
wrong, you'll want to uncomment it again so the reinstated warnings
can help you locate and fix the problem.
</quote>
I am failing to see where this would be preferable except in the case
of also using the "use English" pragma or many of the other options
that would be limited to the enclosing block.
"use English" has nothing to do with it. The dynamically scoped warnings
feature can be enabled via the -w switch or by assigning a true value to
the $^W variable directly. $WARNING is just an alias to $^W under "use
English".
If you really want to understand, I suggest that you read perllexwarn
carefully.
However, not everything in perllexwarn is true. Unlike the impression
that that piece of the docs gives you, it's fully possible to combine
the dynamically scoped warnings feature with "no warnings":
C:\home>type test.pl
$^W = 1;
$x = undef;
sub mysub {
no warnings;
print $x; # line 5
}
print $x; # line 7
mysub();
C:\home>test.pl
Use of uninitialized value $x in print at C:\home\test.pl line 7.
C:\home>
Unless "no warnings" had been effective, line 5 would also have
generated a warning.
--
Gunnar Hjalmarsson
Email: http://www.gunnar.cc/cgi-bin/contact.pl
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