On Fri, 26 Aug 2011 20:08:31 +0100
Rob Dixon <rob.di...@gmx.com> wrote:
> On 26/08/2011 18:12, Jim Gibson wrote:
> > On 8/25/11 Thu Aug 25, 2011 5:20 PM, "Rob Dixon"<rob.di...@gmx.com
> > scribbled:
> >> On 25/08/2011 20:36, Shlomi Fish wrote:
> >>>
> >>> If you want to use $_ so be it, but it can easily introduce
> >>> subtle errors into your code, because $_ is so easy to modify and
> >>> clobber. So I would recommend against these, and still think it's
> >>> a good idea.
> >>
> >> Please substantiate this assertion. I believe your recommendation
> >> deserves to be put in the bin of ill-conceived dogma.
> >
> > Shlomi gave a link to a case where somebody had to find a subtle error
> > caused by $_ being clobbered. I would only disagree with Shlomi's use of the
> > term "easily". I hope we can all agree that $_ can be overwritten
> > inadvertently, but it does not happen very often.
> >
> > Nevertheless, beginning Perl programmers should be cautioned that it can
> > happen, and they should be encouraged to use explicitly-named variables for
> > complex loops and blocks and especially for any block that calls a
> > subroutine or system function.
> >
> > We need to respect the people who post here seeking help. They need to be
> > told about those Perl issues that are not obvious, such as $_ being
> > overwritten. They do not need to be told "never use $_ by default because it
> > can be overwritten". It is up to each programmer to choose whether or not to
> > use $_ as a default variable and risk it getting clobbered by mistake. They
> > do need the information on which to base this decision.
> >
> > They also need to be encouraged to use "best practices". However, what
> > exactly best practices are is open to debate.
>
> I believe the fragility of $_ is apocryphal, and Shlomi's insistence on
> avoiding it is akin to remembering always to close the fridge door to
> keep the elephants out. If nothing else, using named variables
> everywhere adds noise to a program and makes it frustrating to read.
>
> As Randal says, $_ is localized everywhere it is used implicitly by a
> loop, including foreach, map and grep as well as the List::Util
> functions, so I believe that corrupting it is rarely a problem in
> practice. The significant exception is while loops, which do not
> preserve $_ and so could be said to be prone to unexpected errors.
>
> If I was to set rules, it would be that $_ should never be modified
> either explicitly or by the condition in a while loop. The latter most
> commonly appears in a read loop
>
> while (<$fh>) {
> :
> }
>
> which is more safely written as
>
> while (my $record = <$fh>) {
> :
> }
>
> especially now that the semantics of such a read loop now correctly
> check the definedness of the data read, and is equivalent to
>
> while (defined(my $record = <$fh>)) {
> print $record;
> }
>
> But I would be sad to rule against niceties like
>
> not /^#/ and print for <$fh>;
>
> As an aside, the same applies to the common warning against using the
> global values $a and $b 'because they are used by sort'. Once again,
> sort localizes these variables within the comparison code so there is
> very little chance of inadvertently modifying their values.
>
The problem starts to happen when you try to declare $a and $b using my. This
program:
[CODE]
#!/usr/bin/perl
use strict;
use warnings;
my $a = 5;
my $b = 6;
print map { "$_\n" } sort { $a <=> $b } (9,100,5,6,70,3,4,98,28,27);
[/CODE]
Yields this error:
[CODE]
Can't use "my $a" in sort comparison at test.pl line 9.
[/CODE]
This is perl-5.14.1 - previous versions of Perl may behave more
erratically when executing this. $a and $b are built-ins plain and simple, and
should be treated as such, due to their use by perldoc -f sort and other
functions from List::Util, List::MoreUtils, etc.
Another reason not to use them except for those cases is because "a" and "b"
are not very meaningful and indicative identifiers.
Regards,
Shlomi Fish
> Overly-careful warnings can have the opposite of the desired effect,
> especially on beginner programmers, and make it seem like the language
> is rife with pitfalls and gotchas, especially when these apply to
> ubiquitous core concepts like $_. I hope people will think twice about
> the ideas that they are conveying.
>
> Cheers all,
>
> Rob
>
--
-----------------------------------------------------------------
Shlomi Fish http://www.shlomifish.org/
Why I Love Perl - http://shlom.in/joy-of-perl
Sophie: Let’s suppose you have a table with 2^n cups…
Jack: Wait a second! Is ‘n’ a natural number?
Please reply to list if it's a mailing list post - http://shlom.in/reply .
--
To unsubscribe, e-mail: beginners-unsubscr...@perl.org
For additional commands, e-mail: beginners-h...@perl.org
http://learn.perl.org/
