[Bioperl-l] $_ assignment question

[Dave Howorth]

Jonathan Barber wrote:
> On Thu, Jul 31, 2003 at 04:01:51PM -0400, Aaron J Mackey wrote:
>>Of course, this particular brand of pedanticness would indicate that this
>>construct:
>>
>>while(<FH>) {
>>  # ...
>>}
>>
>>... should also never be used.
> 
> Yep. At least not in a module. The reason being if I do this:
> 
> for (qw(different sequence filehandes)) {
>     $Bioperl_object->random_method($_); # method assigns to $_
>     do_something_else_with_fh($_);
> }
> 
> then do_something_else() is not getting what I expect it to get.

I was interested by this, so I ran the code (Perl 5.6.1). Turns out it 
won't run. Perl says:

   Modification of a read-only value attempted at Test.pm line 14.

It does this even with warnings and strict OFF. I'd say this was strong 
supporting evidence that Jonathan is right :) It also seems that:
  (i)  problems of this kind will show up pretty quickly and
  (ii) they can be isolated by putting:

for ('once') { ... }

around the tests of any (all!) method calls.

Cheers, Dave


test.pl
=======
#!/usr/bin/perl

use lib ('.');
use Test;

for (qw(hello world)) {
  print "\nA $_\n\n";
  Test::g();
  print "\nB $_\n";
}

Test.pm
=======
#!/usr/bin/perl

package Test;

require Exporter;

our @ISA       = qw(Exporter);
our @EXPORT    = qw();
our $VERSION   = 1.00;

sub g
{
  open FH, '<', 'test.pl';
  while (<FH>) {
   print $_;
  }
}


-- 
Dave Howorth
MRC Centre for Protein Engineering
Hills Road, Cambridge, CB2 2QH
01223 252960