Re^2: Looking for suitable spells to get open to return a filehandle from a module

[afoken]

package Extra;

use Symbol;

sub open
{
    my ( $fh, $direction, $filename ) = @_;
    
    $$fh = Symbol::gensym;
    open ( $$fh, $direction, $filename );
}

1;

I think this should work without explicit references:

sub open
{
    my (undef, $direction, $filename)=@_;

    $_[0]=Symbol::gensym;
    open ($_[0],$direction,$filename);
}

The trick is not to copy the alias in @_, but to directly use the alias. Maybe you need to extend open to have a prototype of ($$$).

Anyway, I would prefer a modified open function to return a handle or undef instead of a boolean, so:

sub open
{
    my ($direction,$filename)=@_;
    open my $h,$direction,$filename or return;
    # alternatively: or die "Can't open $filename: $!";
    return $h;
}

Alexander

[AnomalousMonk]

What is the effect of the invocation of Symbol::gensym? This code seems to work just fine without it both with aliasing and with an automatic reference via a prototype (also minimally tested):
t_extra_open.pl:

use warnings;
use strict;

use Extra;

{
    my $filename = 'extra-open.txt';

    Extra::open my $fh, '>', $filename
        or die "opening '$filename' for write: $!";
    defined $fh or die "write filehandle not defined";
    print $fh "Extra open works just fine \n";
    print $fh "at ", scalar(localtime), "\n";
    close $fh or die "closing '$filename' after write";

    undef $fh;
    Extra::open $fh, '<', $filename
        or die "opening '$filename' for read: $!";
    defined $fh or die "read filehandle not defined";
    print '<<', <$fh>, '>>';
    close $fh or die "closing '$filename' after read";

}

Extra.pm:

package Extra;

sub open {  # works

    my (undef, $direction, $filename) = @_;

    return open $_[0], $direction, $filename;

    }

# sub open (\$@) {  # works
#
#     my ($sr_fh, $direction, $filename) = @_;
#
#     return open $$sr_fh, $direction, $filename;
#
#     }

1;

(Tested under ActiveState 5.8.9 and Strawberry 5.14.4.1.)

[afoken]

What is the effect of the invocation of Symbol::gensym?

In this case, just a lazily copied expression. Quoting Symbol:

Symbol::gensym creates an anonymous glob and returns a reference to it. Such a glob reference can be used as a file or directory handle.

And that's it. It was needed in ancient perls. Perl 5.6.1 introduced autovivication for file and directory handles (see perl561delta), so Symbol::gensym is technically no longer needed. open my $fh, ... or open local $fh, ... is sufficient.

Alexander

[pryrt]

... so Symbol::gensym is technically no longer needed

But there are times when it's still useful... like when you want open3() to keep STDOUT and STDERR separate (by default, if you pass it undef for both STDOUT and STDERR, STDERR will be directed to the same filehandle that's created for STDOUT)

#!/usr/bin/env perl
use warnings;
use strict;
use IPC::Open3;
use Symbol qw(gensym);
use POSIX qw(:sys_wait_h);

# I'm sometimes using perl 5.8.5, so //'<undef>' unavailable
# verified later on 5.14.2
sub defor($) { defined($_[0]) ? $_[0] : '<undef>' }

foreach my $run (1..3) {
    my ($out, $err) = ('','');
    my ($fhi, $fho, $fhe) =
        ($run == 1) ? (undef, undef, undef) :           # when fhe==un
+def goes to open3, STDERR will be redirected to STDOUT
        ($run == 2) ? (undef, undef, gensym) :          # this keeps S
+TDERR and STDOUT separate
                      (gensym, gensym, gensym) ;        # as does this
+, the first two gensym are unecessary, but the third is needed

    my $pid = open3( $fhi, $fho, $fhe, qq[cat - ; perl -le 'print STDE
+RR "this should be STDERR (run#$run)...\nand so should this (run#$run
+)\n"'] );
    print { $fhi } "[run#$run] This should be STDOUT\n";
    close($fhi);

    $out .= join '', map { s/^/STDOUT> / if $_ ne $/; $_ } readline($f
+ho)     if defined $fho;
    $err .= join '', map { s/^/STDOUT> / if $_ ne $/; $_ } readline($f
+he)     if defined $fhe;
    print "Read STDOUT:\n$out\n" if length($out);
    print "Read STDERR:\n$err\n" if length($err);
    printf "run#%d --- fhi='%s' fho='%s' fhe='%s'\n", $run, defor $fhi
+, defor $fho, defor $fhe;
    print "="x75, "\n\n";
}

__END__
Read STDOUT:
STDOUT> [run#1] This should be STDOUT
STDOUT> this should be STDERR (run#1)...
STDOUT> and so should this (run#1)


run#1 --- fhi='GLOB(0x92fdba4)' fho='GLOB(0x92fdbcc)' fhe='<undef>'
======================================================================
+=====

Read STDOUT:
STDOUT> [run#2] This should be STDOUT

Read STDERR:
STDOUT> this should be STDERR (run#2)...
STDOUT> and so should this (run#2)


run#2 --- fhi='GLOB(0x93811c0)' fho='GLOB(0x932bc60)' fhe='GLOB(0x92fd
+ba4)'
======================================================================
+=====

Read STDOUT:
STDOUT> [run#3] This should be STDOUT

Read STDERR:
STDOUT> this should be STDERR (run#3)...
STDOUT> and so should this (run#3)


run#3 --- fhi='GLOB(0x93811c0)' fho='GLOB(0x932bc60)' fhe='GLOB(0x92fd
+ba4)'
======================================================================
+=====

Yes, I know the OP didn't include open3, but since I was recently learning the ins and outs of open3 filehandles, I thought I'd mention this exception to your general rule, for posterity's sake.

[Laurent_R]

I would prefer a modified open function to return a handle

I agree that creating the filehandle within the sub and returning it to the caller is by far the simplest and the best solution (unless I miss something from the OP requirement).