in reply to SOLVED: Looking for suitable spells to get open to return a filehandle from a module

Hello talexb,

You are passing in $fh as a scalar, then overwriting it in the line $fh = Symbol::gensym;, so $fh works correctly inside Extra::open but remains undef in the caller. You need to pass in a reference, and dereference it accordingly within Extra::open:

{
    my $fh;
    Extra::open ( \$fh, '>', 'extra-open.txt' ) or
        die "2. Open failed: $!";
    defined $fh or die "2. Filehandle not defined";
    print $fh "Extra open works fine.\n";
    close ( $fh );
}

[download]
package Extra;

use Symbol;

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

1;

[download]

In my (minimal) testing, this writes the correct messages to the two output files, and no error is generated.

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

Replies are listed 'Best First'.
Re^2: Looking for suitable spells to get open to return a filehandle from a module
by afoken (Chancellor) on Nov 28, 2016 at 06:17 UTC 
    package Extra;

use Symbol;

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

1;

    [download]

    I think this should work without explicit references:

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

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

    [download]

    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;
}

    [download]

    Alexander

    --
    Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)
[reply]
[d/l]
[select]

      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";

}

      [download]
      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;

      [download]
      (Tested under ActiveState 5.8.9 and Strawberry 5.14.4.1.)

      Give a man a fish:  <%-{-{-{-<

[reply]
[d/l]
[select]
        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

        --
        Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)
[reply]
[d/l]
[select]
      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).
[reply]
Re^2: Looking for suitable spells to get open to return a filehandle from a module
by talexb (Chancellor) on Nov 28, 2016 at 13:34 UTC

    Brilliant. Thanks!

    Alex / talexb / Toronto

    Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.

[reply]

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