IO::Detect(3) is this a frickin' filehandle or what?!

SYNOPSIS


use IO::Detect;

if (is_filehandle $fh)
{
my $line = <$fh>;
}

DESCRIPTION

It is stupidly complicated to detect whether a given scalar is a filehandle (or something filehandle like) in Perl. This module attempts to do so, but probably falls short in some cases. The primary advantage of using this module is that it gives you somebody to blame (me) if your code can't detect a filehandle.

The main use case for IO::Detect is for when you are writing functions and you want to allow the caller to pass a file as an argument without being fussy as to whether they pass a file name or a file handle.

Functions

Each function takes a single argument, or if called with no argument, operates on $_.
"is_filehandle $thing"
Theoretically returns true if and only if $thing is a file handle, or may be treated as a filehandle. That includes blessed references to filehandles, things that inherit from IO::Handle, etc.

It's never going to work 100%. What Perl allows you to use as a filehandle is mysterious and somewhat context-dependent, as the following code illustrates. 

        my $fh = "STD" . "OUT";
        print $fh "Hello World!\n";
"is_filename $thing"
Returns true if $thing is a IO::All object or Path::Class::Entity or ``any non-reference, non-zero-length string with no line breaks''. That's because depending on your operating system, virtually anything can be used as a filename. (In fact, on many systems, including Linux, filenames can contain line breaks. However, this is unlikely to be intentional.)

This function doesn't tell you whether $thing is an existing file on your system. It attempts to tell you whether $thing could possibly be a filename on some system somewhere.

"is_fileuri $thing"
Returns true if $thing is a URI beginning with ``file://''. It allows for URI objects, RDF::Trine::Node::Resource objects, strings and objects that overload stringification.

This function actually returns an ``interesting value of true''. The value returned is a URI::file object.

"as_filehandle $thing, $mode"
Returns $thing if it is a filehandle; otherwise opens it with mode $mode (croaking if it cannot be opened). $mode defaults to ``<'' (read access).

This function is not exported by default, but needs to be requested explicitly: 

        use IO::Detect qw(as_filehandle);

You may even specify a different default mode, or import it several times with different names: 

        use IO::Detect 
          as_filehandle => { -as => 'as_filehandle_read',  mode => '<' },
          as_filehandle => { -as => 'as_filehandle_write', mode => '>' };

Smart Matching

You can import three constants for use in smart matching: 

        use IO::Detect -smartmatch;

These constants are:

"FileHandle"
"FileName"
"FileUri"

They can be used like this: 

        if ($file ~~ FileHandle)
        {
                ...
        }

Note that there does exist a FileHandle package in Perl core. This module attempts to do the right thing so that "FileHandle->new" still works, but there are conceivably places this could go wrong, or be plain old confusing.

Although "is_filehandle" and its friends support Perl 5.8 and above, smart match is only available in Perl 5.10 onwards.

Use with Scalar::Does

The smart match constants can also be used with Scalar::Does: 

        if (does $file, FileHandle)
        {
                ...;
        }
        elsif (does $file, FileName)
        {
                ...;
        }

Precedence

Because there is some overlap/ambiguity between what is a filehandle and what is a filename, etc, if you need to detect between them, I recommend checking "is_filehandle" first, then "is_fileuri" and falling back to "is_filename". 

        for ($file)
        {
                when (FileHandle)  { ... }
                when (FileUri)     { ... }
                when (FileName)    { ... }
                default            { die "$file is not a file!" }
        }

Export

Like Scalar::Does, IO::Detect plays some tricks with namespace::clean to ensure that any functions it exports to your namespace are cleaned up when you're finished with them.

Duck Typing

In some cases you might be happy to accept something less than a complete file handle. In this case you can import a customised ``duck type'' test... 

        use IO::Detect
                -default,
                ducktype => {
                        -as     => 'is_slurpable',
                        methods => [qw(getlines close)],
                };
        
        sub do_something_with_a_file
        {
                my $f = shift;
                if ( is_filehandle $f or is_slurpable $f )
                        { ... }
                elsif ( is_filename $f )
                        { ... }
        }

Duck type test functions only test that the argument is blessed and can do all of the specified methods. They don't test any other aspect of ``filehandliness''.

BUGS

Please report any bugs to <http://rt.cpan.org/Dist/Display.html?Queue=IO-Detect>.

AUTHOR

Toby Inkster <[email protected]>.

COPYRIGHT AND LICENCE

This software is copyright (c) 2012-2014 by Toby Inkster.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.

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