oracular (3) IO::Detect.3pm.gz
NAME
IO::Detect - 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>.
SEE ALSO
This module is an attempt to capture some of the wisdom from this PerlMonks thread
<http://www.perlmonks.org/?node_id=980665> into executable code.
Various other modules that may be of interest, in no particular order... Scalar::Does, Scalar::Util,
FileHandle, IO::Handle, IO::Handle::Util, IO::All, Path::Class, URI::file.
AUTHOR
Toby Inkster <tobyink@cpan.org>.
COPYRIGHT AND LICENCE
This software is copyright (c) 2012-2014, 2017 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.