$_[0] fails for file handle?

sophate has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks,

I have a simple program that passes a file handle as the only argument to a subroutine. <$_[0]> fails to return a line from the file. Instead, it returns something like "GLOB(0x1bcb22a0)". However, if I use another variable to store the file handle. It works!

Anyone has any idea?

#!/usr/bin/perl -w
use strict;
use warnings;
open my $FH, "/tmp/test.txt" || die "Failed to open /tmp/test.txt: $!\
+n";

## NOT WORKING !!
while (my $Line = GetLine($FH)) {
        print "$Line";
}

## NOT WORKING !!
sub GetLine {
       my $line = <$_[0]> ;
       return $line;
}
[download]

If I change the subroutine GetLine to the codes below. It works.

sub GetLine {
        my $FH = shift;
        my $line = <$FH> ;
        return $line;
}
[download]

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Re: $_[0] fails for file handle? by jwkrahn (Abbot) on Jul 17, 2012 at 04:45 UTC
That is because the `<>` operator has two functions. `my $line = <$_[0]> ;` is the same as `my $line = glob "$_[0]" ;` while `my $line = <$FH> ;` is the same as `my $line = readline $FH ;`	[reply] [d/l] [select]
Re^2: $_[0] fails for file handle? by sophate (Beadle) on Jul 17, 2012 at 04:55 UTC
Thanks for your reply. That explains the myth!	[reply]
Re: $_[0] fails for file handle? by AnomalousMonk (Archbishop) on Jul 17, 2012 at 17:03 UTC
`open my $FH, "/tmp/test.txt" \|\| die "Failed to open /tmp/test.txt: $!\ +n";` [download] BTW: This statement from the OP has a bug. The relatively high precedence of the `\|\|` logical-or operator means that the truth value of the file name is logical-ored with the value returned by the die built-in (which, of course, never returns). Because a file name will almost always be true (`'0'` is the only one I can think of that would be false), the short-circuiting of the `\|\|` operator means that `die` will almost never be evaluated and the statement will seem to 'work'. The `or` logical operator has a sufficiently low precedence to make this really work. (See perlop.) So the correct version of this idiom is something like `open my $FH, "/tmp/test.txt" or die "Failed to open /tmp/test.txt: $!\n";` or (less elegantly IMHO) `open(my $FH, "/tmp/test.txt") \|\| die "Failed to open /tmp/test.txt: $!\n";` Update: This bug is hinted at by the fact that in the deparse of toolic's reply, the `die` sub-expression has vanished! This is because the literal file name `"/tmp/test.txt"` is always true, so the compiler softly and silently optimizes the `die` away.	[reply] [d/l] [select]
Re: $_[0] fails for file handle? by toolic (Bishop) on Jul 17, 2012 at 13:12 UTC
You already have your answer, but here is a method to finding that answer. Tip #6 from the Basic debugging checklist: Check to see if your code is what you thought it was: `B::Deparse` `use warnings; use strict 'refs'; open my $FH, '/tmp/test.txt'; while (my $Line = GetLine($FH)) { do { print "$Line" }; } use File::Glob (); sub GetLine { use warnings; use strict 'refs'; my $line = glob($_[0]); return $line; }` [download]	[reply] [d/l] [select]