Question

I am new to the File::Slurp module, and on my first test with it, it was not giving the results I was expecting. It took me a while to figure it out, so now I am interested in why I was seeing this certain behavior.

My call to File::Slurp looked like this:

my @array = read_file( $file ) || die "Cannot read $file\n";

I included the “die” part because I am used to doing that when opening files. My @array would always end up with the entire contents of the file in the first element of the array. Finally I took out the “|| die” section, and it started working as I expected.

Here is an example to illustrate:

perl -de0

Loading DB routines from perl5db.pl version 1.22
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):   0
DB<1> use File::Slurp

DB<2> $file = '/usr/java6_64/copyright'

DB<3> x @array1 = read_file( $file )
0  'Licensed material - Property of IBM.'
1  'IBM(R) SDK, Java(TM) Technology Edition, Version 6'
2  'IBM(R) Runtime Environment, Java(TM) Technology Edition, Version 6'
3  ''
4  'Copyright Sun Microsystems Inc, 1992, 2008. All rights reserved.'
5  'Copyright IBM Corporation, 1998, 2009. All rights reserved.'
6  ''
7  'The Apache Software License, Version 1.1 and Version 2.0'
8  'Copyright 1999-2007 The Apache Software Foundation. All rights reserved.'
9  ''
10  'Other copyright acknowledgements can be found in the Notices file.'
11  ''
12  'The Java technology is owned and exclusively licensed by Sun Microsystems Inc.'
13  'Java and all Java-based trademarks and logos are trademarks or registered'
14  'trademarks of Sun Microsystems Inc.  in the United States and other countries.'
15  ''
16  'US Govt Users Restricted Rights - Use duplication or disclosure'
17  'restricted by GSA ADP Schedule Contract with IBM Corp.'
DB<4> x @array2 = read_file( $file ) || die "Cannot read $file\n";

0  'Licensed material - Property of IBM.
IBM(R) SDK, Java(TM) Technology Edition, Version 6
IBM(R) Runtime Environment, Java(TM) Technology Edition, Version 6

Copyright Sun Microsystems Inc, 1992, 2008. All rights reserved.
Copyright IBM Corporation, 1998, 2009. All rights reserved.

The Apache Software License, Version 1.1 and Version 2.0
Copyright 1999-2007 The Apache Software Foundation. All rights reserved.

Other copyright acknowledgements can be found in the Notices file.

The Java technology is owned and exclusively licensed by Sun Microsystems Inc.
Java and all Java-based trademarks and logos are trademarks or registered
trademarks of Sun Microsystems Inc.  in the United States and other countries.

US Govt Users Restricted Rights - Use duplication or disclosure
restricted by GSA ADP Schedule Contract with IBM Corp.
'

Why does the || die make a difference? I have a feeling this might be more of a Perl precedence question instead of a File::Slurp question. I looked in the File::Slurp module and it looks like it is set to croak if there is a problem, so I guess the proper way to do it is to allow File::Slurp to croak for you. Now I’m just curious why I was seeing these differences.

Answer 1

The || operator puts its left-hand operand in boolean (scalar) context:

C-style Logical Or

Binary || performs a short-circuit logical OR operation. That is, if the left operand is true, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated.

You could also discover this empirically:

#! /usr/bin/perl

sub lhs {
  my $ctx = wantarray;
  if (defined $ctx) {
    if ($ctx) { "list" }
    else      { "scalar" }
  }
  else { "void" }
}

print lhs || 0, "\n";

Output:

$ ./or-ctx.pl
scalar

To fix your program, you could modify it slightly:

my @array = eval { read_file("/etc/issue") };
die unless @array;

This is necessary only if you want to add to the error message (stored in the special variable $@ after the eval) because read_file will die if something goes wrong.