Handling Context: an Example - Page 22

Putting all the above together, here is a final version of list_files that handles both scalar, list, and void contexts, along with a sample program to test it out in each of the three contexts:

#!/usr/bin/perl
# listfile.pl
use warnings;
use strict;

sub list_files {
  die "Function called in void context" unless defined wantarray;
  my $path = shift;

  return wantarray?():undef unless defined $path;
  chomp $path;
  # remove trailing linefeed, if present
  $path.='/*' unless $path =~/\*/;
  # add wildcard if missing
  my @files = glob $path;
  return wantarray?@files:\@files;
}

print "Enter Path: ";
my $path = <>;

# call subroutine in list context
print "Get files as list:\n";
my @files = list_files($path);
foreach (sort @files) {
  print "\t$_\n";
}

# call subroutine in scalar context
print "Get files as scalar:\n";
my $files = list_files($path);
foreach (sort @{$files}) {
  print "\t$_  \n";
}

# to get a count we must now do so explicitly with $#...
# note that 'scalar would not work, it forces scalar context.
my $count = $#{list_files($path)}+1;
  print "Count: $count files\n";

# call subroutine void context - generates an error
list_files($path);

The name wantarray is something of a misnomer, since there is no such thing as 'array context'. A better name for it would have been wantlist.

Determining and Responding to the Calling Context - Page 21
Professional Perl Programming
Closures - Page 23