Day: May 11, 2006

A colleague at work just found this little gem in File::Copy::Recursive:

  my $two = join '-', ( stat $cur )[0,1] || '';

That should pick out the device and inode, join them with a hyphen and set $two to the empty string if the stat failed. However, there’s a precedence problem:

  # Intended code.
  my $two = join( '-', ( stat $cur )[0,1] ) || '';
  # Actual code.
  my $two = join '-', ( ( stat $cur )[0,1] ) || '';

Spot the difference? The || bit applies to the result of the list slice. But what happens when you use || on a list slice? Let’s look at the debugger…

    DB<1> x @statinfo = stat '/etc/hosts'
  0  2050
  1  327794
  2  33188
  3  1
  4  0
  5  0
  6  0
  7  332
  8  1089379886
  9  1134738486
  10  1134738486
  11  4096
  12  8
    DB<2> x @statinfo[0,1]
  0  2050
  1  327794
    DB<3> x @statinfo[0,1] || ''
  0  327794
    DB<4> x @statinfo[0,1,2] || ''
  0  33188

So it appears that putting a list slice into scalar context always returns the last element of the list. Weird. More info (including examples at perldoc/C-style Logical Or).

Hopefully, he’ll file a bug report as it was actually tripping him up…

Update: RT#19205