How can I set a default file handle in a perl subroutine signature?

Question

I have a subroutine,

use experimental "signatures";

sub foo {
  my $fh = +shift;
  open ($fh, '>', 'default_file') unless defined $fh;
}

I would like to port that to subroutine signatures. Is it possible to set $fh to default to a filehandle that points to default_file .

Something like this

sub foo ($fh=open($fh,">",foo")) {}

or even with a do{} block,

sub foo ($fh=do { open($fh,">",foo"), $fh } ) {}

I know I can get this syntax if I use or create a wrapper. But it just seems like there should be a way of some sort to get this done without invoking IO::File and the like.

Answer 1

Can also cheat, just a little, and use a name-less and value-less optional argument

use experimental "signatures";

sub foo ($=) {
    my $fh = shift;
    open $fh, '>', "foo.txt" or die $!  if not defined $fh;
    say $fh 42;
    return $fh;
}

This fakes it in the sense that it processes arguments "normally" (as it would without signatures). However, one of nice things with signatures is that the @_ is kept pristine and can also be used

When using a signature, the arguments are still available in the special array variable @_ , in addition to the lexical variables of the signature.

I don't see a good reason to not use that. ^† Even as it pushes the argument processing back into the sub's body, you still get other benefits of signatures -- and an added measure of flexibility.

---

^† The stylistic point of handling arguments in the signature, including their defaults, is denied here anyway, merely by how much there is to do for this default.

Answer 2

Pending a better answer, I think the only method is to use a wrapper,

use IO::File;                                                              
sub foo ( $fh=IO::File->new("default_file", "w") ) {                                
  print $fh 42                                                             
}

Answer 3

The second attempt you showed is almost valid, you just need another variable to stand in since $fh doesn't exist yet when assigning the default; and to return it in a separate statement from where it's declared. You could abuse global variables to do this of course but that's ugly and leaky.

sub foo ($fh=do { open(my $d,">","foo"); $d } ) {}

This of course should have error handling.

sub foo ($fh=do { open(my $d,">","foo") or die "Failed to open 'foo': $!"; $d } ) {}

A proposed extension to signatures could be used to do it slightly differently, but I'm not sure if this is any better:

sub foo (?$d, $fh=(open($d,">","foo") || die "Failed to open 'foo': $!", $d) ) {}