Rewriting a recursive function in perl so that it can be used in list context

Consider the binary tree developed in Moose::Cookbook::Basics::Recipe3

To retrieve all nodes in preorder, I could add the following subroutine to the BinaryTree package

sub pre_order {
  my ($self,$aref) = @_;

  push @$aref, $self-开发者_运维百科>node;

  pre_order($self->left,$aref) if $self->has_left;
  pre_order($self->right,$aref) if $self->has_right;
}

The sub would have to be used like this:

my $btree = BinaryTree->new;
#add some nodes

#then later...
my @nodes_in_preorder;
$btree->pre_order(\@nodes_in_preorder);

How would i have to change the subroutine to be able to use syntax like the below:

my @nodes_in_preorder = $btree->pre_order();

in order to be able to do things like

for ($btree->pre_order()) { #bla bla } 

later on.

Does this make sense, or am I being to pedantic?

How about:

sub pre_order {
  my $self = shift;
  return ($self->node,
    $self->has_left ? $self->left->pre_order : (),
    $self->has_right ? $self->right->pre_order : ());
}

You could just change the caller:

for ( do { my @preorder; $btree->pre_order(\@preorder); @preorder } ) {

Or simply change the code like so:

sub pre_order {
    my ($self,$aref) = @_;

    push @$aref, $self->node;

    $self->left->pre_order($aref) if $self->has_left;
    $self->right->pre_order($aref) if $self->has_right;

    return @$aref if wantarray;
}

No need to pass an array reference to the outer call; one will be created automatically, so your code will Just Work (TM):

for ( $self->pre_order() ) {