I just want to point out that in this case, using prototypes is unnecessary (and actually not even used), and it's advised that you don't use them unless you know why you're using them. They're good for emulating and/or overriding built-in operators for instance.

In the following:

sub collectLeaves ($) {
    my ( $ds, $leaves ) = @_;

[download]

You've said "I take a single scalar", but then extract from @_ after calling the sub with two distinct params, but it works. Why? Because when you call your subs the old fashioned way with &, the prototype is ignored. If you remove the & from the calls, your @_ will only contain the very last scalar element of all the params you've passed in. In fact, you'd get an error in your case, because you're sending in two params when the prototype says it only accepts one:

sub this ($){
    my @this = @_;
    print @this;
}

this(1, 1);

__END__
Too many arguments for main::this at ./sub.pl line 10, near "1)"
Execution of ./sub.pl aborted due to compilation errors.

[download]

Back to the calls with &. It is legacy code, and other than a few rare cases, is really only needed when dereferencing a code reference. In your case, it's actually doing more harm than good as it makes the code very confusing for future maintainers. I'd have a look at the code and remove the &, but then prototypes may break. I'd then have to hunt down and figure out whether the prototypes are required, and remove them one at a time if they aren't. Or, a newer Perl programmer may add a sub call without & and not even understand why errors are thrown on their call, but not the others.

Also, upon first glance at the sub definition, I may miss the $ prototype, but a my ($x, $y) = @_; will be abundantly clear. I would then be thinking why for the love of all things good is $y undef after calling the sub like func(@a);, where @a is a list with two elements. I'm passing in a list of two items, but $x is the last element of the list and $y is completely borked. Prototypes are a very uncommon thing, and unless you've dealt with them (most haven't), the errors and especially the silent modifications can be quite the treat to figure out.

In reply to Re^2: howto map/grep a complex structure by stevieb
in thread howto map/grep a complex structure by Anonymous Monk

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