Nice writeup. People keep getting confused by context and it's a fairly simple idea, even if their are a few edge cases that throw people off.

And while on the topic of context, if I keep responding to posts like this, people will think I'm planting shills in the audience :) Naturally, that means that what follows is a shameless plug for a module that makes dealing with context much easier (for subroutines).

Context is a great thing, but it trips up many people. It trips use people using other's code:

    my $foo   = some_func();
    my ($foo) = some_func(); # may or may not behave the same!
[download]

It also trips up people writing code for others to use:

    sub some_func {
        # do stuff
        return wantarray ? @results : \@results; # is void context OK?
    }
[download]

To solve this problem from the perspective of those writing code for others to use, I posted an RFC for Sub::Attributes. After some great feedback, I uploaded a module to the CPAN named Attribute::Context. It's easy to use. If you want to return an array, but return a reference in scalar context and warn about using it in void context, it used to be that you would have to write something like this:

    sub some_func {
        # do stuff
        return wantarray
            ? @results
            : defined wantarray
                ? \@results
                : warn "Useless void context";
    }
[download]

Not only is that ugly, it's also confusing and easy to write incorrectly. The if-else chain is hardly better:

    sub some_func {
        # do stuff
        if (wantarray) {
            return @results;
        elsif (defined wantarray) {
            return \@results;
        else {
            warn "this is tedious";
        }
    }
[download]

If you want all of your subroutines to have this behavior, it's going to get awfully tedious to code that every time. Now, you can just write this:

    use Attribute::Context;

    sub some_func : Arrayref(WARNVOID) {
        # do stuff
        return @results;
    }
[download]

That's much easier to write, easier on the eyes, and is less likely to be buggy!

I also got rid of the nasty Iterator in exchange for a Custom attribute that returns an object in scalar context:

    sub some_func : Custom(Some::Class) {
        # do stuff
        return @results;
    } # scalar context returns Some::Class->new(\@results)
[download]

I'll update the Custom attribute to be more flexible if people find it useful. There are also attributes for First (like CGI::param), Last and Count. More attributes may be added if requested.

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