Day 20 – Helping Santa with Roles

On Day 14 of the Advent calendar I stopped lurking and started learning Perl 6 by converting a Perl 5 Timer object into its Perl 6 equivalent. Day 14 ends with a ‘challenge’ to convert the Perl 6 Timer class into a role.

Perl 6 roles encapsulate functionality for ready reuse by classes and objects.

The Timer class is a good candidate for ‘role-ification’ – it’s simple and is potentially useful to all sorts of objects.

Santa wants to go Lean this year, don’t worry he’s still rotund – he just wants to apply Lean principles to the process of present giving. The Timer role will help identify bottlenecks so Santa Inc. can improve.

To turn the Timer class into a role we just swap class for role (i.e., s/class/role/):

role Timer {

    has $!start-time;   # private scalar storing the starting time
    has @!event-log;    # private list of events
                        # dashes-in-identifiers save keystrokes too

    method record ($event_description) {
        # initialise the start-time if this is the first time through
        $!start-time //= now;

        @!event-log.push({
            timestamp   => now - $!start-time, 
            description => $event_description
        });
    }

    method report {
        my @report_lines;

        for @!event-log -> %event {
            @report_lines.push("[%event<timestamp>.fmt("%.3f")] %event<description>");
        }
        return @report_lines.join("\n");
    }
}

So that’s the role declared. Now I need a class or object to compose it into – something that does the role. How about a Timer for Santa’s Helpers?

use Timer;

class SantasHelper does Timer {
    has @!presents = qw<train lego doll> ;

    method wrap-present($child's_name) {
        return @!presents.pick ~ " for " ~ $child's_name;            
    } 
}

A quick check in the REPL shows:

> use SantasHelper;
> my $bruce = SantasHelper.new;    # this helper has an Australian accent
> $bruce.^methods
record report wrap-present

Pointing to the Higher Order Workings (HOW) of the $bruce object via .^ shows the Timer role’s methods (record, report) are now combined with the methods for SantasHelper (wrap-present). Let’s start wrapping presents:

use SantasHelper;

my $helper = SantasHelper.new;

my @children = qw<Hugo Honey Willow>;

# using {@children} to interpolate the list
$helper.record("started wrapping presents for {@children}"); 

for @children -> $child {
    my $present = $helper.wrap-present($child);
    $helper.record("wrapped $present");
}
say $helper.report; 

The report shows:

[0.006] started wrapping presents for Hugo Honey Willow
[0.013] wrapped train for Hugo
[0.016] wrapped doll for Honey
[0.020] wrapped doll for Willow

Nice.

But now there’s a problem! Santa’s Lean approach has exposed a bottleneck. To wrap all the presents in time Santa will need to roll up his sleeves and start wrapping too. We need to refactor the wrap-present method into its own role so Santa can wrap-presents too:

role WrappingPresents {
    has @!presents = qw<doll train lego> ;

    method wrap-present($child's_name) {
        return @!presents.pick ~ " for " ~ $child's_name;            
    } 
}

It turns out we can dynamically add a role to an existing object with does:

> use Santa
> my $santa = Santa.new
Santa.new()
> use WrappingPresents
> $santa does WrappingPresents
Santa+{WrappingPresents}.new(name => Any)
> $santa.wrap-present('Emma')
lego for Emma
> $santa.wrap-present('Claire')
lego for Claire

Santa Inc is now back on track.

It’s been fun playing with Perl 6 roles. Coding in Perl 6 feels malleable and expressive – and I’ve only just started!

jnthn++ has more details about parameterised roles and what happens when you compose multiple roles with the same method signature.

For now I’ll leave the last word to Perl 6’s Santa:

> $santa.greeting();
> Happy Christmas!