Day 7 – that’s how we .roll these days

This is a public service announcement.

Since Wolfman++ wrote his blog post about .pick back in the year two thousand nine (nine years after the distant future), we’ve dropped the :replace flag, and we’ve gained a .roll method. (Actually, the change happened in September 2010, but no-one has advent blogged about it.)

So this is the new way to think about the two methods:

.pick    like picking pebbles out of an urn, without putting them back
.roll    like rolling a die

I don't remember how I felt about it back in 2010, but nowadays I'm all for keeping these methods separate. Also, it turns out, I use .roll($N) a whole lot more. In that light, I'm glad it isn't spelled .pick($N, :replace).

Just to give a concrete example:

> say <Rock Paper Scissor>.roll
Rock                    # good ol' Rock

.roll is equivalent to .roll(1). When we're only interested in one element, .roll and .pick work exactly the same since replacement doesn't enter into the picture. It's a matter of taste which one to use.

I just want to show two more tricks that .roll knows about. One is that you can do .roll on a Bag:

> my $bag = (white-ball => 9, black-ball => 1).Bag;
bag(white-ball(9), black-ball)
> say $bag.roll         # 90% chance I'll get white-ball...
white-ball              # ...told ya

Perfect for all your weighted-randomness needs.

The other trick is that you can .roll on an enumeration:

> enum Dwarfs <Dipsy Grumpy Happy Sleepy Snoopy Sneezy Dopey>
> Dwarfs.roll
Happy                   # me too!
> enum Die <⚀ ⚁ ⚂ ⚃ ⚄ ⚅>
> Die.roll
⚄
> Die.roll(2)
⚀ ⚀                     # snake eyes!
> Die.roll(5).sort
⚀ ⚀ ⚁ ⚂ ⚃

The most frequent use I make of this is the idiom Bool.roll for randomly tossing a coin in my code.

if Bool.roll {
    # heads, I win
}
else {
    # tails, you lose
}

Bool is an enum by spec, even though it isn't one in Rakudo (yet) for circularity saw reasons.

> say Die.HOW
Perl6::Metamodel::EnumHOW.new()
> say Bool.HOW
Perl6::Metamodel::ClassHOW.new()    # audience goes "awwww!"

This has been a public service announcement. Enjoy your holidays, and have a merry Perl 6.

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