D&D Rolls in Perl 6
Last week, I played my first D&D session with a couple of old friends from high school theatre. It was super fun and I haven’t really gotten it off of my mind since.
I was reading up on the game, and I found out that the dice notation that they use is actually rather intricate. Might be old news for most of my audience, but it’s new to me, and it piqued my interest.
A basic dice roll in dice notation looks like MdN
, where M
is the number of dice to roll and N
is the number of sides on the dice. So, something like 4d10
means “roll 4 dice with 10 sides”.
I took that and defined a custom operator in Perl 6 to roll any number of nsided dice. It looks like this:
sub infix:<d>(Int $n, Int $max) { (1..$max).pick xx $n }
That operator can be called just as the notation would suggest.
say 3 d 20;
# this rolls 3 dice with 20 sides
# when I ran it, I got
# => (17 1 12)
If you just wanted the final outcome of the roll, it’s easy enough to call .sum
on the output of the roll.
say (3 d 20).sum;
# when I ran this, it gave me
# => 16
D&D lets you add modifiers to your rolls as well. You can multiply and add together dice rolls or constants. For example, 2*(2d4+1)
will add 1 to the result of rolling 2d4, then multiply the whole roll by two.
This is super easy to replicate in Perl 6:
say 2 * ((2 d 4).sum + 1);
# gave me a result of 6
Even more exciting is the idea of exploding dice or open ended rolling. Some games represent it as a postfix !
, and it means “keep rolling and summing dice as long as you roll the max value”.
For example, if you were to roll 4d20 and get (1, 8, 20, 6)
, you’d add up the values 1, 8, 6 and 19 (it’s not possible to roll a 20 with this method). Once you’ve added all those, you can roll the 20 again. Say you got a roll of 11
. Then, your total roll for 4d20!
would be 1 + 8 + 6 + 19 + 11 = 45.
To set that up in Perl 6, I created a new infix operator b
(short for b
ang, since the bang symbol !
was taken). On the left side, it takes a list of dice rolls, and on the right side it takes the number to explode on. So, using it is a little more verbose than simply appending a !
to your dice roll, but it’s also more flexible and allows me to preserve the simplicity of the d
operator doing nothing but producing a list of numbers.
sub infix:<b>(@ds, Int $max) is assoc<left> {
( @ds, { .map: > $d { $d ~~ $max ?? ($d  1, (1 d $max)) !! $d } } ... * == * ).tail
}
(thanks to timotimo for an idea of how to write this function!)
It looks complicated, but it’s really just a lazy sequence that terminates when two elements are the same (that’s what ... * == *
signifies). It starts with the list of dice rolls @ds
, then iterates the function { .map: > $d { $d ~~ ($max) ?? ($d  1, (1 d $max)) !! $d }
until the dice list no longer changes. That function makes the dice explode: it maps the list of dice and appends new die rolls if it finds a die that’s equal to the max roll.
It can be used as follows:
say 3 d 4 b 4;
# on my machine, my
# first roll got me
# => (3 1 1 2 1)
Another fun thing to do is to remove the .tail
at the end of the definition of b
to actually see how the dice explode over time:
sub infix:<b>(@ds, Int $max) is assoc<left> { ( @ds, { .map: > $d { $d ~~ $max ?? ($d  1, (1 d $max)) !! $d } } ... * == * ) }
say 2 d 2 b 2;
# my first roll was
# => ((2 1) (1 2 1) (1 1 1 1) (1 1 1 1))
The initial roll was (2 1)
, and 2 is the max roll you can roll on a hypothetical d2 labelled 1 through 2. So, we go ahead and explode on the 2.
After exploding the first dice in the first roll, we actually end up rolling another 2. Now our rolls total (1 2 1)
and we can explode the 2 yet again.
Finally, we’re left with (1 1 1 1)
, our final roll.
Got any other cool ideas to implement and/or more fun facts about dice? Email or tweet me below.
Comments

/u/aaronsherman on Reddit made a much more in depth dice roll parser using Perl 6’s grammars. I think it’s a fantastic little bit of code. Here’s a permalink on GitHub.

Damian Conway commented that the correct semantics of an
MdN
dice roll was to roll M dice labelled 1 through N, not 0 through N  1. This was corrected throughout the post. He also offered up this alternate definition for thed
function, which is very succinct and I like it very much:sub infix:<d>(Int $n, Int $max) { (1..$max).roll($n) }