Executable library

The library can be written as a module:

module Hailstone {
    our sub hailstone($n) is export {
	$n, { $_ %% 2 ?? $_ div 2 !! $_ * 3 + 1 } ... 1
    }
}

sub MAIN {
    say "hailstone(27) = {.[^4]} [...] {.[*-4 .. *-1]}" given Hailstone::hailstone 27;
}

It can be run with:

$ raku Hailstone.rakumod

Output:

hailstone(27) = 27 82 41 124 [...] 8 4 2 1

It can then be used with a program such as:

use Hailstone;
my %score;
(1 .. 100_000).race.map: { %score{hailstone($_).elems}++ };
say "Most common length is {.key}, occurring {.value} times." given max :by(*.value), %score;

Called with a command line as:

$ RAKULIB=. raku test-hailstone.raku

The environment variable RAKULIB might be necessary if the file Hailstone.rakumod is not in the standard library path for Raku.