偶数斐波那契数列

作者：Mark A. Hershberger

https://projecteuler.net/problem=2

斐波那契数列中的每一项都是前两项的和。从 1 和 2 开始，前 10 项是

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

考虑斐波那契数列中值不超过四百万的项，求所有偶数项的和。

源代码： prob002-hexmode.pl

use v6;

# code exposes a bug in r34733 rakudo, but works in pugs

class Fibonacci {
    has @!list = (0, 1);

    method next() {
        @!list[2] = [+] @!list;
        shift @!list;
        return @!list[1];
    }
}

my $fibber = Fibonacci.new;
my $f;

my @r = gather {
    $f = $fibber.next;
    while $f < 4000000 {
        take (0+$f) if $f % 2 == 0;
        $f = $fibber.next;
    }
}

say [+] @r;