Fibonacci sequence

List Generator

This constructs the fibonacci sequence as a lazy infinite list.

constant @fib = 0, 1, *+* ... *;

If you really need a function for it:

sub fib ($n) { @fib[$n] }

To support negative indices:

constant @neg-fib = 0, 1, *-* ... *;
sub fib ($n) { $n >= 0 ?? @fib[$n] !! @neg-fib[-$n] }

Iterative

sub fib (Int $n --> Int) {
    $n > 1 or return $n;
    my ($prev, $this) = 0, 1;
    ($prev, $this) = $this, $this + $prev for 1 ..^ $n;
    return $this;
}

Recursive

proto fib (Int $n --> Int) {*}
multi fib (0)  { 0 }
multi fib (1)  { 1 }
multi fib ($n) { fib($n - 1) + fib($n - 2) }