Perl6 also has been able to do this for a couple of years: sub fib(Int $n) { return $n if $n < 2; my $t = start { fib($n - 2) } fib

Perl6 also has been able to do this for a couple of years: sub fib(Int $n) { return $n if $n < 2; my $t = start { fib($n - 2) } fib($n - 1) + await($t); }