Should it mutate or not? YES!

On Discord Hydrazer was looking for a list concatenation operator. That leaves the question if it should mutate like Array.push or return a new list of Slips.

sub infix:«|<<»(\a, \e) {
    Proxy.new(FETCH => method { |a, |e },
              STORE => method (\e) {})
              but role { method sink { a.push: e } };
}

my @a = 1,2,3;
@a |<< 4;
dd @a;
my @b = @a |<< 5;
dd @a, @b;

In sink-context returning a new list would not make sense. With a Proxy that provides a sink-method we can answer the question with “YES!”.

This made me wonder if Proxy should have the optional argument SINK. Right now there is no way to define containers in pure Raku. Even with subclassing we would have to decent into nqp-land. As newdisp has shown, it tends to be quite brittle to make modules depend on use nqp;`.