p6.p6

class Proxee {
use MONKEY-GUTS;

proto method new(|) { * }

class Proxee::X::CannotBindStore is Exception {
    method message {
        'A Proxee cannot use :BIND and :STORE at the same time'
    }
}

multi method new(\coercer where {.HOW ~~ Metamodel::CoercionHOW}) {
    my \from     = coercer.^constraint_type;
    my \to       = coercer.^target_type;
    my $to-name := to.^name;

    my $STORAGE;
    Proxy.new: :FETCH{ $STORAGE }, STORE => -> $, \v {
        die X::TypeCheck.new: :got(v.WHAT), :expected(from), :operation<Proxee>
            unless nqp::istype(v, from); # on 2017.11 about 13x faster than ~~

        nqp::istype(v, to) ?? ($STORAGE := v)
                           !! ($STORAGE := v."$to-name"());
        $STORAGE
    }
}

multi method new (:&BIND, :&STORE, :&FETCH) {
    &BIND and &STORE and die Proxee::X::CannotBindStore.new;
    my $*PROXEE;
    my &final-store := &STORE || &BIND || { $*PROXEE := $_ };
    my &fetch = &FETCH || { $*PROXEE };
    Proxy.new: :FETCH{ fetch }, STORE => -> $, \v is raw { final-store v }
}}

my $v := Proxee.new: :STORE{ $*PROXEE := $_² };
$v = 42;