gistfile1.txt

use v6;

use Slang::Tuxic;

class C {

    has Str $!foo = "A";
    has Str $!bar = "B";
    has Str $!baz = "C";        # Cannot be controlled from outside of class

    method foo (Str $s) { $!foo = $s; }
    method bar (Str $s) { $!bar = $s; }

    submethod BUILD (*%init) {
        for keys %init -> $attr {
            my @can = self.can (lc $attr) or die "Unsupported attribute: $attr";
            .(self, %init{$attr}) for @can;
            }
        self!sanity_check;
        }

    method !sanity_check {
        $!foo eq $!bar and die;
        }
    }

 C.new (foo => "D");
#C.new (box => "A"); # crash
 C.new (bar => "A"); # crash


=>

use of uninitialized value $!bar of type Str in string context  in method sanity_check at t.pl:22

use of uninitialized value $!foo of type Str in string context  in method sanity_check at t.pl:22