FWIW, with:
http://modules.perl6.org/dist/Hash::Restricted:cpan:ELIZABETH
one can restrict access to a hash to a certain set of keys:
use Hash::Restricted;
my %h is restricted = a => 42, b => 666; # restrict to keys at initialization
my %h is restricted<a b c>; # restrict to keys a, b, c
> On 18 Jul 2018, at 17:32, Alastair Douglas via RT
> <perl6-bugs-follo...@perl.org> wrote:
>
> On Mon, 19 Jun 2017 09:30:41 -0700, brad wrote:
>> The way Moose in Perl 5 works around this is to give it a subroutine
>
>> there currently isn't as far as I know, a way to do what you intended.
>
> I'd like this feature as well. I was in IRC asking about whether we could
> restrict a hash in the same way python does, such that %hash<missing-value>
> dies.
>
> It was noted that one can do
>
> my %h is default(Failure.new);
>
> This would put a Failure in anything that didn't exist, which would detonate
> whenever accessed. Presumably, this would be the same Failure each time, but
> that's probably OK.
>
> It means there is no way of generating a default based on access. I think
> that would look something like:
>
> my %h is default(-> $key { Failure.new("$key not provided") });
>
> But then how would it know to run the Callable to generate the default,
> rather than simply providing the Callable as the default? I have no answer
> for that.
