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Re: [perl #131599] [RFC] Allow a Callable for `is default` that willgenerate default values

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Elizabeth Mattijsen via RT
July 21, 2018 06:57
Re: [perl #131599] [RFC] Allow a Callable for `is default` that willgenerate default values
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FWIW, with:

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 <> 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(;
> 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(-> $key {"$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.

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