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Re^9: Moose and default values

by morgon (Deacon)
on Feb 20, 2013 at 23:28 UTC ( #1019864=note: print w/ replies, xml ) Need Help??


in reply to Re^8: Moose and default values
in thread Moose and default values

Were getting here :-)

Ideally I would want to declare everything at the attriute-level, e.g.:

has foo => (is => "ro", default => { value => "whatever", when => "fal +se" });
Can you do that as well?

I am not so much interested in the feature itself, but it is interesting to see some examples of Moose-metaprogramming at work...

Thanks!


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Re^10: Moose and default values
by tobyink (Abbot) on Feb 20, 2013 at 23:54 UTC

    In this case what I'd suggest is a custom type constraint.

    use Moose::Util::TypeConstraints; subtype 'TrueStr', as 'Str', where { !!$_ }; coerce 'TrueStr', from 'Any', via { $_ || "whatever" }; has foo => (is => "ro", isa => "TrueStr", coerce => 1);

    Then any value which is not a TrueStr will be coerced into one via the code $_ || "whatever".

    Of course, this also applies to accessors, not just the constructor. (Not a problem for a read-only attribute though!) And also, it means that if you've got a bunch of attributes like this, each with different defaults, you need to define a type constraint for each one.

    The alternative - attribute metaprogramming - starts off seeming simple enough, but becomes more complicated when you start thinking about immutability. (Attribute metaprogramming can be a real bitch when it touches on immutability.)

    A compromise would be to hook has to automatically build per-attribute type constraints and coecions. I submitted a patch to MooseX::AttributeShortcuts for precisely this feature some months ago, but it was only partly accepted: per-attribute constraints are now supported, but not coercions.

    package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

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