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Re^3: Moose "unions" and inheritance

by tobyink (Abbot)
on Nov 30, 2012 at 12:06 UTC ( #1006465=note: print w/replies, xml ) Need Help??

in reply to Re^2: Moose "unions" and inheritance
in thread Moose "unions" and inheritance

"Starting to grok the code, I 1st wondered why the type constraints do not need to be written in a string. It seems use MooseX::Types::Moose is responsible for being able to do that."

Moose type constraints are not strings, they are instances of Moose::Meta::TypeConstraint. has (and indeed the Moose::Meta::Attribute class that powers has) just gives you a little sugar allowing you to indicate type constraints as strings. But internally it translates those to type constraint objects.

MooseX::Types basically gives you functions defined along these lines:

# It's actually a lot more complex than this, but just # pretend it's this simple... # sub HashRef () { ### empty prototype return Moose::Meta::TypeConstraint->lookup('HashRef'); }

So that you can use:

isa => HashRef

It also plays some fancy tricks overloading bitwise operators so that HashRef | ArrayRef "just works".

So everything I did in the previous example would work without MooseX::Types; you'd just need to do a bit of work with Moose::Meta::TypeConstraint objects, so it wouldn't look as pretty.

"Indicating, that only the 1st one in the inheritance hierarchy gets propagated. So I assume for it to work independent of how many parent classes were passed, one would have to loop over them in _existing_constraint (how?) and buildup the type constraint before returning it."

Multiple inheritance is always going to make things messier. For example, if you're inheriting from classes A and B where:

package A; has data => (is => 'ro'); package B; has data => (is => 'rw');

... what on earth would you expect to happen?!

In cases like this, I'd want the class that's doing the multi-inheritance to define as much of the data attribute as it can, and rely on its base classes' definitions as little as possible.

perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'

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Re^4: Moose "unions" and inheritance
by PetaMem (Priest) on Nov 30, 2012 at 13:49 UTC
    Moose type constraints are not strings, they are instances of Moose::Meta::TypeConstraint.
    Sure, I assumed they have semantics beyond 'being a String'. I just meant from the syntactic perspective, Moose::Manual::Types pretty much makes it look like the types are strings. At least
    ... isa => Str | ArrayRef, ...

    doesn't work without MooseX::Types (Bareword "Str" not allowed while "strict subs"...)

    Anyway, I tried to make this inherited union constraints "do what I mean" for multiple inheritance and came up with
    package ValAtom; use strict; use warnings; use 5.10.1; use Moose; use MooseX::Types::Moose -all; extends 'KeyAtom', 'JustTesting'; has '+data' => ( isa => __PACKAGE__->_existing_constraint('data') . ' | ArrayRe +f | HashRef', ); sub _existing_constraint { my ($class, $attr) = @_; return join ' | ', map { $_->meta->find_attribute_by_name($attr)->type_constraint } $class->meta->superclasses; } 1;
    Back to string again - obviously - as I couldn't figure out how to do it otherwise. But this seems to work.

        All Perl:   MT, NLP, NLU

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