porton porton - 2 months ago 12
Perl Question

How to change attribute types? (Perl Moo)

We use Perl Moo.

Let there is defined a set of attributes:

package C;
use Moo;
use Types::Standard qw(Str Int Num Maybe);

has 'x' => (is=>'rw', isa=>Str);
has 'y' => (is=>'rw', isa=>Int);
has 'z' => (is=>'rw', isa=>Int);

# here to insert make_optional() described below

1;


I want to write a routine which will replace T with Maybe[T] for some attributes. For example:
make_optional(qw(x y))
should make type of
x
Maybe[Str]
and type of
y
Maybe[Int]
.

How to do it with Moo?

Answer

You can't.

Moo does not have a Meta Object Protocol. Without it, you cannot go back and alter stuff.

There is no meta object. If you need this level of complexity you need Moose - Moo is small because it explicitly does not provide a metaprotocol.

Furthermore, the types are just code refs.

There is no built-in type system. isa is verified with a coderef; if you need complex types, Type::Tiny can provide types, type libraries, and will work seamlessly with both Moo and Moose.


What you could do is maybe write a type that access some kind of singleton somewhere else to decide if it behaves like Maybe[Str] or Str, but that's a long shot and probably ugly and crazy and you shouldn't do it.

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