What's the proper way to create a BUILD method in MooseX::Declare?

Question

I am having difficulty with the BUILD method in MooseX::Declare. If I say:

#!/usr/bin/perl

use MooseX::Declare;

class Foo {
    has foo => (is => "rw", isa => "Str", default => "foo");

    method BUILD {
     print "I was called\n";
    }
}

Foo->new;

I get the following less than helpful error message:

Reference found where even-sized list expected at /Users/cowens/perl5/lib/perl5/MooseX/Method/Signatures/Meta/Method.pm line 335.
Validation failed for 'MooseX::Types::Structured::Tuple[MooseX::Types::Structured::Tuple[Object],MooseX::Types::Structured::Dict[]]' failed with value [ [ Foo=HASH(0x804b20) ], { HASH(0x8049e0) => undef } ], Internal Validation Error is: Validation failed for 'MooseX::Types::Structured::Dict[]' failed with value { HASH(0x8049e0) => undef } at /Users/cowens/perl5/lib/perl5/MooseX/Method/Signatures/Meta/Method.pm line 365
        MooseX::Method::Signatures::Meta::Method::validate('MooseX::Method::Signatures::Meta::Method=HASH(0xb8aab0)', 'ARRAY(0xb8ab30)') called at /Users/cowens/perl5/lib/perl5/MooseX/Method/Signatures/Meta/Method.pm line 139
        Foo::BUILD('Foo=HASH(0x804b20)', 'HASH(0x8049e0)') called at generated method (unknown origin) line 25
        Foo::new('Foo') called at test.pl line 13

But if I say:

#!/usr/bin/perl

use MooseX::Declare;

class Foo {
    has foo => (is => "rw", isa => "Str", default => "foo");

    sub BUILD {
     my $self = shift;
     print "I was called\n";
    }
}

Foo->new;

everything works just fine (but is ugly and out of place with the rest of the code).

Answer 1

Perl understands sub and so an entry into the current package's symbol table is made. &Foo::BUILD, after Devel::Declare and other magic has created a package scope from the class closure.

Moose specifically looks for the BUILD sub to allow you to manipulate constructor logic. My guess (although I haven't traced it all the way through) is that the MooseX modules stay out of the way of what Moose is trying to do. So that a native BUILD is forever passed to the Moose magic to determine constructor logic.

On the other hand the method keyword is more Devel::Declare magic to create methods in the meta class structure.

Answer 2

It's failing because BUILD requires a one-arg method signature. By default, MooseX::Declare creates a signature which is not compatible with the way BUILD is called. (The details are murky to me.) I know because I ran into a similar error once. I certainly agree the error message could be more enlightening; that's true with a lot of Moose stuff.

Anyway, I got it to work like this:

use MooseX::Declare;

class Foo {
    has foo => (is => "rw", isa => "Str", default => "foo");

    method BUILD(Item $href) {
        print "I was called\n";
    }
}

Foo->new;

Hope that helps.

You can fiddle with the signature and try more specific types; I think Moose sends a hashref of the as-yet-unblessed object as the parameter.

Answer 3

BUILD takes an arg, if you don't need it, just say:

method BUILD($) { ... }