Defining a MX::Declare method as a attribute trigger - perl

Question

The following code works as I'd expect. The cached lazy attribute gets cleared and rebuilt when the foo attribute it depends on is changed.
use MooseX::Declare;
use 5.010;
class Test {
has foo => ( isa => 'Str', is => 'rw', trigger => sub {my $self = shift; $self->clearer}, default => '' );
has lazy => ( isa => 'Str', is => 'ro', lazy => 1, clearer => 'clearer',
default => method { say 'building lazy'; return "foo is '".$self->foo."'"; },
);
method say ( ) {
say $self->lazy;
}
}
my $inst = Test->new( foo => 'baz' );
$inst->say;
$inst->say;
say $inst->foo();
$inst->foo('bar');
$inst->say;
output:
building lazy
foo is 'baz'
foo is 'baz'
baz
building lazy
foo is 'bar'
How do I, however, use the MX::Declare sugar for the trigger subroutine? Defining foo as:
has foo => ( isa => 'Str', is => 'rw', trigger => method {$self->clearer}, default => '' );
Results in the class dying on compilation (below). Am I doing something wrong with my anonymous method declaration?
Trigger must be a CODE ref on attribute (foo) at
C:/Strawberry/perl/site/lib/Moose/Meta/Attribute.pm line 423
Moose::Meta::Attribute::_process_trigger_option('Moose::Meta::Attribute',
'foo', 'HASH(0x2a5d14c)') called at C:
/Strawberry/perl/site/lib/Moose/Meta/Attribute.pm line 299
Moose::Meta::Attribute::_process_options('Moose::Meta::Attribute',
'foo', 'HASH(0x2a5d14c)') called at C:/Strawb
erry/perl/site/lib/Moose/Meta/Attribute.pm line 88
Moose::Meta::Attribute::new('Moose::Meta::Attribute', 'foo', 'trigger', 'MooseX::Method::Signatures::Meta::Metho
d=HASH(0x39a421c)', 'isa', 'Str', 'definition_context',
'HASH(0x3452184)', 'default', '', 'is', 'rw') called at C:/Straw
berry/perl/site/lib/Moose/Meta/Attribute.pm line 114
Moose::Meta::Attribute::interpolate_class_and_new('Moose::Meta::Attribute',
'foo', 'trigger', 'MooseX::Method::S
ignatures::Meta::Method=HASH(0x39a421c)', 'isa', 'Str', 'default', '',
'definition_context', 'HASH(0x3452184)', 'is', 'r w') called at
C:/Strawberry/perl/site/lib/Moose/Meta/Class.pm line 704
Moose::Meta::Class::_process_new_attribute('Moose::Meta::Class=HASH(0x38c79d4)',
'foo', 'trigger', 'MooseX::Meth
od::Signatures::Meta::Method=HASH(0x39a421c)', 'isa', 'Str',
'default', '', 'definition_context', 'HASH(0x3452184)', 'is ', 'rw')
called at C:/Strawberry/perl/site/lib/Moose/Meta/Class.pm line 697
Moose::Meta::Class::_process_attribute('Moose::Meta::Class=HASH(0x38c79d4)',
'foo', 'trigger', 'MooseX::Method::
Signatures::Meta::Method=HASH(0x39a421c)', 'isa', 'Str', 'default',
'', 'definition_context', 'HASH(0x3452184)', 'is', ' rw') called at
C:/Strawberry/perl/site/lib/Moose/Meta/Class.pm line 566
Moose::Meta::Class::add_attribute('Moose::Meta::Class=HASH(0x38c79d4)',
'foo', 'trigger', 'MooseX::Method::Signa
tures::Meta::Method=HASH(0x39a421c)', 'isa', 'Str', 'default', '',
'definition_context', 'HASH(0x3452184)', 'is', 'rw') called at
C:/Strawberry/perl/site/lib/Moose.pm line 77
Moose::has('Moose::Meta::Class=HASH(0x38c79d4)', 'foo', 'isa', 'Str', 'is', 'rw', 'trigger', 'MooseX::Method::Si
gnatures::Meta::Method=HASH(0x39a421c)', 'default', '') called at
C:/Strawberry/perl/site/lib/Moose/Exporter.pm line 356
Moose::has('foo', 'isa', 'Str', 'is', 'rw', 'trigger', 'MooseX::Method::Signatures::Meta::Method=HASH(0x39a421c) ',
'default', '') called at mx_declare.pl line 5
main::ANON() called at C:/Strawberry/perl/site/lib/MooseX/Declare/Syntax/MooseSetup.pm line
81
MooseX::Declare::Syntax::MooseSetup::ANON('CODE(0x38c3a94)')
called at mx_declare.pl line 13

Answer 1

The method keyword returns an instance of the MooseX::Method::Signatures::Meta::Method class, which is a subclass of Moose::Meta::Method, which is a subclass of Class::MOP::Method.
Moose allows a method object for default, but not for trigger, which must be a regular coderef.
If you really want to use the method keyword there, you could probably do:
trigger => method { $self->clearer }->body,
But it's probably easier (and saner) to do what #cjm suggests and just use a regular coderef:
trigger => sub { shift->clearer },

Answer 2

You can't. method returns an object, not a plain coderef. However, you can write this even more concisely than method would allow:
has foo => ( isa => 'Str', is => 'rw', trigger => sub {shift->clearer}, default => '' );
That's 3 characters shorter than method {$self->clearer}. And it has less overhead.