Looks like a misunderstanding of the problem of indirect object, and definitely not a demonstration

Its amazing to see it go back all the way to http://search.cpan.org/~stevan/Moose-0.10/lib/Moose/Util/TypeConstraints.pm#Slightly_Less_Important_Caveat

See 2001 indirect object in camel book , Unconfuse filehandles and classes, Indirect object syntax (RE: Beginnings of Online CGI Course), Why was indirect object syntax inferred here?o

Thanks for your response.

It sounds like you also think this could be a documentation error. Thanks for digging up the original doco: I wouldn't necessarily assume a misunderstanding, it could simply be a typo.

Re "See 2001 ...": unless you're merely indicating that indirect object syntax has been misunderstood for a long time, the significance of the 2001 links eludes me. (The 3rd link is actually 2000, if that's important.) The eariest version of Moose on CPAN (Moose 0.10), referenced in your second paragraph, is dated 05 Jul 2006.

-- Ken

Re "See 2001 ..." ... the significance of the 2001 links eludes me.

Well, it was just link to explain about indirect object pitfall for anyone interested, the "2001" isn't important, I just found it curious, and it slipped into my post :)

#!/usr/bin/perl;

use Moose;
use Moose::Util::TypeConstraints;

use DateTime;
use Test::More tests => 1;

subtype 'DateTime',
    => as 'Object', 
    => where { $_->isa('DateTime') };

has 'datetime' => (is => 'ro', isa => 'DateTime', required => 1);

my $type = find_type_constraint('DateTime');
ok( $type,        'made a DateTime constraint' );
done_testing();
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Interestingly, the only thing really needed to be quoted is DateTime, and only if the line ends after it.

Thanks for your reply, Khen1950fx.

I wasn't suggesting not quoting. Rather, I was saying that I thought subtype DateTime => ... should perform the same quoting as subtype 'DateTime', ...: the B::Deparse tests, showing subtype('DateTime', ... for both forms, suggest this is correct.

My subsequent example without quoting (in "Perhaps this is a documentation error. ...") was intended to show what would actually fail silently. With this form, B::Deparse gives 'DateTime'->subtype, ....

In your test code, I note you've doubled up on the commas (i.e. 'XXX', => ...) in two places. With your code as is, I get

1..1
ok 1 - made a DateTime constraint
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Removing the quotes around DateTime (i.e. subtype DateTime,\n  => ...), I get

1..1
Can't locate object method "subtype" via package "DateTime" at pm_moos
+e_quote.pl line 9.
# Looks like your test exited with 255 before it could output anything
+.
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Then removing the comma after DateTime (i.e. subtype DateTime\n  => ...), I get

1..1
ok 1 - made a DateTime constraint
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The fat comma is now quoting DateTime.

-- Ken

Let's reduce that to the simpler:

subtype DateTime => as 'Object';
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... because that's all that's necessary to see what's going on. The problem is that Perl parses this in two different ways depending on whether DateTime has been loaded. (Or more formally, depending on whether there exists a stash called DateTime::.) If DateTime is not loaded, it gets parsed effectively as:

subtype('DateTime', as('Object'));
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But if DateTime is loaded, it can (at least potentially - these things are tricky to predict) be parsed as an indirect method call, i.e. like this:

DateTime::->subtype( as('Object') );
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This is in fact one of the reasons why it recommends picking a name like "MyDateTime" for your datetime datatype. Differentiating between the Moose type constraint name and the Perl package name.

FWIW, MooseX::Types gives you a much more sugary syntax for types.

Thanks tobyink. I was only referring to instances with DateTime loaded.

The documentation states that this fails silently:

use DateTime;
subtype DateTime => as Object => where { $_->isa('DateTime') };
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I am unable to reproduce this failure.

One idea was that if the behaviour of the fat comma quoting mechanism had changed since Perl 5.8, then maybe the failure only happens with earlier versions of Perl. If this wasn't the case, my only other thought was that this was a documentation error - I provided an example of what would fail silently:

use DateTime;
subtype DateTime, as Object => where { $_->isa(q{DateTime}) };
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-- Ken