POSTLUDE
MooseX::Declare would no longer be recommended by anyone as it relies on Devel::Declare which served its purpose but is itself obsolete. At this point if anyone wants MX::D they should look at Moops
ORIGINAL
Assuming I already have a decent knowledge of old-style Perl OO, and assuming I am going to write some new code in some flavor of Moose (yes, I understand there is a performance hit), I was wondering if deeper down either rabbit hole, am I going to wish that I had chosen the other path? Could you SO-monks enlighten me with the relative merits of Moose vs. MooseX::Declare (or some other?). Also how interchangeable they are, one for one class and the other for another, should I choose to switch.
(p.s. I would be ok cw-ing this question, however I think a well formed answer might be able to avoid subjectivity)
MooseX::Declare is basically a sugar-layer of syntax over Moose. They are, for everything past the parser, identical in what they produce. MooseX::Declare just produces a lot more of it, with a lot less writing.
Speaking as someone who enjoys the syntax of MooseX::Declare but still prefers to write all of my code in plain Moose, the tradeoffs are mostly on the development & maintainability side.
The basic list of items of note when comparing them:
MooseX::Declare has much more concise syntax. Things that take several hundred lines in plain old perl objects (POPO?), may take 50 lines in Moose, may take 30 lines in MooseX::Declare. The code from MooseX::Declare is to me more readable and elegant as well.
MooseX::Declare means you have MooseX::Types and MooseX::Method::Signatures for free. This leads to the very elegant method foo(Bar $bar, Baz $baz) { ... } syntax that caused people to come back to Perl after several years in Ruby.
A downside to MooseX::Declare is that some of the error messages are much more cryptic than Moose. The error to a TypeConstraint validation failure may happen several layers deep in MooseX::Types::Structured and getting from there to where in your code you broke it can be difficult for people new to the system. Moose has this problem too, but to a lesser degree.
The places where the dragons hide in MooseX::Declare can be subtly different than where they hide in Moose. MooseX::Declare puts in an effort to walk around known Moose issues ( the timing of with() for example) but introduces some new places to be aware of. MooseX::Types for example have a wholly different set of problems from Moose's native Stringy types[^1].
MooseX::Declare has yet another performance hit. This is known to the MooseX::Declare developers and people are working on it (for several values of working I believe).
MooseX::Declare adds more dependencies to Moose. I add this one because people complain already about Moose's dependency list which is around 20 modules. MooseX::Declare adds around another 5 direct dependencies on top of that. The total list however according to http://deps.cpantesters.org/ is Moose 27, MooseX::Declare 91.
If you're willing to go with MooseX::Declare, the best part is you can swap between them at the per-class level. You need not pick one over the over in a project. If this class is better in Moose because of Performance needs, or it's being maintained by Junior programmers, or being installed on a more tightly controlled system. You can do that. If that class can benefit from the extra clarity of the MooseX::Declare syntax you can do that too.
Hope this helps answer the question.
[^1]: Some say fewer, some say more. Honestly the Moose core developers are still arguing this one, and there is no right answer.