# If You Give a Mouse A Christmas

Jerrad Pierce wrote a Perl Advent Calendar entry that has sparked some discussion in the Moose community. He has revised it two or three times, and the final version is disappointing but not blatantly wrong. As a Moose contributor who has deployed Mouse into production I wanted to give some perspective on the issues that came up.

Jerrad suggests strongly in the tone of his article that Mouse is everything you want in Moose but faster. There are some strong issues with this implication, the first being that there is no way to really know if that is true. Jerrad quotes the Mouse documentation by saying

Mouse is “Moose without the antlers” i.e; lacking the thorny dependencies and added heft giving you a pain in the neck.

But the truth is that the antlers the Mouse documentation is talking about isn’t the dependencies (which I’ve blogged about previously), it is the Meta Object Protocol (the MOP). To quote Jerrad again (from one of his re-writes)

[Mouse] can be a nicer, gentler introduction to the world of Moose. Like the cervine form, the rodent provides a simple means of providing accessors which are more explicit than a generalized AUTOLOAD mechanism, while still eliminating redundant code. Plenty of other fancy OO features come along for the ride, but no “metaprotocol stuff,” which some would argue is the raison d’être of Moose.

Mouse implements much of the sugar from Moose, but implements the bare-minimum of an Object System to back up that sugar.

You may be willing to make such a trade-off, but what if you’re not writing the code, and instead run into some other module that foists Moose upon you? That author may or may not need all of Moose, but chances are good they don’t.

Here is where my problem with the solution provided in this calendar entry comes. Chances are that the developer who chose to use Moose has no clue which pieces of the MOP they rely upon. The MOP in Moose is a kind of an iceberg, everything in Moose is build around it. The thin sugar layer that most people interact with is only the surface of what is going on inside.

This is the power of Moose. You don’t need to know what is going on for 90% of the things you do, you just need to know that Moose works, and MooseX::Aliases, or MooseX::Getopt, or MooseX::Storage can hook into the proper places in the MOP and get things done.

Jerrad mentions this almost as an aside though in his calendar entry.

Note that even if other code will compile correctly with Mouse, it’s possible the code could be doing some deep introspection and you may end up with Chet rather than Comet. it is therefore recommended that you run the code’s test suite against Mouse whether you force it through Package::Alias or substituting Any::Moose.

How confident are you that your test suite includes the proper coverage for your application that you can detect subtle bugs in the MOP? I have a few applications that have high 90%+ test coverage, and I’m not sure they would deal well with having their object system replaced underneath them.

The fact of the matter is that unless the upstream module author designed their application with possible Mouse usage in mind, it simply cannot be a safe guaranteed drop in replacement for Moose. This is why the Mouse documentation say to use Moose instead.

This leads to my last, and sadly personal, issue with this calendar post. I’m an active member in the Moose community. I have been for about three years now. I am one of the people who will have to support the kinds of failure that this well intentioned but reckless advice will cause.

If Jerrad had simply talked to us when writing this calendar post, we wouldn’t have reacted so strongly. We could have pointed out the issues in the advice he was giving, and helped him to write a better article that focused on the important part of advocating Mouse as a reasonable replacement for Moose in some circumstances, and the interesting hack he performed with Package::Alias, or Alias.pm, or even the import hack someone in the Moose community suggested.