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This is the design document for the Mo module.

It is intended to help people understand the thinking behind Mo.

Like Mo, this document is a work in progress. Nothing here is in stone. Everything is up for discussion. This document just explains the current understanding, so you'll know where to start.

Design Goals

Mo has the following design goals. These goals are all important and have to be balanced against each other.

Provide usable OO for Perl modules.

This includes at least the following. The current implementation contains a bit more than this, but that is due to the other design goals.

Single Inheritance

Mo should provide a mechanism for inheritance. At least single inheritance.

Object Construction

Mo needs to provide a new() object constructor. It should take a set of name/value pairs and return a new object reference, blessed into the class' package name.

Attribute Declaration

Mo needs to provide a way to declare attribute accessing methods. They need to be able to set and get values.

Be minimal

Mo only adds core features if they are considered very desirable and can be implemented in a very small amount of code. Most features are in external feature modules now.

Be useful

Mo wants to be the first module that Perl programmers reach for when they need simple OO. To that end, it needs to be extremely useful and support common idioms, even if they are not strictly minimal.

Be fast

Mo should be about as fast as if you rolled your own OO. It should be fast at both startup (compile) time and at run time. It should especially strive to have fast accessors. Speed optimizations should be simple and minimal.

Be embeddable

There may be situations where you want to inline Mo into your code. For this reason, Mo will try to be in one minimal pure Perl file, with no comments or documentation. See Mo::Inline.

Easy upgrade/downgrade path with Moo

Moose has become the accepted style of OO in perl. Mo will attempt to not do the things it does in an incompatible style to the Moose family.

This is not to say that all Mo code can be switched to Moo, or vice versa. This is to say that you should be able to find a style of coding using the full capabilities of Mo, that you can switch to Moo (or Mouse or Moose), if you want to.

This is a difficult design goal, and might sometimes lose out to the other goals. However, this document will attempt to explain all the decisions.

Current Status

This section will go into detail on all the current aspects of Mo, why decisions were made and any known concerns being thought about.


Mo uses extends to name its (single inheritance) parent.

In the past, Mo supported multiple inheritance. This was considered suboptimal for a few reasons. MI is generally frowned upon in Perl. It has problems that are better solved by roles. It also makes the BUILD call sequence much more difficult, which makes Moose compatibility hard. For these reasons, we removed MI as a Mo feature. As a result, the code became much simpler.

It is highly doubtful that roles will be supported either. One should upgrade to Moo or higher when MI or roles become needed.


Mo uses a new method for construction. It is super minimal and fast. It does no calling of the BUILD sequence. To get that, use:

    use Mo 'build';

Mo uses has for generating accessors. Like Moose, it takes a name and a list of option/value pairs.

All options are silently ignored. Options like default and builder are available as feature modules:

    use Mo qw'default builder';

The default getter/setter is optimized for speed. It does no checks and is always 'rw'.

No runtime checks

Mo will not check or validate its usage. It is so minimal, that it will leave this up to the code author. Run time checks don't offer any gain when the usage is already correct. In a dynamic language like Perl, they only serve to make code slower. Plus, runtime checks would bloat the Mo code. Where would you draw the line? The best option is to leave them out, document things well, and let people write tests for their code, if they really need to. Upgrading from Mo is another alternative.

Size matters

Mo.pm and its feature modules are golfed, compressed and unreadable by mere mortals. It has no comments or pod. The documentation is in Mo.pod and the comments are all in here. This makes Mo.pm a little black box of code that you can use anywhere.

When we hack on Mo.pm we do it on src/. When we are done we run make -C src/ and it compresses stuff into lib/.

The package declaration and $VERSION are on their own lines. That way if someone inlines Mo, they can make their own package line more easily, and just grab line 3, the code.

strict and warnings

We turn on strict and warnings for the user of Mo because it is really easy and offers great value. Also, all the other Moose family does it.

Topics, Concerns and Issues

This is a list of everything else.

The feature system

Mo now has a decent feature system so that almost anything can be added piecemeal. To use features you say:

    use Mo qw'foo bar';

This loads Mo::foo and Mo::bar. It calls the e() subroutine on each.

1024 Size Limit

Mo.pm has a goal to never be larger than 1024 bytes. It has almost never been larger than 500 bytes so far. It is currently under 450 and should only go down (assuming we have the core features nailed down).

Automated Golfing

I want to start a policy that all of the golfing efforts will be made via a script to sane code. We should automate testing both the ungolfed and golfed code.

Golfing has now been automated using the power of PPI.

Moose and Moo creators like Mo

I can't (and am certain that I don't) speak for all the Moose community, But I(ngy) personally know that stevan (creator of Moose) and mst (creator of Moo) approve of Mo. They hang out on the #mo irc channel, and mst has made a large portion of the code commits.

This is a sign of a healthy project, because hopefully we can all forge a clear idea of how all these modules relate to each other and support each other.

Why not Moose?

The sad fact of Perl 5 is that there's no object model built in.

Moose not only added an elegant, usable object model, it took OO to a new level. It is indeed a postmodern system. Unfortunately this comes at a (often hefty) performance price.

Attempts to make something similar but less hefty came in the forms of Mouse, Mousse and Moo. Mo is just the next attempt. It is a bare minimum OO framework, that still looks like Moose.

I've heard people argue that Mo doesn't belong in the Moose family, but those same people often feel the same way about Mouse and Moo. It won't stop me from trying to make something wonderful, that can be fairly easily upgraded to something possibly more wonderful.

With each of these attempts, less of the original Moose power is implemented. This annoys the hardcore Moose developers. But hopefully it pushes them towards making Moose better and better. I can imagine the day when Moose is a compiled in part of the perl interpreter and thus faster then Mo. At that point, all the others will become remnants of the past.

In the meantime, I hope that Mo et al, helps people to get past their Moose inhibitions, and start using the Mo* that makes sense. I(ngy) have authored other OO base modules like Spiffy and Gloom. There are things about those that I sorely miss in the Moose family, but I have decided to stopping fighting the Moose. I for one, welcome our new giant antlered overlord.