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roles, advice, and BUILD in Moose

by rjbs, created 2010-11-05 10:36
last modified 2011-02-22 18:22
tagged with: @markup:md journal moose perl programming

A very common complaint on #moose is, "BUILD is broken. I put BUILD methods in my code and they never got called." There are a lot of variations on this. They tend to come from the fact that BUILD is not called like almost any other method. Imagine the following class hierarchy:

  ParentClass   <--(does)--<   Role1, Role2
  ChildClass    <--(does)--<   Role3

Normally if we call a method on an object of GrandChild, we first look for that method in GrandChild itself. If we find it, we call it, and we're done. If not, we look in ChildClass. Maybe it's defined there, or maybe it came from Role3. If we don't find it there, we look in ParentClass, which again may have defined its own or gotten it form a role. If we don't find it there, we give up and throw an error. This is all pretty well-understood, even with the addition of roles. They're invisible to the method resolver, so you can just pretend they're not there and that their contents have been stuffed into the classes. That's how they work, after all.

The problem is that BUILD doesn't work like this, which triggers the "this is strange magic" response in the programmer's reptile brain. BUILD is called before new returns, and it's called like this:

  1. BUILD is called on ParentClass, if ParentClass has a BUILD method
  2. BUILD is called on ChildClass, if ChildClass has a BUILD method
  3. BUILD is called on GrandChild, if GrandChild has a BUILD method

Nobody needs to call super in BUILD, something else makes sure each one is called. Now, notice that the three-step process above didn't mention roles. If Role1 (for example) has a BUILD method, then it only gets called because it's included in ParentClass. This is where people start to get confused. It seems like, "everything that is part of the object's composition gets called." Heck, if you put BUILD in Role1, it gets called! So, it seems like roles are "part of the object's composition" and BUILD gets called in them.

This is an illusion, though. For example, if we have a BUILD method in both ParentClass and Role1, the one from Role1 isn't called. This should not be a surprise. The BUILD method is called on every class in the hierarchy, so maybe of of those classes has a BUILD method from a role. It doesn't call them in the role. Here are some of the things that get interpreted as bugs:

So, to let roles contribute their own independent behavior to BUILD, this has emerged:

package Role1;
use Moose::Role;

after BUILD => sub { ... real behavior goes here... };

So, after the BUILD behavior on the class is called, the role's behavior is called. This is generally good enough for the kind of thing people want in a role's BUILD, since role application order is not fully defined. The problem is that maybe no class has defined a BUILD method, so the after advice can't attach to anything and you get a compile error. To combat this, the actual pattern used is this:

package Role1;
use Moose::Role;

sub BUILD {}
after BUILD => sub { ... real behavior goes here... };

We create a stub BUILD and attach to that. If the class using Role1 already has a BUILD, it is not replaced. If it doesn't, we get that basic stub for the advice to apply to. This isn't ideal, though. The next problem is that Role2 might be using the same trick, and when ParentClass includes both Role1 and Role2, their BUILD methods conflict, so we need to go make sure that neither does anything so we can resolve the conflict by adding a stub BUILD to ParentClass, too. Gross.

Wanting to be able to have a role contribute some behavior to an operation without replacing it is pretty reasonable, and there are plenty of use cases. The BUILD stub and its potential conflicts seem, at first, like an edge case, caused by the funky nature of how BUILD is called on every superclass. Unfortunately, there are simlar problems with other methods.

Imagine that ParentClass has a send_message method, and that Role1 and Role2 both contribute behavior to it by using advice. Everything works as expected. Then Role3 adds more advice to the method, and now there are four pieces of code being called when the method is invoked -- still, just as we expect. In GrandChild, however, we override send_message. Now, the advice in our roles was applied to the method defined in ParentClass. When we override that method in GrandChild, the advice is also overridden. This is often a source of confusion, and rightly so: the question of what should happen isn't necessarily clear, so one of two mental models is usually formed by the programmer:

  1. Methods and advice are distinct, and both are inherited distinctly from one another, so that altering one leaves the other intact.
  2. Advice is applied to the first relevant class method as soon as possible and is then no longer advice.

The second model is correct. Is it a fundamentally better model? I don't think it is either necessarily better, but a choice had to be made, and Moose made one. [Update: Shawn Moore reminded me that CLOS made the opposite decision!] If it didn't function this way, we would have had to add a new kind of method declarator, like "override," that meant "and don't apply any previous advice, either." Because it does function this way, we might want ways to say "call the previous method including its advice," and "call my new method body only, but do call previous advice." The former exists, as override paired with super. The latter does not yet exist. So far, I haven't needed it very badly, and it seems safe to believe that neither has anybody else, since it has not yet been coded.

Roles and advice, like most of the rest of Moose, are pretty simple concepts, but they're also pretty new to many programmers. What they are not is magic. When you encounter behavior that's contrary to your expectations, finding out what the actual model in use is can be dramatically helpful, not just with your current problem, but in future work using the system.

In my next post, I will write about my need for other ways to combine code blocks spread across class composition structures, different from either plain old methods, advice, or BUILD.