composing your own behavior across Moose class structures

In my last entry, I wrote about how role composition and advice and BUILD interact. A number of times, I’ve wanted to get behavior that was like BUILD, but without needing the stub method hacks that are needed to get roles to participate in the method call. A very simple example came when I was writing Throwable::X, which had a mechanism for all of its contituent parts to contribute tags. The idea was that any class or role that was part of your class hierarchy could implement an x_tags method that would return a list of tag strings. These methods would all get called and the resulting set of tags would be uniqued and returned.

The only problem was that it totally didn’t work. I tried to implement it by copying from BUILDALL, the mechanism by which BUILD methods are called. This had all the problems that BUILD has (like needing stubs and advice to make things in roles all get called), but I didn’t notice because (a) I was not thinking very hard about it and (b) I was not testing very thoroughly. (Of these, I fault myself more for (b).)

This is what BUILDALL looks like, boiled down:

  my ($self, $params) = @_;
  foreach my $method (
    reverse( class_of($self)->find_all_methods_by_name('BUILD'))
  ) {
    $method->{code}->execute($self, $params);

So, find the BUILD method in every class that has it (started with the least derived class) and then call it. For tags, I also had to gather up return values, but this was simple: I just pushed the results of each method call onto an array and returned it.

My first thought for fixing my implementation was, “I can walk up my class hierarchy (like BUILDALL) but at each class, also look at all of the roles it composed for the method.” Not only was this a bad idea because it violated the notion that roles should not be considered as entities external to a class, after composition, but it was a terrible idea, because it didn’t even solve the problem of including two roles that provide an x_tags method!

What I wanted was a way for a role or a class to add a hunk of code that would be called, no matter what, and then I wanted a way to decide how to interpret the results. In other words, something like this:

package TagProvider;

use MooseX::ComposedBehavior {
  sugar_name   => 'add_tags',
  compositor   => sub {
    my ($self, $results) = @_;
    return uniq( map { @$_ } @$results );
  method_name  => 'tags',

package Foo;
use Moose::Role;
use TagProvider;

add_tags { qw(foo bar) };

package Bar;
use Moose::Role;
use TagProvider;

add_tags { qw(bar quux) };

package ParentClass;
use Moose;
use TagProvider;
with qw(Foo Bar);

add_tags { qw(parent) };

package ChildClass;
use Moose;
extends 'ParentClass';
use TagProvider;

add_tags { qw(child) };

Phew! So, the TagProvider library is generated mostly by this “MooseX::ComposedBehavior” thing, which I was going to have to write. We told it that we wanted add_tags sugar in anything using TagProvider. Later, we’ll call that sugar with a block that we want called on every piece of a class. We’d get all those blocks to be called by calling a method named tags, and it would compose all the results of the add_tags blocks by flattening them and picking the unique values.

Then we get this behavior:

my $obj = ChildClass->new;

$obj->tags;  # set of: foo bar quux parent child

This is just one use for this kind of “call everything everywhere!” behavior. We could, for example, have each sub-block return a deep hashref and then have the compositor perform a deep merge, possibly dying on any conflict. We could have merged our list of arrayrefs-of-tags into a hash telling us how many time each tag occurred. Further, the sub-blocks are all passed the object on which tags was called, along with any arguments passed. Each block could perform analysis on the arguments, for example, and the compositor could reduce the analyses to a single report.

One of my other use cases is similar to that final example. We don’t want to use the compositor for reducing reports – there are other facilities better for this, already, like delegation. We don’t want the sub-block calls to even do the analysis. Delegation handles that better, too. Instead, I have code where each sugar block returns a hash of delegates to be used for performing analysis. The hashes are merged, and conflicts on keys (the delegate’s identifier) are fatal. When instances are constructed, they can replace delegates by name. The “composed behavior” here is a distribued system to allow class components to contribute to a composite default value.

I will note, for now, just one of things to be wary of when using like MooseX::ComposedBehavior – which I did implement and release. Because every part of the class can contribute behavior unconditionally, there is no easy way to say that you want to “reset” the behavior at a certain point in the class hierarchy. You either get all the behavior or none of it.

Moose provides a lot of good tools for reusing code, and many of them focus on ways to compose units of code together: roles and traits, advice, the behavior-building facilities of attributes, and so on. Being able to decompose your code down into simple, easy to understand pieces is one of the most important skills for a programmer to have, and that means both being able to make the most out of the existing, well-known patterns of reuse and also being able to design new patterns of reuse as a last resort when the existing ones don’t quite solve your problem.

Written on November 5, 2010
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